Hubble Space Telescope

Faint Object Camera

Instrument Handbook

[Post-COSTAR]

Version 6.0

June 1995 The  Space  Telescope  Science  Institute  is  operated  by  the
Association  of  Universities  for Research in Astronomy, Inc., for the
National Aeronautics and Space Administration.

STScI Contacts for Assistance

Function Contact Name Title Phone e-mail

General instrument questions Antonella Nota FOC Group Lead 410-338-4943
nota@stsci.edu

Robert Jedrzjewski FOC Instrument Scientist 410-338-4817 rij@stsci.edu

Warren Hack Data Analyst 410-338-4943 hack@stsci.edu

Proposal scheduling questions Presto Program Coordinators and Liaison
Scientists 410-338-5080 hstquery@stsci.edu

Any question SSD "Hotseat" help@stsci.edu

FOC Instrument Handbook Revision History

Version Date Of Issue Editors

1.0 May 1985 F. Paresce

2.0 April 1990 F. Paresce

3.0 April 1992 F. Paresce

4.0 February 1993 A. Nota, R. Jedrzejewski, W. Hack

5.0 May 1994 A. Nota, R. Jedrzejewski, P. Greenfield, W. Hack

6.0 June 1995 A. Nota, R. Jedrzejewski, W. Hack

This  FOC  Instrument  Handbook  supersedes  all  previous  post COSTAR
versions. If a conflict exists between this document and another,  the
document  with  the  latest  date  of  issue  should  be accepted.

FAINT OBJECT CAMERA INSTRUMENT HANDBOOK [Post-COSTAR]

A. Nota R. Jedrzejewski W. Hack

Space Telescope Science Institute 3700 San Martin Dr. Baltimore, MD 21218

Version 6.0 June 1995

FOC Instrument Handbook Version 6.0 i

CONTENTS

1.0 HANDBOOK REFERENCE GUIDE 1
1.1 What is the FOC? . . . . . . . . . . . . . . . . . . . . . . . .  1
1.2 Why Use the FOC?  . . . . . . . . . . . . . . . . . . . . . . . .  1
1.3 What other imaging capabilities does HST offer? . . . . . . . . . . . . . .
2
1.4 What is the actual performance of the FOC? . . . . . . . . . . . . . . . .
3
1.5 How do I calculate the FOC exposure times?  . . . . . . . . . . . . . . . .
3
1.6 What features of the FOC have been calibrated? . . . . . . . . . . . . . .
3
1.7 Where can I get updated information on the FOC? . . . . . . . . . . . . . .
3
1.8 Where is information about pre-COSTAR FOC data?  . . . . . . . . . . . . .
3
1.9 What's New with the FOC Handbook? . . . . . . . . . . . . . . . . . . . .
4
1.10 What considerations should be made in preparing an FOC observing proposal?
5

2.0 COSTAR OVERVIEW 8

3.0  INSTRUMENT OVERVIEW 11

4.0 DETAILED INSTRUMENT DESCRIPTION 16
4.1 TRANSFER OPTICS  . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.2 FOCAL PLANE APERTURES . . . . . . . . . . . . . . . . . . . . . . 17
4.3 INTERNAL CALIBRATION SYSTEM . . . . . . . . . . . . . . . . . . .  23
4.4 FILTER WHEELS . . . . . . . . . . . . . . . . . . . . . . . .  23
4.4.1 Bandpass and Neutral Density Filters . . . . . . . . . . . . .  32
4.4.2 Objective Prisms . . . . . . . . . . . . . . . . . . . . . . .  32
4.4.3 Polarizers . . . . . . . . . . . . . . . . . . . . . . . .  . . 35
4.5 LONG SLIT SPECTROGRAPHIC FACILITY  . . . . . . . . . . . . . . . . . 37
4.6 DETECTORS . . . . . . . . . . . . . . . . . . . . . . . .  . . 39
4.6.1 Image Intensifier and Coupling Lens  . . . . . . . . . . . . . . . 39
4.6.2 TV Tube  . . . . . . . . . . . . . . . . . . . . . . . .  . . 40
4.7 VIDEO PROCESSING UNIT . . . . . . . . . . . . . . . . . . . . . . . .41
4.8 SCIENCE DATA STORE . . . . . . . . . . . . . . . . . . . . . . . . . 42

5.0 OBSERVING ARRANGEMENTS 43
5.1 IMAGING, OCCULTATION AND SPECTROGRAPHIC MODES . . . . . . . . . . . 43
5.2 TARGET ACQUISITION MODES  . . . . . . . . . . . . . . . . . . . . . 46
5.2.1 Mode I Target Acquisition - INTeractive ACQuisition  . . . . . . . 46
5.2.2 Mode III Target Acquisition - Blind Pointing  . . . . . . . . . . . 46
5.2.3 EARLY ACQuisition  . . . . . . . . . . . . . . . . . . . . . . .  . 47
5.3 THE FOC TARGET ACQUISITION APERTURES  . . . . . . . . . . . . . . . . 47

6.0 INSTRUMENT PERFORMANCE 48
6.1 THE POINT SPREAD FUNCTION (PSF) . . . . . . . . . . . . . . . . . . . 48
6.1.1 Image quality and Field Dependence of the PSF  . . . . . . . . . .  52
6.2 DYNAMIC RANGE  . . . . . . . . . . . . . . . . . . . . . . . .  . . . . .
52
6.2.1 Uniform Illumination  . . . . . . . . . . . . . . . . . . . . . . . 53

ii FOC Instrument Handbook Version 6.0

6.2.2 Non-Uniform Illumination  . . . . . . . . . . . . . . . . . . . . . .54
6.3 ABSOLUTE QUANTUM EFFICIENCY . . . . . . . . . . . . . . . . . . . . . .56
6.3.1 Format-dependent Effects . . . . . . . . . . . . . . . . . . . . . . 60
6.4 DETECTOR BACKGROUND  . . . . . . . . . . . . . . . . . . . . . . . . . 60
6.5 STRAY LIGHT  . . . . . . . . . . . . . . . . . . . . . . . .  61
6.6 DETECTOR OVERLOAD  . . . . . . . . . . . . . . . . . . . . . . . .  63
6.7 OVERHEAD TIMES AND MULTIPLE EXPOSURES  . . . . .  . . . . . . . . . . . .
64
6.8 GUIDING MODES WITH THE FOC . . . . . . . . . . . . . . . . . . . . . . . 64
6.9 UNIFORMITY OF RESPONSE (FLAT FIELDING)  . . . . . . . . . . . . . . . . 64
6.10 VISIBLE LEAKS  . . . . . . . . . . . . . . . . . . . . . .  . . . . . . 68
6.11 GEOMETRIC DISTORTION AND STABILITY  . . . . . . . . . . . . . . . . . . 70
6.12 PLATE SCALE  . . . . . . . . . . . . . . . . . . . . . . . .  . 73
6.13 CURRENT F/48 PERFORMANCE  . . . . . . . . . . . . . . .  . . . . . . . 73

7.0 OBSERVER'S GUIDE (PRESCRIPTION FOR ESTIMATING EXPOSURE TIMES) 76 7.1 POINT
SOURCES . . . . . . . . . . . . . . . . . . . . . . . .  79
7.1.1 Imaging . . . . . . . . . . . . . . . . . . . . . . . .  . . . 79
7.1.2 Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . .  . 81
7.2 EXTENDED SOURCES . . . . . . . . . . . . . . . . . . . . . . . .  . . 82

8.0 FOC EXPOSURE TIME SIMULATORS 85
8.1  SYNPHOT  . . . . . . . . . . . . . . . . . . . . . . . .  . . . 85
8.2 FOCSIM . . . . . . . . . . . . . . . . . . . . . . . .  . . . . . . 85
8.3 WWW Form for FOCSIM . . . . . . . . . . . . . . . . . . . . . . . .  . 86
8.4 Limiting Magnitudes  . . . . . . . . . . . . . . . . . . . . . . . .  . 87
8.5 Objective Prism Image Simulation  . . . . . . . . . . . . . . . . . . . 87

9.0 FOC WWW PAGES 89

10.0  FOC DATA ANALYSIS AND PRODUCTS 91
10.1 PIPELINE PROCESSING . . . . . . . . . . . . . . . . . . . . . . . .  91
10.2 GENERAL PROCEDURES  . . . . . . . . . . . . . . . . . . . . . . . . . . 93
10.2.1Dark-Count Subtraction . . . . . . . . . . . . . . . . . . . . . . . . 93
10.2.2Format-Dependent Photometric Correction (ITF) . . . . . . . . . . . . .
93
10.2.3Correct For Zoom Mode  . . . . . . . . . . . . . .. . . . . . . . . . .
93
10.2.4Compute Absolute Sensitivity . . . . . . . .. . . . . . . . . . . . . .
93
10.2.5Geometric Correction . . . . . . . . . . . . . . . . . . . . . . . .  .
93
10.2.6Relative Calibration or Flat Field Correction (normal images only) . .
.94
10.2.7Spectrographic Detective Efficiency Correction . . . . . . . . . . . .
.94

11.0 FOC CALIBRATION: STATUS AND ACCURACIES 95
11.1 CURRENT CALIBRATION STATUS . . . . . . . . . . . . . . . . . . . . . . .
95
11.1.1The Routine Calibration Programs  . . . . . . . .. . . . . .  . . . . .
96
11.1.2The Special Calibration Programs . . . . . . . . . . . . .  . . . . . .
96
11.2 CALIBRATION ACCURACIES  . . . . . . . . . . . . . . . . . . . . . . . . 96
11.2.1Absolute Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . 96

FOC Instrument Handbook Version 6.0 iii

11.2.2Geometric Distortion . . . . . . . . . . . . . . . . . . . . . . . . . 97
11.2.3Flat Fields  . . . . . . . . . . . . . . . . . . . . . . . .  97
11.2.4Pointing Accuracy  . . . . . . . . . . . . . . . . . . . . . . . . . . 98
11.2.5Objective Prisms  . . . . . . . . . . . . . . . . . . . . . . . . . . .98
11.2.6Polarization  . . . . . . . . . . . . . . . . . . . . . . . . . 99
11.2.7F/48 Long-slit Calibrations  . . . . . . . . . . . . . . . . . . . . . 99

12.0 ACKNOWLEDGMENTS 100

iv FOC Instrument Handbook Version 6.0

LIST OF FIGURES

Figure 1. Diagram showing the COSTAR correction principle for the F/96 relay.
.8
Figure 2. A Schematic View of COSTAR showing the FOC light paths. . . . . . .
10
Figure 3. FOC Operational and Data Flow Block Diagram. . . . . . . . . . . .
12
Figure 4. Schematic Drawing of the FOC . . . . . . . . . . . . . . . . . . .
.13
Figure 5. The Transfer Optics Block Diagram. . . . . . . . . . . . . . . . .
17
Figure 6. The schematic optical layout of the two cameras.  . . . . . . . . .
18
Figure 7. FOC entrance apertures on the HST focal plane. . . . . . . . . . .
19
Figure 8. The camera entrance aperture for the F/96 relay. . . . . . . . . .
20
Figure 9. The camera entrance aperture for the F/48 relay. . . . . . . . . .
21
Figure 10. Normalized Emission Spectra of the Calibration LEDs.  . . . . . .
24
Figure 11. Transmittance of the wide band filters on the F/96 filter wheels.
27
Figure 12. Transmittance of the visible medium band filters on the F/96 filter
wheels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
28
Figure 13. Transmittance of the UV medium band filters on the F/96 filter
wheels.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . 29
Figure 14. Transmittance of the neutral density filters on the F/96 filter
wheels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  .
.30
Figure 15. Transmittance of all the filters on the F/48 filter wheels. . . .
.31
Figure 16. Optical layout of the focal plane of the F/96 relay with the
FUVOP.34
Figure 17. The Physical Layout of the FOC Polarizers.  . . . . . . . . . . .
35
Figure 18. Image configurations of the F/96 relay for the three polarizers..
.36
Figure 19. The major and minor principal transmittances of the three FOC
polarizers. . . . . . . . . . . . . . . . . . . . . . . . . . . . .  . . . . .
36
Figure 20. Optical layout of the F/48 focal plane in the spectrograph mode.  .
39
Figure 21. Physical Layout of the Imaging Photon Counting Detectors . . . . .
40
Figure 22. Schematic Drawing of a Typical Raster Scan Output of the Detectors
41
Figure 23. Radial profiles of pre-COSTAR aberrated PSF and COSTAR-corrected PSF
at 4860A*.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.48
Figure 24. The encircled energy fraction and PSF profile for the
COSTAR-corrected F/96 and pre-COSTAR F/96 relays. . . . . . . . . . . . . . . .
. . . . . . . . 50
Figure 25. Variation of FWHM with wavelength for the F/96 relay of the FOC  .
51
Figure 26.  Images of PSFs taken with the COSTAR-corrected F/96 camera. . . .
.52
Figure 27. Flat-field linearity plots for the pre-COSTAR F/48 and F/96
detectors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . .55
Figure 28. Linearity relation for point sources. . . . . . . . . . . . . . . .
55
Figure 29. Baseline overall (OTA + COSTAR + FOC) absolute quantum efficiency.
58
Figure 30. Stray light illumination at the OTA focal plane.. . . . . . . . . .
62
Figure 31. The Earth's daylight radiance as a function of wavelength. . . . .
63
Figure 32. Contour plots of the vignetting function for the F/48 relay. . . .
.66
Figure 33. Contour plots of the smoothed flat field for both relays. . . . . .
66
Figure 34. Plots across the UV flat field for both relays.. . . . . . . . . .
67
Figure 35. Ratio between external UV flat field and internal flat field for
both relays.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . 67
Figure 36. The expected count rate as a function of wavelength for the F231M
filter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
69
Figure 37. The 512z * 1024 format distortion field for the F/48 relay. . . . .
72
Figure 38. The 512z * 1024 format distortion field for the F/96 relay. . . . .
72
Figure 39.  Mosaic of F/48 Images taken during the November 1994 test. . . . .
74
Figure 40. F/48 camera background as a function of time after camera
switch-on.75
Figure 41. Residual 1216 and 1304A* airglow contribution to the FOC
background.77
Figure 42. Zodiacal light contribution to the FOC background.  . . . . . . . 79

FOC Instrument Handbook Version 6.0 v

Figure 43. Exposure time required to reach a S/N = 10 on a B5V star.. . . . .
88
Figure 44. Exposure time required to reach a S/N = 10 on a B5V extended
source.88
Figure 45. Flow diagram of FOC imaging data through the Routine Science Data
Processing System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
Figure A1. External flat-field image taken with the pre-COSTAR F/48 relay.
.101
Figure A2. External flat-field image taken with the pre-COSTAR F/96 relay.
.102
Figure A3. Extended format image taken with the pre-COSTAR F/48 relay in
spectrograph mode of an extended external object. . . . . . . . . . . . . .
.103
Figure A4. Central region of an FUVOP image taken with the COSTAR-corrected
F/96 relay.  . . . . . . . . . . . . . . . . . . . .  . . . . . . . . . . . . .
104

List of Tables

Table 1: Summary of FOC Performance Characteristics I. Imaging  . . . . . . .14
Table 2: Summary of FOC Performance Characteristics II. Spectroscopy  . . . .15
Table 3: F/96 Optical Element Characteristics Ordered by Peak Wavelength . . 25
Table 4: F/48 Optical Element Characteristics Ordered by Peak Wavelength . . 26
Table 5: FOC Objective Prism Characteristics . . . . . . . . . . . . . . . . 33
Table 6: F/96 IMAGE or OCC Modes . . . . . . . . . . . . . . . . . . . . . . 44
Table 7: Target Acquisition Formats  . . . . . . . . . . . . . . . . . . . . 47
Table 8: Measured Energy Fraction for the F/96 Relay . . . . . . . . . . . . 49
Table 9: Calculated Flat-Field Linearity Parameters  . . . . . . . . . . . . 53
Table 10: Overall (OTA+FOC+COSTAR) Absolute Quantum Efficiency . . . . . .   57
Table 11: Format-Dependent Sensitivity Ratios.  . . . . . . . . . . . . . . .60
Table 12: Zodiacal Light Intensities in S10 Units  . . . . . . . . . . . . . 78
Table 13: Summary of Cycle 5 Calibration Programs  . . . . . . . . . . . . . 95

FOC Instrument Handbook Version 6.0 1

1.0 HANDBOOK REFERENCE GUIDE

The basic aim of this handbook is to make relevant information about the
performance of the FOC+COSTAR system available to a wide group of astronomers.
This handbook must be used when applying for HST time, and later on to
calculate and plan FOC exposures.The information here provided has been
obtained from in-flight characterization of the FOC , combined with COS TAR
ground and flight calibrations and represents our best present knowledge of the
performance of  the  FOC+COSTAR  system.  This  handbook  covers  not  only
the  description  of  the  camera (hardware and performances), but also
describes how to calculate exposure times in the prepara tion of a proposal,
what features of the FOC have been calibrated, and also how to retrieve the lat
est FOC calibrations files. The following sections, organized through questions
and answers, will help  the  reader  to  locate  where  the  individual  items
are  discussed  throughout  the  handbook.  In addition, at the end (Section
1.10), we provide a summary c

1.1 What is the FOC?

The  Faint  Object  Camera  (FOC)  is  one  of  the  two  imaging  cameras  on
board  the  Hubble Space Telescope (HST). It is a long-focal-ratio,
photon-counting imager capable of taking high resolution two-dimensional images
of the sky up to 14 * 14 arcseconds 2  in size with pixel dimen sions as small
as 0.014 * 0.014 arcseconds 2  in the 1150 to 6500A* wavelength range. Its
perfor mance  approaches  that  of  an  ideal  imaging  system  at  low  light
levels.  The  FOC  is  the  only instrument on board the Hubble Space Telescope
(HST) to fully use the spatial resolution capabil ities of the Optical
Telescope Assembly (OTA) and is one of the European Space Agency's contri
butions to the HST program. Chapter 3 provides a complete overview of the
camera itself, with COSTAR and its functions described in Chapter 2. More
technical details about the specifics of the detectors, such as the apertures,
filters, and electronics are given in Chapter 4.

1.2 Why Use the FOC?

Primarily  the FOC serves  as  an  imaging  camera,  but  the possibility  to
use  objective prisms and the F/48 long slit facility provide also some
spectrograhic capabilities.The following are just some of the unique properties
of the FOC.

a.  Spatial  Resolution:  The  FOC  offers  imaging  capabilities  with  a
pixel  size  of  0.014" and a FWHM of less than 2 pixels (less than 0.6
Planetary Camera pixel). The dependence of the FWHM on wavelength is described
more fully in Section 6.1.

b. UV Sensitivity: The FOC provides peak sensitivity at 3400A* and better
sensitivity than the WFPC2 for all observations with * < 4000A*, down to
1150A*. Section 6.3 discusses the overall throughput of the FOC, while the
large selection of UV filters are described in Section 4.4.

2 FOC Instrument Handbook Version 6.0

c. Polarizers: The FOC provides three polarizer filters with pass directions of
0*, 60* and 120*.  These  filters  provide  a  straight-through,  low
reflection-angle  system  which  intro duces  less than  5% intrinsic
polarization. The specifics  of these filters and their calibra tions can be
found in Section 4.4.3.

d. Objective Prisms: There are two objective prism filters for use with the
FOC, operat ing  in  the  near-UV  and  far-UV  regions.  Objective  prism
images  allow  spectra  to  be obtained  for  different  regions  of  a
spatially  resolved  object  at  the  same  time  with  high throughput, and
low- to medium- resolution from 1700A* - 6000A* for the near-UV prism and
1150A*-6000A*  for  the  far-UV  prism. Section  4.4.2  discusses  the
resolution  and throughput of the prisms in more detail.

e. Deep Photometry of Point Sources: The low detector background and its
insensitivity to cosmic rays allows for long exposures providing very deep
photometry of point sources in the field of view, reaching a S/N of 10 for a
V=26 B5V star in a 45 minute exposure. The limiting magnitudes are discussed in
more detail in Section 8.4.

f. F/48 Long Slit Spectroscopy: The F/48 relay contains a long-slit
spectroscopy facility for  obtaining  spectra  with  a  resolving  power  of
~1150  in  4  orders  spanning  from  3600- 5400A*, 1800-2700A*, 1200-1800A*,
and 1150-1350A*. Problems with the F/48 relay have hampered its use in the last
three cycles, but long slit spectroscopy will be offered as the sole  use  of
the  F/48  relay  in  Cycle  6. Section  4.5  discusses  this  facility  in
more  detail, while Section 6.13 describe the current performance of the F/48
detector itself.

1.3 What other imaging capabilities does HST offer?

In addition to the FOC, HST offers the WFPC2 camera for wide field imaging. The
two cam eras  compliment  each  other.  In  comparison  to  the  FOC,  the
WFPC2  can  offer  the  following advantages:

a.  Area  Coverage:  The  WF  CCDs  have  an  imaging  plate  scale  of  ~0.1"
per  pixel  over each of the three 800*800 chips, while the PC offers a plate
scale of ~0.045" per pixel. This results in a combined field of view of nearly
160*160", compared with the 14"*14" (maximum)  for  the  FOC.  Due  to  the
large  size  of  some  objects,  it  may  be  necessary  to forgo  the
increased  resolution  of  the  FOC  in  order  to  image  the  entire  object
with  the WFPC2.

b. Red Sensitivity: The WFPC2 CCDs have a peak sensitivity in the red portion
of the spectrum with filters all the way out to 1u. In contrast, the FOC cuts
off at about 6000A* resulting in the WFPC2 having better sensitivity than the
FOC for * > 4000A*.

c. High Precision Photometry: Photometry with the FOC suffers from variations
in the background and other effects which limit the errors to about 5-10%.
However, the WFPC2 CCDs  provide a  better  understood  background  that can
result  in  photometry  with  lower errors.

FOC Instrument Handbook Version 6.0 3

1.4 What is the actual performance of the FOC?

Table 1 summarizes the basic performance characteristics of the FOC, while
Chapter 6 breaks down each aspect in more detail. Specific information
regarding the PSF, the sensitivity, the back ground and flat-fields, plus many
other characteristics, is given in the individual sections in that chapter.
This  information  can  then  be  used  to  estimate  the  exposures
necessary  to  carry  out  an observing program.

1.5 How do I calculate the FOC exposure times?

Chapter  7  is  devoted  entirely  to  calculating  the  exposure  time  by
hand  for  any  given  FOC exposure using information given in the Handbook. In
contrast, Section 8.2 describes how FOC SIM  has  been  designed  to
automatically  use  that  same  information  to  calculate  exposure  times and
to simulate FOC images. For point source exposure times calculations, FOCSIM
can actually be run by anyone under the FOC WWW pages (Section 8.3) using a
very simple graphical form. Objective prism observations can now be simulated
using the STSDAS package FOCPRISM as described  in Section  8.5.  This
combination  of  options  should  allow  every  user  to  easily  model their
FOC exposure.

1.6 What features of the FOC have been calibrated?

Chapters  6 and  11 are  both  devoted  to  describing  what aspects  of the
FOC have  been  cali brated and to what accuracy. In general, the most effort
has been spent on determining the abso lute  sensitivity  of  the  FOC,  the
aperture  location  (for  target  acquisitions),  the  geometric distortions
and, to a lesser degree, the flat-field behavior of the F/96 relay. At the
moment of writ ing, very little is known on the post-COSTAR performances of the
F/48. Chapter 11 details cur rent knowledge and future development of the F/48
calibration.

1.7 Where can I get updated information on the FOC?

This handbook can only report on calibrations which were analyzed prior to its
publication, whereas new calibration data continues to be taken throughout each
cycle. As a result, there may be updated information, or maybe new calibration
files, coming out after the publication of this handbook. This up to date
information will be made available via the FOC team's WWW pages (see Chapter 9
for more details) at: http://www.stsci.edu/ftp/instruments.html These WWW pages
provide access to electronic copies of calibration reports, calibration files,
answers on data reductions, and even this handbook. In addition to the WWW
pages, the same information can also be accessed via ftp at:
ftp.stsci.edu/instrument_news/FOC Through these two channels and the usual
documentation, the FOC team will insure that the latest calibration data will
always be readily available.

1.8 Where is information about pre-COSTAR FOC data?

This handbook applies only to post-COSTAR observations. Therefore, for
observers analyz

4 FOC Instrument Handbook Version 6.0

ing pre-COSTAR FOC data, Version 3.0 of the handbook should be referenced. If
there are any difficulties in obtaining a copy of Version 3.0 of the FOC
Instrument Handbook, please contact the FOC team.

1.9 What's New with the FOC Handbook?

A great deal of effort has been put into making this version of the handbook a
more complete reference for FOC. This chapter itself represents a new addition,
not only providing some guid ance in using the handbook, but also summarizing
the latest information about the FOC. Through out  the  handbook,  references
have  been  included  to documentation produced  by  the  FOC  team where it is
available, most of which are available on-line. New information is provided
on:

a. F/48 performance and availability: Problems with the F/48 kept it from being
used in the last three cycles, however, recent tests have shown that the
situation is finally improv ing so that it can be used for science again. As a
result, the F/48 relay will be available for long-slit spectroscopy ONLY in
Cycle 6. The latest information describing the recent performance of the F/48
can be found in the new Section 6.13.

b. Updated DQE:  Previous versions of the handbook have  included a tabulation
of the overall throughput curve for the FOC+COSTAR+OTA system (Section 6.3)
which had the obscuration by the OTA secondary mirror system folded in. This,
however, caused confu sion when it was compared with the curves used by
SYNPHOT, which accounts for the obscuration separately. Therefore, the DQE
curve was re-tabulated to not account for the obscuration. In addition, the
discussion in Chapter 7 which explicitly describes how to use this curve to
estimate exposure times was also updated to account for this change in con
vention. In addition, we have also incorporated the latest calibration results
from Cycle 4 to  insure  that  the  best  available  sensitivity  information
is  provided  (see Section  6.3  for more details).

c.  Behavior  of  PSF  with  wavelength:  New  information  showing  how  the
FOC+COS TAR PSF FWHM changes with wavelength has been added to Section 6.1, so
that users can better understand their FOC images.

d. Improved Bright Object Acquisition Procedure: As promised in Version 5.0 of
the Instrument  Handbook,  a  fully  automatic  procedure  for  acquiring
bright  objects  is  now available. This new procedure is described more fully
in Section 5.2.

e.  Improved  Geometric  Distortion  Calibrations: A  new  method  for
determining  the geometric distortion of the FOC has resulted in a significant
improvement of the distortion correction. This new model along with its effects
on the plate scale and on our understand ing of the stability of the geometric
distortions are described in Section 6.11.

f.  Calibration  Accuracies: The  results  of  the  calibration  activities
have  always  been implicitly incorporated into each previous version of the
handbook, however, this time a new chapter (Chapter 11) has been added to
describe in more detail the present calibration program (Section 11.1), along
with the current accuracies involved in the current calibra

FOC Instrument Handbook Version 6.0 5

tions (Section 11.2).

g. On-line documentation: This handbook along with all other calibration
products and documents produced by the FOC team have been made available
through WWW browsers through  the  STScI  pages.  These  pages  and  what
they  offer  are  described  in  more  detail later in this chapter (Section
1.7).

h. Updated Information on FOC simulators: The FOC Handbook has always contained
a chapter describing the software package FOCSIM, which was primarily
accessible only through an account at the Institute. This chapter, however, has
been enhanced to describe the new software that has been added to simulate
objective prism exposures (Section 8.5) and  to  describe  how  people  can
run  FOCSIM  through  the  FOC  WWW  pages  (Section 8.3).

There are many other less significant changes throughout the handbook as well,
but primarily they only served to update what was already present in earlier
versions, rather than adding any thing new. These updates, along with the new
material listed above, combine to make this hand book the most up-to-date and
complete version possible.

1.10 What considerations should be made in preparing an FOC observing
proposal?

The  proposal  instructions  provide  the  details  of  the  syntax  for
preparing  an  HST  observing proposal, but it is this handbook which provides
the details necessary for putting together a feasi ble program. Unfortunately,
it is a difficult task to keep all the details of the FOC in mind when
preparing a proposal. The following summary describes the most common problems
in using the FOC and where the problems are discussed in the handbook.

a.  Bright  Target  Acquisition:  The  FOC  must  take  great  care  not  to
be  exposed  to  an excessive amount of light. The specific bright limits are
described in Section 6.6. Bright targets, defined as any target of magnitude 9
or brighter, must be observed using the new Bright Object Acquisition Procedure
(described in Section 5.2). This procedure checks to make sure that proper
filters are in place to assure a low enough count rate from the star. The
filter combination, however, must be verified by an FOC Instrument Scientist
prior to execution of the proposal, necessitating a careful simulation of the
expected observation.

b. Photometric Linearity and Image formats: The FOC offers a wide range of
formats for imaging, each with its own photometric characteristics. As a rule,
the smaller the for mat, the higher the count rate can be and still maintain a
photometric linearity in the image (see Section 6.2). The smaller formats are
best suited for imaging brighter objects that are not very large, while the
largest formats provide the best field of view for imaging faint sources. In
general, though, point sources should not exceed 1 count sec -1  pixel -1  for
the central  pixel  (Section  6.2.2)  and  extended  sources  should  not
exceed  0.15  counts  sec -1

pixel -1  in the 512x512 format (Section 6.2.1 and Table 9) in order to obtain
a photometri cally linear image.

c. Small Image Format and INT ACQs: Sometimes it is necessary to use the
smallest

6 FOC Instrument Handbook Version 6.0

formats, such as the 256*256 format. In these cases, it becomes critical to
have pinpoint target acquisition since the field of view is only 3.7"*3.7". An
error of even 1" in the tar get coordinates could result in the target missing
the field of view, since guide star errors are typically 0.6" and the FOC
apertures positions are only known to about 0.2" (see Sec tion 11.2).
Therefore, anytime a format with a dimension less than 512 pixels is used, an
INT ACQ must be taken to insure that the target is acquired within the small
format.

d.  Inaccurate  Target  Coordinates:  The  default  acquisition  mode  is  a
blind  pointing, described in Section 5.2.2. The small field of view for the
FOC places strict constraints on the accuracies for the target coordinates.
Even errors as large as 1" can seriously jeopar dize the observation given the
small fields of view for the FOC. Therefore, all FOC target coordinates should
be measured using GASP in order to provide the most consistent set of
coordinates for this blind pointing.

e. Wrap-around in 8-bit formats: The largest format for the F/96 relay provides
a field of view of 14.7"*14.7". Unfortunately, due to memory constraints
(Section 4.8), the larg est  formats  (512z*1024  and  512*1024)  only  have
8-bit  deep  pixels,  i.e.  they  can  only count up to 255 counts per pixel
(Table 6). For a point source with a count rate of 0.25 counts sec -1  pixel -1
, the peak pixel of the star will count up to 255 in 1000 seconds then reset to
0 on the next count. The image has lost track of those counts, resulting in a
loss of photometry. In order to use this format, short exposures should be
taken, then co-added, as long  as  the  sources  in  the  image  maintain
count  rates  that  are  linear  (see Table  9  for extended sources, Section
6.2.2 for point-sources).

f. Format sensitivity: Each FOC format has a different photometric sensitivity
(Section 6.3.1).  This  usually  causes  no  problem, except in cases  where
images  taken in different formats are compared. If different fields are to be
compared, then taking the images in the same format will insure a consistent
photometric response due to the image format, elimi nating one more source of
error in the photometry.

g. PSF Artifacts: A great advantage of the FOC is to be able to discern detail
on much smaller scales than with any other instrument on HST. However, for the
smallest details, differences in the shape of the PSF must be known in order to
differentiate source struc ture  from  instrumental  structure.  A
description  of  some  of  the  most  obvious  features  of FOC PSFs is given
in Section 6.1, including a jet-like feature in the F372M PSF and some
degradation in the F320W PSF. A couple of other filters (F501N and F502M) both
show faint ghost images, as well. These might be important for programs trying
to detect faint sources near brighter ones. A full library of observed PSFs can
be retrieved using the FOC WWW pages (Chapter 9) to provide a better
understanding of them prior to selecting the filter for use in the proposal.

h.  Background  noise,  stray  light  and  flat-field  features:  Although
the  FOC  benefits from having very low background noise (Section 6.4), there
are still limitations as to what can  be  detected  in  that  faint
background.  Detection  of  very  faint,  background-limited sources relies
heavily on knowing the flat-field response of the FOC (Section 6.9 and Sec tion
11.2.3) and the effect of the geometric correction on the background (Section
6.11). In

FOC Instrument Handbook Version 6.0 7

addition, requirements can be specified in the proposal to minimize the effect
of stray light (Section 6.5) in images of extremely faint sources.

i. Overhead times for images: The use of RPS2 in preparing observing proposals
now allows observers to more efficiently schedule their observations in each
orbit. This system automatically  incorporates  all  the  overhead  times  for
taking  FOC  images  (Section  6.7). One  overhead  which  can  be  minimized
is  the  selection  of  the  filter  for  the  image.  By selecting the filters
one after the other as they are situated on the filter wheels (see Tables 3
and  4  for  filter  wheel  positions),  the  time  spent  selecting  the
filters  can  be  minimized, allowing for more exposure time.

j. Visible Leaks for filters: Selecting the proper filter for the exposure
usually focuses on obtaining a good count rate. However, each filter has its
own transmission characteristics (Section 4.4.1) with their own visible leak
concerns. For some observations, a small per centage of light from the visible
(for a UV filter) does not make a difference. For other sci ence, the precise
amount of light from outside the desired passband can seriously affect the
results.  This  problem  is  addressed  in Section  6.10,  with  FOCSIM  also
providing  a means of judging how serious this problem would be for a given
observation (Section 8.2).

k.  Geometric  Stability:  Long  series  of  FOC  images  have  been  noticed
to  suffer  from small, residual variations in the geometric stability, as
described in Sections 6.11 and 6.12. This can present some problems from
programs needing astrometric conditions for their science. The accuracies in
the distortion which can be expected are discussed in Section 11.2.2, along
with recommendations on how to best account for this in the proposal.

We advise all the users who finally decide to write a proposal using the FOC to
examine all the items presented in this checklist. This will help them to
optimize the scientific return from the instrument, and to obtain excellent
images.

8 FOC Instrument Handbook Version 6.0

2.0 COSTAR OVERVIEW

The HST was placed in orbit on April 24, 1990. A few months later, it was
realized that the 2.4m diameter primary mirror suffered serious optical
degradation due to a manufacturing error at Perkin-Elmer. The primary mirror of
the HST OTA was incorrectly figured with the wrong conic constant,  which
produced  severe  spherical  aberration  at  the  OTA  image  plane  (Burrows
et  al. 1991, Ap.J.Letters, 369,L21.). This aberration, which caused the light
from a star to be spread out into a  circular  Point  Spread  Function  (PSF)
of  2.5 arcseconds radius, could not be  removed by changing the secondary
mirror focus position or by moving the primary mirror actuators. In the fall
of  1990,  a  Strategy  Panel  was  convened  to  investigate  possible
methods  for  removing  the spherical aberration, and made the following
recommendations: 1) replace the Wide Field Plane tary Camera (WF/PC) with the
WFPC2 at the earliest possible opportunity, with the optics of the WFPC2
re-designed to counteract the OTA sp

COSTAR (Corrective Optics Space Telescope Axial Replacement) has restored the
two prime scientific objectives of the FOC: deep imagery and photometry of very
faint celestial objects and imagery of bright objects at the highest possible
resolution available from HST. The FOC is now capable  of  detecting  a  star
of  U  magnitude  27.5  in  a  5  hour  exposure  with  a  S/N  of  5  and  of
resolving bright sources in the near UV up to an effective angular resolution
of 0.03 arcseconds. COSTAR replaced the High Speed Photometer in the Axial bay
of HST, in the V2, +V3 quad rant (see Figure 7). It is a ``passive''
instrument, in that it has no detector of its own, its sole pur pose  being
to  deploy  a  set  of  mirrors  in  front  of  the  other  Axial  Scientific
Instruments  (ASI). These mirrors, and their associated mounts and arms, serve
only to block the aberrated OTA beam from entering the ASI entrance apertures
and to correct the spherical aberration of a different part of the OTA field of
view befor

Figure  1. Diagram  showing  the  COSTAR  correction  principle  for  the
F/96  relay.  The  aberrated OTA beam (dashed line) is blocked from reaching
the FOC aperture by the M2 mirror.

FOC Instrument Handbook Version 6.0 9

beam that would have formed an image 4.658 arcminutes from the V1 axis is
re-directed by the spherical  M1  mirror  to  form  an  image  of  the  OTA
exit  pupil  on  the  M2  mirror.  This  mirror becomes the exit pupil of the
OTA+COSTAR optical system, and the anamorphic aspheroidal fig ure fulfills
three functions:

*  it re-directs the corrected beam into the FOC, forming an image at the FOC
focal plane aper ture;

* it corrects for the OTA spherical aberration;

* it increases the astigmatism from that present at 4.658 arcminutes to that
appropriate to 6.565 arcminutes because the FOC is designed to correct for
exactly the latter amount of astigma tism. However, there is one major
difference between the COSTAR-corrected beam and the unaber rated OTA: the
F/ratio is increased from F/24 to F/37. This results in a change in the
F/number of the FOC relays from F/48 to F/75.5 and from F/96 to F/151. Because
the names ``F/48'' and ``F/96'' are deeply-rooted in the HST ground system at
all levels, from proposal entry to data archiving, we have been forced to
retain these names despite the fact that they do not describe the true focal
ratios of the cameras anymore. As a result, the user must take partic ular care
in reading this handbook, for whenever we mention the F/48 and the F/96 relays,
we are really referring to the relays with focal ratios of F/75.5 and F/151
respectively. A schematic diagram of COSTAR when deployed is shown in Figure 2.
The FOC M1 mirrors are mounte

10 FOC Instrument Handbook Version 6.0

not  suffer  from  vignetting.  For  this  reason,  it  was  decided  to  make
the  F/96  channel  the  `pre ferred' channel of the FOC, and attempt to
optimize the image quality of this channel.

Figure 2. A Schematic View of COSTAR, after deployment, showing the FOC light
paths. For clarity, the FOS and GHRS M2 arms are omitted.

FOC Instrument Handbook Version 6.0 11

3.0  INSTRUMENT OVERVIEW

The Faint Object Camera (FOC) is one of the four axial scientific instruments
sharing the HST focal plane. It is located in the -V2, -V3 quadrant (see Figure
7), has overall dimensions of 0.9 * 0.9 * 2.2 meters, weighs 320 kg and
consumes 130W of power on average in operation. An over all operational and
data flow block diagram of the instrument is shown in Figure 3 with the FOC
itself contained within the dashed line. Radiation from an astronomical source
focused onto the OTA  focal  plane  is reimaged  by  COSTAR and  fed  into
either  of  two  separate  and independent cameras each with its own entrance
aperture, imaging optics and detector system.  One camera magnifies the image
on the OTA focal plane by a factor of two to an effective focal ratio of F/75.5
while the other magnifies the focal plane by a factor of four to an effective
focal ratio of F/151. This transfer is accomplished mainly in order to match
the OTA resolution performance with the available detector pixel size. Each
optical relay consis

counts sec -1  pixel -1  for the F/96 relay and * 10 -2  counts sec -1  pixel
-1  for the F/48 relay (see also Section 6.13 for the current performance).
The SDS storage capacity is adapted to an image area of 512 * 512 resolution
elements and provides a 16 bit data word for each pixel. This results in a
memory capacity of 256K words of 16 bits each or 0.5 Mbyte in total. The word
length can also be commanded to 8 bits to store data in a 512 * 1024 pixel
format with reduced dynamic range. Because of operational constraints, a time
interval of at least 3.9 minutes must elapse between the end of an exposure and
the start of the next (see Section 6.7 for more details). The detectors are
sensitive to radiation between 1150 and 6500A*, the lower limit being set by

12 FOC Instrument Handbook Version 6.0

the  MgF 2   input  window  and  the  upper  limit  by  the  bialkali
photocathode  material.  The  useful photocathode area is 40 millimeters in
diameter while the size of an independent resolution ele ment  (pixel)  is  on
average  normally *  24 *  24  microns  squared,  but  one  dimension  can  be
stretched (zoomed) to * 48 microns. The longer pixel dimension is in the TV
frame scan direction and  perpendicular to  the dispersion  direction  of all
but  one  (the  FOPCD)  of the dispersing ele ments.  The  plate  scales  for
the  F/48  and  the  F/96  relays  are  1.131  and  0.569  arcseconds  mm -1

respectively. These parameters, coupled to the quoted maximum SDS capacity,
imply that the F/48 camera has a maximum achievable field of view of * 28 * 28
arcseconds squared imaged at a pixel size of 0.056 * 0.028 arcseconds squared
(512 zoomed * 1024, 48 * 24 micron squared pixels with 8- bit words). The
corresponding values for the F/96 relay are * 14 * 14 and 0.028 * 0.014 arcsec
onds  squared.  Smaller  fields  can  be  imaged  at  higher  spatial
resolution  and  extended  dynamic

Figure 3. FOC Operational and Data Flow Block Diagram

FOC Instrument Handbook Version 6.0 13

range (see Table 6 for a partial list, and Section 5.1 for more details). In
summary, the key operating features of the FOC are its low noise, high angular
resolution, high sensitivity in the UV range and extreme versatility due to its
occultation, polarization, and objective prism capabilities. Its most
significant limitations, on the other hand, are its relatively small field of
view, further restricted by COSTAR, and the non-linearity of response at high
count rates which limits its useful application to objects yielding less than *
2 counts sec -1  pixel -1 , cor responding  to  a  U *  18.5  A0V  star
observed  through  the  F342W  filter  with  the  F/96  relay,  for example. A
summary of the most important performance characteristics of the FOC as
presently known is given in Tables 1 and 2 for the imaging and spectrographic
modes, respectively.

Figure 4. Schematic Drawing of the FOC

14 FOC Instrument Handbook Version 6.0

a. The F/48 mode is offered in Cycle 6, limited to long slit spectroscopy ONLY.
b. S/N = 5, 5 hour integration, U band. c. Same as b. over 0.1" * 0.1" area. d.
2 counts sec -1 pixel -1 upper limit e. 0.5 counts sec -1 pixel -1  upper limit
f. 0o,60o, 120o direction of polarization

Table 1: Summary of FOC Performance Characteristics I. Imaging

Optical Modes F/48 a F/96

Focal Ratio 75.5 151

Operating Range (A*): 1150-6500 1150-6500

Number of Bandpass Filters: 11 39

Bandpass FWHM (A*) Max: 2700 2300

Bandpass FWHM (A*) Min: 300 34

Maximum ND attenuation (mag.): 0 9

Field of View (arcsec) Max: 28x28 14x14

Field of View (arcsec) Min (128x128): 3.6x3.6 1.8x1.8

Unzoomed Pixel Size (arcsec): 0.028 0.014

Minimum Wavelength for Critical Sampling (A*): 6500 3250

Peak Efficiency (%): 6.6 7.9

Peak Wavelength (A*): 3400 3700

Limiting Magnitude, Point Source b : 27 27.5

Limiting Magnitude Arcsec -2 , Extended Source c : 23.5 23

Dynamic Range, Point Source d  (mag): 20-27 19-27.5

Dynamic Range, Extended Source e  (mag arcsec -2 ): 18-25 17-25

Overload Magnitude: 9 9

Number of Polarizing Prisms f : 0 3

FOC Instrument Handbook Version 6.0 15

a. SP = Spectrograph; SP-CD = Cross Dispersion Spectrograph; F/48 is Available
in Cycle 6 ONLY for long slit spectroscopy(SP). b. Spectrograph characteristics
are given at the following wavelengths for each order: (1) 5000A*, (2) 2500A*,
(3) 1500A*, (4) 1250A* c. S/N = 5, 3 hour integration, flux in emission line at
1500A*(FUVOP) and 2500A*(NUVOP). d. Same as c. over 0."063 * 0."063 area.

Table 2: Summary of FOC Performance Characteristics II. Spectroscopy

Spectral Modes a : F/96-FUVOP F/96-NUVOP F/48-SP F/48 SP-CD

Operating Range b : 1150-3000 1700-4000 3600-5400 (1) 1800-2700 (2) 1200-1800
(3) 1150-1350 (4)

3600-5400 (1) 1800-2700 (2) 1200-1800 (3) 1150-1350 (4)

Maximum Field of View (arcsec): 14*14 14*14 0.06*12.5 0.06*12.5

Number of Bandpass or Order Sorting Filters Available: 33 33 11 5

Number of ND Filters Available: 1 1 0 0

Spatial Resolution (arcsec): 0.05 0.05 0.063*0.1 0.063*0.1

Resolving Power: 300 250 1150 1150

Wavelength (A*): 1500 2500 (1) - (4) (1) - (4)

Spectrum Length (pix): 160 450 1024 1024

Spectrum width (pix): 3 3 <=450 <=450

Maximum Spectral Resolution (A*): 5 10 4 2 1.3 1.0

4 2 1.3 1.0

Peak Efficiency (%): 6.4 6.4 1.1 (1) 0.55 (2) 0.15 (3) 0.13 (4)

1.1 (1) 0.53 (2) 0.13 (3) 0.08 (4)

Limiting Flux, Point Source c  (ergs cm -2 s -1 ): 6*10 -16 1*10 -16 3*10 -16
(1) 7*10 -16 (2) 6*10 -15 (3) 2*10 -14 (4)

3*10 -16  (1) 7*10 -16  (2) 7*10 -15  (3) 3*10 -14  (4)

Limiting Flux, Extended Source d  (ergs cm -2 s -1  arcsec -2 ): 2*10 -14 4*10
-15 3*10 -14  (1) 7*10 -14  (2) 6*10 -13  (3) 2*10 -12  (4)

3*10 -14  (1) 7*10 -14  (2) 7*10 -13  (3) 3*10 -12  (4)

16 FOC Instrument Handbook Version 6.0

4.0 DETAILED INSTRUMENT DESCRIPTION

4.1 TRANSFER OPTICS

 A component block diagram of the FOC transfer optics is shown in Figure 5. A
 conceptual schematic optical layout in a plane containing the V1 axis and the
 chief ray is shown in Figure 6. Radiation from the COSTAR corrected OTA enters
 the FOC through a baffled tube that leads to a field-defining entrance
 aperture located in  a plane tangential to  the  OTA focal surface and cen
 tered on or near the best focus point at the position of each relay. Just
 beyond the entrance aper ture,  the  radiation  encounters  a  light  tight
 shutter  mechanism  that,  in  its  closed  position, introduces a
 calibration  mirror into the beam  to  intercept light  emitted by  an
 internal source of visible radiation and to uniformly illuminate the FOC
 object plane. Once past the shutter, radiation impinges on a two element
 aplanatic optical system consisting of  a  spherical  concave  primary  and
 an  elliptical  convex  secondary  mirror.  This  optical  system magnifies
 the OTA focal plane by a factor of two for the F/48 r

FOC Instrument Handbook Version 6.0 17

of course, due to the MgF 2  cut-off of the detector. Wavelength range
selection is accomplished by introducing suitable bandpass filters into the
optical path of the F/48 relay or by using the objec tive prism (FOPCD) whose
dispersion axis is oriented at * 90* to the grating dispersion direction as a
cross disperser.

4.2 FOCAL PLANE APERTURES

 The two FOC field defining entrance apertures are each located in a plane
 tangent to an OTA focal surface at the center point of the aperture. The
 center of each aperture is located at a linear distance of 110 millimeters
 from the V1 axis. Each FOC channel of COSTAR redirects part of the OTA field
 of view into an FOC entrance aperture. For the F/96, the field is centered on
 a point

Figure  5. The  Transfer  Optics  Block  Diagram.  The  removable  components
are  shown  in  the dashed frames.

18 FOC Instrument Handbook Version 6.0

4.658 arcminutes from the V1 axis, while for the F/48 the field is centered on
a point 4.312 arc minutes from V1. The projections of these apertures onto the
plane of the sky are shown in Figure 7; the dotted circles show the projection
through the OTA only (i.e. the pre-COSTAR positions), while the solid circles
are the positions through the COSTAR+OTA optics. In this figure, the V1 axis
runs into the paper at the center of the WFPC2 field of view and V1, V2, V3,
U2, and U3 are the HST axes defined in the Call for Proposals and Proposal
Instructions. The observant reader will notice that in Figures 8 and 9 the
V2-V3 directions are reversed with respect to pre-COSTAR versions of the FOC
Handbook: this is because COSTAR forms an intermediate image between the M1 and
M2 mirrors. At the locations of the FOC entrance apertures, the OTA focal plane
makes an angle of 10.05* with  the  normal  to  the  ST  axis.  This  plane
is  the  object  plane  for  the  FOC  optical  relays.  The COSTAR-corrected
focal plane

Figure 6. The schematic optical layout of the two cameras in the planes
containing the V1 axis and the chief rays.

FOC Instrument Handbook Version 6.0 19

cameras D96 and D48 that run through the center of the apertures, perpendicular
to and intersect ing on the V1 axis form an angle of 30*. The D96 axis forms an
angle of 30* with the +V2 axis and D48 an angle of 30* with the +V3 axis.

An expanded view of the two apertures in exactly the same perspective is shown
in Figures 8 and 9. The camera aperture for the F/96 relay is a circular
diaphragm of 10.5 millimeters in diam eter corresponding to 24 arcseconds on
the sky in the COSTAR-corrected field, centered at point O with two 2
millimeter-long protruding opaque metal fingers oriented * 30* to the D96 line
and parallel to the V2 axis. The finger on the right is 0.112 millimeters thick
(0.25 arcsecond in the sky)  while  the  other  is  0.223  millimeters  thick
(0.5  arcsecond  in  the  sky).  The  directions  of increasing sample (S) and
line (L) numbers for the extended SDS format define the image coordi nate
system with its center at point C 96 . This system is aligned with the X, Y
reference system

Figure 7. The location of the FOC entrance apertures on the HST focal plane
projected onto the plane of the sky. In this perspective V1 is directed into
the paper at the center of the WFPC2  pattern.  V1,  V2  and  V3  form  the
HST  right  handed  coordinate  system defined in the Call for Proposals.

20 FOC Instrument Handbook Version 6.0

used to designate the orientation of the apertures on the sky in the Proposal
Instructions. The cor ners ABCD of the 512 * 1024 zoomed format are marked on
Figure 8. The large 14 * 14 arcsec ond  square  marks  the  limit  of  the
extended  format.  The  opaque  coronographic  fingers  are indicated by the
hatched regions. The V1 axis is 4.658 arcminutes from O in the direction indi
cated to V1. The fully corrected field point for the F/96 relay coincides with
C 96 . Due to the pres ence of a reseau mark at C 96 , the reference position
for F/96 images has been moved off the center to R 96 , located at (S,L) =
(556,536) and shown in Figure 8.


Figure 8. The camera entrance aperture for the F/96 relay projected onto the
sky.

R96

FOC Instrument Handbook Version 6.0 21

Figure 9. The camera entrance aperture for the F/48 relay projected onto the
sky. KI along the spectrograph slit coincides with the reference axis x that is
used to designate the ori entation of the apertures on the sky in the Proposal
Instructions.  The point P is the optimally corrected field point for the F/48
relay and the new 512 * 512 imaging for mat is shown outlined with a bold
line.

22 FOC Instrument Handbook Version 6.0

The F/48 entrance aperture is shown in Figure 9. The center O of the main
aperture coincides with the center of the extended SDS format and lies on the
sagittal focus while the center J of the slit lies on the tangential focus of
the OTA. Because of vignetting induced in the FOC by moving the OTA exit pupil
from 7 meters to 530 millimeters from the F/48 entrance aperture, it is not
possible to have the center of the extended field unvignetted as well as all of
the slit. The best compromise, which maintains the utility of the long slit as
well as providing an imaging field useful for spectrograph acquisitions, was to
move the optimally corrected field point from the center of the extended format
to the point P, which is where the hypothetical extension of the slit would
meet the edge of the extended imaging format. The default 512 * 512 imaging
format has therefore been moved to that corner of the extended format, and has
been outlined in Figure 9. In this way, over half of the slit suffers less than
20% vign

FOC Instrument Handbook Version 6.0 23

TAR  F/48  spectrograph  fields  obtained  in  orbit  with  external  flat
field  illumination  of  the  FOC entrance apertures are shown in Figures
A1--A3 in the Appendix. The extended format 512z * 1024 pixel images have been
dezoomed and displayed in a 1024 * 1024 pixel format. The occult ing fingers,
spectrograph slit, reseau marks appear clearly together with some blemishes and
large scale response inhomogeneities. The latter are discussed in more detail
in Section 6.9 and 11.

4.3 INTERNAL CALIBRATION SYSTEM

When the shutter is closed, an Al + MgF 2  mirror (see Figure 6) reflects the
light beam from a light emitting diode (LED) calibration source into the
optical path of the relay. The position of the source and the curvature of the
mirror insure a quasi flat field illumination of the object plane. The unit
consists of seven LEDs (two red, two yellow, two green and one blue)
illuminating an inte grating sphere. Their normalized emission spectra are
shown in Figure 10. The unit is capable of illuminating  both  calibration
mirrors  simultaneously.  Each  LED  output  can  be  set  to  256  sepa rately
commandable intensity levels. The calibration system will be used to determine
the detec tor's intensity transfer function, the uniformity of response, the
FOC response to visible light and the geometric distortion. A ground-based
comparison between external and LED flat field illumi nation of the detectors
at the same wavelengths shows that the spatial variations of LED illumina tion
are less than +-3% peak to pe

4.4 FILTER WHEELS

The FOC has six commandable rotating filter wheels holding 58 optical elements
and six clear apertures. Four wheels are on the F/96 relay and two on the F/48
relay. The filter wheels of the F/96 camera have 3 long pass, 9 wide band, 20
medium band, 2 narrow band and 5 neutral density filters. They also contain 3
polarizers and 2 objective prisms. The filter wheels of the F/48 relay contain
3 long pass, 8 wide band, and 3 objective prisms. A complete list of the
optical elements ordered by increasing peak wavelengths * o  is given in Table
3 for the filters of the F/96 relay and in Table 4 for the filters of the F/48
relay. In these tables, ID indicates the filter identification code, FW
indicates the filter wheel number (1-4 for the F/96, 1-2 for the F/48), POS
indicates the posi tion of the filter on the wheel [information useful in
accounting for exposure overhead times], * 0 the wavelength at the peak of the
curve representing the product of the filter transmission (T) with the OTA +
FOC response function

24 FOC Instrument Handbook Version 6.0

these  will  find  a  useful  astronomical  application.  Observing
configurations  requiring  more  than one filter on the same wheel are not
possible, of course. Filter positions on the wheels were care fully selected in
order to minimize this possibility. The time required to change some filter
combi nations may reach 3 minutes. This implies a considerable expense in
overhead time for programs requiring extensive cycling between filters.

Figure 10. Normalized Emission Spectra of the Calibration LEDs.

FOC Instrument Handbook Version 6.0 25

Table 3: F/96 Optical Element Characteristics Ordered by Peak Wavelength

ID FW Pos. * 0 (A*) ** 0 (A*) T(* 0 ) QT(* 0 ) Comments

F120M 3 3 1230 82 0.10 0.0008 Medium band

F130M 3 6 1280 88 0.10 0.0010 Medium band

F140W 2 6 1370 298 0.21 0.0028 Wide band

F140M 3 9 1400 178 0.08 0.0010 Medium band

F152M 3 4 1500 184 0.08 0.0010 Medium band

F165W 3 11 1700 910 0.28 0.0036 Wide band

F170M 3 8 1760 184 0.18 0.0021 Medium band

F175W 2 7 1730 716 0.24 0.0033 Wide band

F195W 3 10 2110 946 0.42 0.0079 Wide band

F190M 3 7 2000 276 0.15 0.0024 Medium band

F210M 3 2 2156 214 0.18 0.0042 Medium band

F220W 2 8 2280 480 0.39 0.0109 Wide band

F231M 3 12 2330 228 0.18 0.0058 Medium band

F253M 4 9 2550 236 0.18 0.0075 Medium band

F275W 2 9 2770 594 0.40 0.0196 Wide band

F278M 4 10 2800 316 0.26 0.0132 Medium band

F307M 4 11 3080 328 0.26 0.0161 Medium band

F130LP 4 7 3750 2202 0.92 0.0630 Long pass (*>= 1300A*)

F320W 2 10 3360 844 0.89 0.0599 Wide band, Not Available (see Fig 25)

F342W 2 3 3410 702 0.81 0.0546 Wide band, U filter

F346M 4 2 3480 434 0.58 0.0390 Medium band, u filter

F372M 4 6 3710 406 0.73 0.0494 Medium band, bar in PSF (see Fig 26c.)

F430W 2 4 3940 832 0.74 0.0469 Wide band, B filter

F370LP 2 2 4020 984 0.83 0.0500 Long pass (*>= 3700A*)

F410M 4 3 4100 194 0.58 0.0337 Medium band, b filter

F437M 4 12 4290 438 0.71 0.0350 Medium band, ghost

F470M 4 4 4710 212 0.79 0.0222 Medium band, v filter

F486N 2 11 4870 34 0.63 0.0155 Interference filter centered on H *  line

F502M 4 8 4940 530 0.82 0.0192 Medium band, ghost

F501N 2 12 5010 74 0.68 0.0151 Interference filter centered on [O III],ghost

F480LP 2 5 5100 744 0.82 0.0183 Long pass (*>= 4800A*), V filter

F550M 4 5 5460 188 0.77 0.0093 Medium band, y filter

F600M 1 5 5800 410 0.8 0.0059 Medium band

26 FOC Instrument Handbook Version 6.0

F630M 1 6 6382 208 0.67 0.0009 Medium band

CLEAR 1,2,3,4 1 3700 2208 1.0 0.0680 Clear aperture

F8ND 1 2 3440 2100 9*10 -4 6 *10 -5 Neutral density, *m = 8.0

POL120 1 3 3700 2180 0.91 0.0626 Polarizer, 120o

F2ND 1 4 3400 2176 0.19 0.0135 Neutral density, *m = 2.0

POL0 1 7 3700 2180 0.92 0.0626 Polarizer, 0o

F4ND 1 8 3400 2176 0.03 0.0021 Neutral density, *m = 4.0

F6ND 1 9 3200 2348 0.005 0.0003 Neutral density, *m = 6.0

PRISM2 1 10 3700 2124 0.94 0.0640 Near UV objective prism (NUVOP)

POL60 1 11 3740 2076 0.92 0.0626 Polarizer, 60o

PRISM1 1 12 3700 2232 0.94 0.0638 Far UV objective prism (FUVOP)

F1ND 3 5 3400 2162 0.39 0.0259 Neutral density, *m = 1.0

Table 4: F/48 Optical Element Characteristics Ordered by Peak Wavelength

ID FW Pos. * 0 (A*) ** 0 (A*) T(* 0 ) QT(* 0 ) Comments

F140W 1 5 1320 300 0.20 0.0047 Wide band

F150W 1 3 1400 628 0.23 0.0059 Wide band

F175W 1 6 1730 678 0.18 0.0035 Wide band

F195W 1 4 2110 1076 0.36 0.0112 Wide band

F220W 1 8 2250 480 0.36 0.0155 Wide band

F275W 2 6 2750 656 0.29 0.0163 Wide band

F130LP 2 7 3400 2722 0.94 0.0536 Long pass (*>= 1300A*)

F180LP 2 5 3400 2702 0.92 0.0528 Long pass (*>= 1800A*)

F342W 2 3 3400 706 0.83 0.0475 Wide band, U filter

F305LP 1 7 3450 1692 0.92 0.0509 Long pass (*>= 3050A*)

F430W 2 4 3950 938 0.75 0.0372 Wide band, B filter

CLEAR 1,2 1 3400 2732 1.0 0.0571 Clear aperture

PRISM3 1 2 3360 2722 0.96 0.0550 Far UV Cross Disperser Objective Prism (FOPCD)

PRISM2 2 2 3400 2704 0.88 0.0504 Near UV Objective Prism (NUVOP) Not Available

PRISM1 2 8 3400 2738 0.96 0.0548 Far UV Objective Prism (FUVOP) Not Available

Table 3: F/96 Optical Element Characteristics Ordered by Peak Wavelength

ID FW Pos. * 0 (A*) ** 0 (A*) T(* 0 ) QT(* 0 ) Comments

FOC Instrument Handbook Version 6.0 27

Figure 11. Transmittance of the long pass and wide band filters on the filter
wheels of the F/96 relay as a function of wavelength.

28 FOC Instrument Handbook Version 6.0

Figure 12. Transmittance of the visible medium and narrow band filters in the
filter wheels of the F/96 relay as a function of wavelength.

FOC Instrument Handbook Version 6.0 29

Figure 13. Transmittance of the UV medium band filters on the filter wheels of
the F/96 relay as a function of wavelength. The F120M and F130M filter
transmission curves remain essentially flat at 10 -4  beyond * 2500A*.

30 FOC Instrument Handbook Version 6.0

Figure 14. Transmittance of the neutral density filters on the filter wheels of
the F/96 relay as a function of wavelength.

FOC Instrument Handbook Version 6.0 31

Figure 15. Transmittance of all the filters on the filter wheels of the f/48
relay as a function of wavelength.

32 FOC Instrument Handbook Version 6.0

4.4.1 Bandpass and Neutral Density Filters

In general, the long pass filters are Schott colored glass combined with a low
pass filter, the wide  band filters are  metallic  UV filters,  the  medium
band filters  are multi-dielectric  multi-ele ment with ZnS-Th F 4  layers,
and  the interference  filters are multi-dielectric multi-element with ZnS
chiolithe  layers.  The  measured  transmission  versus  wavelength  curves
for  all  filters  and attenuators for the F/96 relay and the F/48 relay are
shown in Figures 11--15. In order to suppress ghost images, the external faces
of all mono-element filters are parallel to within 5 arcseconds or better. For
multi-element filters the tolerance is 1 arcminute. The cemented elements have
a wedge angle of 1* or less. In order to minimize losses in the modulation
transfer function,  the  external  faces  are  flat  to */5  peak  to  peak
at  6300A*  and  the  internal  faces  in  the multi-element filters are flat
to */2 peak to peak. The refractive index is homogeneous to a level of *n< 2 *
10 -5  to be con

4.4.2 Objective Prisms

The  objective  prisms  consist  of  either  a  single  30  millimeter
diameter,  3  millimeter  thick wedged crystal of MgF 2  (the FUVOP and FOPCD,
called PRISM 1 and PRISM 3 in the Instruc tions) or a combination of two wedged
crystals of MgF 2  and SiO 2  glued together (the NUVOP called PRISM 2 in the
Instructions). The former operates down to 1150A* with a wavelength reso lution
*/*** 50  at  1500A*  while  the  latter  has  a  dispersion */***   100  at
2500A*  but  transmits only above * 1600A*. All of the prisms disperse in a
direction oriented roughly anti-parallel to the increasing line number (L)
direction except FOPCD on FW # 1 of the F/48 camera that, instead, disperses
in  a  direction  roughly  perpendicular  to  L  or  about  90*  to  the
others.  This  last  one  is meant as a cross disperser (CD) for the long slit
spectrograph (see Section 4.5). The MgF 2  prisms (FUVOP  and  FOPCD)  on  the
F/48  relay  (Prisms  1  and  3)  are  both  preceded  by  a  3  mm.  thick CaF
2  window in o

I

I

FOC Instrument Handbook Version 6.0 33

Table 5: FOC Objective Prism Characteristics

F/96 FUVOP * = 8o x(pix); R(A*/pix) T

F/96 NUVOP * = 11o x(pix); R(A*/pix) T

F/48 FUVOP * = 12o x(pix); R(A*/pix) T

F/48 NUVOP * = 12o x(pix); R(A*/pix) T

F/48 FOPCD * = 100o x(pix); R(A*/pix) T *(A*)

-431;1.7 0.44 1200

 -374;2.4 0.58  -183;5.1  0.61  -208;5.1 0.62 1300

-350;4.1 0.64  -168;8.8 0.69  -191;8.8 0.70 1400

-325;5.6 0.73  -159;13.7 0.76  -181;13.7 0.76 1500

-314;9.3 0.85  -153;19.5 0.82  -174;19.5 0.82 1600

-303;11.7 0.88  -149;25.9 0.86  -169;25.9 0.86 1700

-297;15.9 0.91 -353;0.7 0.77  -145;33.0 0.9  -50; 0.78  -165;33.0 0.9 1800

-290;19.1 0.92 -238;1.1 0.79  -142;40.7 0.92  -46; 0.79  -161;40.7 0.91 1900

-286;23.9 0.93 -161;1.7 0.8  -140;49.2 0.92  -32;15.7 0.8  -159;49.2 0.92 2000

-282;29.6 0.95 -108;2.1 0.82  -138;58.6 0.94  -19;10.5 0.82  -157;58.6 0.93
2100

-280;38.8 0.95 -69;3.0 0.83  -137;69.0 0.95  -11;11.6 0.84  -156;69.0 0.94 2200

-272;50.4 0.96 1.6;4.4 0.86  -133;107.6 0.96 5;20.9 0.86 -151;107.6 0.96 2500

-265;92.3 0.96 62;11.7 0.88  -130;203 0.96 23;41.8 0.88  -148;203 0.96 3000

-261;161 0.94 99;18.7 0.94  -128;348 0.97 33;67.9 0.89  -145;348 0.96 3500

-259;279 0.94 123;25.4 0.94  -127;55 0.98 40;99 0.89  -144;554 0.97 4000

-257;526 0.94 153;38.5 0.94  -125;1192 0.98 47;168 0.89  -142;1192 0.97 5000

-255;1562 0.92 168;70.4 0.9  -125;2192 0.98 52;247.5 0.85  -142;2197 0.85 6000

34 FOC Instrument Handbook Version 6.0

+L), positive below it (towards -L) consistent with the directions shown in
Figure 16. A blow-up of this spectrum extending from 1200 to 6000 A* as
dispersed by the FUVOP is shown on the right hand side of this figure where the
solid curve gives the position x along of any wavelength for this case. The
reciprocal of the slope of this curve yields the resolution R in A*/pixel given
in the figure for several representative wavelengths. Values of the linear
coordinate x(*), R(*), T(*) the transmission of the prism and the value of *
for each prism is listed in Table 5  as  a  function  of  wavelength.  Please
note  that  the  angle *  for  the  prisms  of  the  F/96  relay increases
clockwise  from  -L  while  it  increases  counterclockwise  from  -L  for
the  prisms  of  the F/48 relay due to the different orientation of the F/48
format shown in Figure 9. The position of the entire dispersed FOV with respect
to the undispersed FOV is also shown in Figure 16. The former is displaced
upwards by 5.88 millimet

A composite image showing the central 256 * 512 pixels of the undispersed image
of a star in the post-COSTAR F/96 256 * 1024 image and its associated Far-UV
objective prism spectrum is shown in Figure A4 in the Appendix. In this 256 *
1024 centered format, the star was placed well below  the  center  of  the
format  so  that  in  the  next  image,  the  dispersed  FUVOP  spectrum  is
roughly  centered.  The  two  images  were  then  co-added  to  produce
Figure  A4.  It  should  also  be apparent from an inspection of Figure 16 and
Table 5 that careful consideration must be given to the positioning of the
format and/or the target object in the format in order to ensure that the ensu
ing spectrum falls on the correct part of the frame. This is especially
critical for the FUVOP's that have a large offset and a spectrum length which
is a sizable fraction of a typical field of view. The simplest way to handle
this problem is through judicious use of the POS TARG special require ment
described in the Proposal I


Figure 16. Optical layout of the focal plane of the F/96 relay with the FUVOP
inserted in the beam.  The star is assumed located at C 96  in the entrance
aperture of Figure 7.

FOC Instrument Handbook Version 6.0 35

ticularly interesting feature in the spectrum of an object located at * 1500 A*
close to the center of the image for the F/96 relay listed in Table 6 using the
FUVOP. According to the data given in Table 5 and the situation illustrated in
Figure 16, one would specify a POS TARG 0, -4.6 because 1500A* falls 325 pixels
or 4.64 arcseconds from the undispersed position of the object in the posi tive
Y(or L) direction specified in the Proposal Instructions.

4.4.3 Polarizers

The FOC polarimeter consists of three MgF 2  double Rochon prisms located on
FW1 in the F/96 relay. Each prism consists of an optically contacted double
Rochon prism combination act ing as a three element birefringent beam splitter.
The pass directions of the prisms are at 0*, 60*, and  120*,  counterclockwise
from  the  image  X  axis  (-S  direction)  as  projected  onto  the  sky.  A
schematic drawing of the device is shown in Figure 17. These values are known
to an accuracy of approximately 3* (see Section 11.2.6 for more details). The
optical axes of the outer components A and B are oriented perpendicular to the
optical beam  axis  while  the  central  component  C  has  its  optical  axis
parallel  to  the  beam  axis.  The entrance face is at the base of the central
prism. In this configuration, the ordinary ray is transmit ted without
deviation while the extraordinary rays are deviated by the interface between
the outer and central prisms. Thus, three exit beams emerge from the polarizer.
The orien

The  major  principal  transmittance  (T MAJ )  of  the  undeviated  beam
through  the  three  prisms and the minor principal transmittance (T MIN ) of
the normal nonpolarized light are given in Figure 19. Notice that one of the
polarizers (POL60) does not transmit below * 1800A*. There are limita


Figure 17. The Physical Layout of the FOC Polarizers. Dimensions are in
millimeters.

36 FOC Instrument Handbook Version 6.0

Figure 18. Image configurations on the focal plane of the F/96 relay for the
three polarizers.


Figure 19. The major principal transmittance T MAJ  and the minor principal
transmittance T MIN of the three FOC polarizers as a function of wavelength.

FOC Instrument Handbook Version 6.0 37

tions on the accuracy which is attainable with this facility, but current
calibrations have attained accuracies for relative photometry of approximately
5% (1*), with more details available in Sec tion 11.2.6. A major factor is that
the three different polarizers have somewhat different through puts,  even
longward  of  3000  Angstroms.  While  the  filter  transmissions  have  been
measured, filters do change with time, and color variations in the source will
result in small differences in the actual throughput. In addition, the light
reaches the polarizers only after reflection off the OTA, COSTAR and the FOC
primary and secondary mirrors (but before the fold mirror). Each of these six
reflections is at a non-zero angle of incidence, ranging from a few minutes of
arc for the OTA primary to about 11.5* for the FOC secondary. Such reflections
introduce a phase shift in incident polarized light and a slight instrumental
polarization which Cycle 4 calibrations have determined to be less than 5%
based on obse

4.5 LONG SLIT SPECTROGRAPHIC FACILITY

Due the encouraging results of the last tests (see Section 6.13), the F/48
camera will be made available in Cycle 6, FOR LONG SLIT SPECTROSCOPY ONLY. This
is to be considered a limited resource, with minimum amount of calibration
avail able (See section 11 for more details). This facility consists of the
following four elements: 1.a rectangular (0.063 * 12.5 arcsecond) slit placed
on the F/48 camera entrance aperture at the OTA tangential focus as shown in
Figure 9, 2.order sorting bandpass filters and/or a cross dispersing objective
prism on the filter wheels of the F/48 relay, 3.a removable toroidal convex
mirror which picks off the F/48 beam between the folding mir ror and the DHU
and reflects it towards 4.a fixed spherical concave reflection grating which
reimages the slit spectrum onto the detec tor photocathode as shown in Figure
6. The last two optical elements are slightly tilted and decentered with
respect to the optical relay axis of the F/48 in order to center the spectrum
of the slit onto the pho

38 FOC Instrument Handbook Version 6.0

Function (PSF) that correspond to * two to three 24 micron pixels. Using the
Rayleigh resolution criterion, the actual resolving power of the instrument is
* 1150 in all orders with a spectral reso lution  of  4,  2,  1.3,  and  1A*
for  first,  second,  third,  and  fourth  orders  respectively.  These  values
have been confirmed by ground-based calibration using line source stimulation.
Both the spectrograph mirror and the grating work with unit magnification. The
convex mirror corrects the astigmatism introduced by the spectrograph's optical
elements. The resulting image is nearly free of astigmatism and image tilt with
respect to the photocathode plane. The fixed grating configuration of the long
slit spectrograph implies that light from all orders falls simultaneously on
the same area of the detector. Because of the limitations of the UV bandpass
filters, any order may be contaminated with light from another, resulting in
possible ambiguities in line identifica tion and degradation of achievable
signal

FOC Instrument Handbook Version 6.0 39

4.6 DETECTORS

The transfer optics described in Section 4.1 relay the image produced on the
OTA focal plane to the photocathode of a two dimensional photon counter drawn
schematically in Figure 21. The detector consists of three basic parts: a three
stage image intensifier tube, a coupling lens, and a TV camera tube.

4.6.1 Image Intensifier and Coupling Lens

The intensifier is an EMI 9614 three stage tube magnetically focused by means
of a permanent magnet.  The  first-stage  photocathode  (like  the  following
two)  is  a  hot  bialkali  for  the  highest quantum efficiency in the UV-blue
region and the lowest dark-count rate at 17*C. It has a useful diameter of 40mm
and is deposited on a MgF 2  input window.

Figure 20. Optical layout of the F/48 focal plane in the spectrograph mode and
the FOPCD in the beam.  The coordinates are line numbers (L) as ordinates and
sample numbers (S) as abscissas. Notice the different scales for S and L.



40 FOC Instrument Handbook Version 6.0

The photoelectrons generated at the first stage are accelerated by a 12
kilovolt potential and impinge on a P11 phosphor layer coupled by a 4 micron
thick mica membrane to the second pho tocathode.  This  amplification  process
is  repeated  in  the  second  and  third  stages  to  achieve  an overall
photon gain of 1.3 * 10 5 . Focusing of the intensifier electrons is
accomplished with a care fully shaped permanent magnet assembly and a trimcoil
is added around the third stage for fine adjustments. The limiting spatial
resolution of the intensifier is 35 line-pairs per millimeter. The dark cur
rent at an ambient temperature of 17* C is less than 10 counts cm -2  s -1  (10
-4  counts pixel -1  s -1  in the  normal  mode).  Both  of  these
characteristics  are  essentially  limited  by  the  first  stage  of  the
intensifier tube. A lens assembly consisting of 9 components in a double
Gaussian design is used to transfer the image from the output phosphor of the
intensifier to the fiber-optic faceplate of th

4.6.2 TV Tube

The camera tube is a Westinghouse WX32719 low-light TV tube. This is a
high-sensitivity, high resolution EBS tube (Santilli and Conger in
Photo-Electronic Devices, AEEP, ed. L. Marton, 33A, 1972) with a 25 millimeters
square diode array target, magnetic focus and deflection coils and an
electrostatically focused image section with a 40 millimeter diameter useful
photocathode area. The S-20 photocathode is evaporated onto the concave inner
surface of the input fiber-optic faceplate.  The  emitted  photoelectrons  are
accelerated  by  a  potential  of  up  to  12  kilovolts  and focused onto the
target which is an N-type silicon wafer with diffused P-type regions arranged
in an hexagonally-packed diode array. During  operation  each  diode  is
reverse  biased.  Incoming  photoelectrons  generate  electron

Figure 21. Physical Layout of the Imaging Photon Counting Detectors

FOC Instrument Handbook Version 6.0 41

hole pairs which discharge the diodes. An amplified charge pattern
corresponding to the image is then stored in the diodes. The charge flowing in
the target lead, when the scanning beam recharges the diodes, is the signal
current. The target gain is about 2500 and the modulation transfer func tion is
50% at 8 line-pairs per millimeter. The video signal coming from the TV tube is
amplified by the preamplifier and then transmitted to the Video Processing Unit
(VPU).

4.7 VIDEO PROCESSING UNIT

Each  camera  has  its  dedicated  VPU  which  accepts  the  amplified
signals  from  the  camera preamplifier. The purpose of the VPU is to determine
the x-y centroid of each event, determine if a true photon event has occurred
and to increment the SDS memory address corresponding to the location where the
photon event was detected. During any one scan of a frame of duration of 30
milliseconds for the 512 * 512 format down to 520 microseconds for the smallest
64 * 64 format there will only be a few scattered photon events. A photon event
is typically a spot with a diame ter of 3 or 4 pixels. It is read by the
scanning beam on successive lines of the raster scan. Figure 22  illustrates
how  such  a  signal  would  look  using  the  z  axis  to  represent  the
magnitude  of  the charge. As a line is scanned, a gaussian shaped pulse is
produced. As successive lines are scanned, additional pulses, corresponding to
slices of the event, increase in peak amplitude until a maxi mum is reached.
The pulse amplitud

Figure 22. Schematic Drawing of a Typical Raster Scan Output of the Detectors

42 FOC Instrument Handbook Version 6.0

4.8 SCIENCE DATA STORE

As an event is detected and classified as valid, the video processing unit
causes the science data store (SDS) to increment by one the memory location
corresponding to the event centroid. The image is gradually built up over the
exposure time. After stopping the exposure, the SDS can be read out without
disturbing the stored image. The SDS can accept a 512 * 512 line image in the
16 bit word mode or a 512 * 1024 line image in the 8 bit word mode. The cycle
time for the SDS is compatible with the camera scan rate of >=10 6  pixels per
second. This rate is maintained for all formats and zoom. In this latter mode,
the camera read beam scans the target twice as fast in the line direction as it
does in the normal imaging mode but the pixels are twice as long. The detector
generates  an  increment  command  for  every  pixel  in  which  a  photon
event  has  been detected. A scan of 256K pixels occurs in less than 30
milliseconds while a scan of 4K pixels (i.e. a 64 * 64 pixel squared format)
takes place in appr

FOC Instrument Handbook Version 6.0 43

5.0 OBSERVING ARRANGEMENTS

5.1 IMAGING, OCCULTATION AND SPECTROGRAPHIC MODES

Operationally, the observation of an astronomical source with the FOC is
defined once the fol lowing physical parameters are specified in terms of RPS2
Exposure parameters: 1.The optical relay or configuration (Config): FOC/96 or
FOC/48 2.The choice of filters or spectral elements(Sp_Element) chosen from the
4 filter wheels of the F/96 or 2 filter wheels of the F/48 camera: See Tables 3
and 4. 3.The optical mode (Opmode) of the camera: ACQ, IMAGE, OCC, or SPEC
[spectrograph mirror in the F/48 beam] 4.The S*L imaging format (Aperture):
S<=512, L<=1024 pixels with S the number of SDS pix els in the line scan
direction and L the number of SDS pixels in the increasing line direction 5.The
pixel size (Optional Parameter: Pixel Size): 25 * 25 microns squared (default)
or 50 * 25 microns squared (for zoomed formats) 6.The position of the target in
the chosen format (Special_Requirement: POS TARG). Teleme try monitoring,
however, can only verify the starting pixel with an accuracy of 32 pixels.
7.The exposure time f

46 FOC Instrument Handbook Version 6.0

5.2 TARGET ACQUISITION MODES

Two  acquisition  modes  are  available  for  use  with  the  FOC  as
described  in  the  Proposal Instructions: INTeractive ACQuisition (Mode I) and
blind pointing (Mode III). On-Board Acqui sition (Mode II) is not available due
to technical limitations. In addition, an EARLY ACQuisition can be specified
where an examination of the field is necessary prior to science exposures to
help measure a target in a crowded field or to determine a slit alignment
angle, for example.

5.2.1 Mode I Target Acquisition - INTeractive ACQuisition

Designed  to  be  the  most  accurate  acquisition  procedure  for  use  with
the  coronographic  fin gers, the slit or small image formats, this procedure
requires the involvement of the observer, in real-time, to identify the field
and measure the center of the target on an FOC image.   First, using a blind
pointing acquisition, the target is placed in a standard field of view and an
image is taken, with instrument parameters such as filters and exposure time
selected by the user. The resulting image  of  the  field  is  then  read
down  for  immediate  display  in the  Observation Support  System (OSS)  area
at  STScI.  Once  the  observer  identifies  the  target using  an
interactive  image  display system and measures the target position from the
display screen, a slew request is generated and up-linked, and the telescope is
maneuvered to place the target in the selected destination. There is no
verification of the subsequent field unless specifically requested by the
observer in their Phase II pro

5.2.2 Mode III Target Acquisition - Blind Pointing

Mode III is the default acquisition procedure for the FOC. When no target
acquisition is spec ified in the Special Requirements section on the proposal
exposure logsheets, the telescope per forms  a  straight-forward  blind
pointing  on  the  coordinates  provided  by  the  user.  After  the

FOC Instrument Handbook Version 6.0 47

acquisition,  no  explicit  verification  of  the  target  position  in  the
fields  of  view  aperture  is  per formed. No overhead time in addition to the
guide stars lock time is charged to the user for this acquisition. Mode III is
expected to be the standard acquisition procedure for IMAGE mode in all instru
ment  configurations,  except  for  those  formats  smaller  than  the  F/96
512  x  512.  The  user  must ensure  that  the  target'  coordinates  have
been  measured  with  the  Guide  star  selection  system Astrometric Support
Package (GASP) ensuring best accuracy with respect to the guide star astro
metric  catalogue.  The  procedure  will  not  be  useful  for  point  sources
in  either  OCC  or  SPEC modes, because blind pointing is unlikely to provide
the precision needed to accurately place a target on the slit of the new F/48
relay (width = 0.06 arcseconds) or on one of the coronographic fingers.
However, for extended objects where accurate pointing is not important, blind
pointing c

5.2.3 EARLY ACQuisition

When  necessary,  it  is  possible  to  take  an  acquisition  image  some
time  before  the  scientific observation. The acquisition image can be used
for a better identification of the field, or a better evaluation of the source
flux, etc. In order to update the observation parameters, a minimum turn around
time of two months is necessary between the acquisition and the science
exposure. Presently,  the  system  does  not  have  the  capability  to
select  for  the  science  observation  the same pair of guide stars
successfully used for the acquisition exposure, thus eliminating the possi
bility  of  using  the  acquisition  image  to  measure  the  target
coordinates  at  the  accuracy  level required to perform, for example, a blind
pointing on the F/48 0.06 arcseconds slit.

5.3 THE FOC TARGET ACQUISITION APERTURES

For convenience, a number of special formats to be used in the Mode I
acquisition exposures have  been  defined.  These  formats  (listed  in  Table
7)  are  calibrated  and  maintained  in  order  to ensure good pointing
accuracy (better than 0.2''). This applies currently to the F/96, however, the
F/48 pointing accuracy has not been established yet. Different  sizes  are
available,  but  it  is  strongly  recommended  to  use  the  larger  images
for acquisition purposes, due to the initial position error of the target's
coordinates with respect to the Guide Stars (*0.33 arcseconds).

Table 7: Target Acquisition Formats

Relay Format Name Size USE

F/48 512z * 1024 1024 * 1024 spectrograph slit

F/48 512 * 512 512 * 512 spectrograph slit

F/96 512z * 1024 1024 * 1024

F/96 512 * 512 512 * 512

48 FOC Instrument Handbook Version 6.0

6.0 INSTRUMENT PERFORMANCE

6.1 THE POINT SPREAD FUNCTION (PSF)

Before the installation of COSTAR, the HST+FOC PSF suffered from severe
spherical aber ration, which meant that a circular aperture of 0.1 arcsecond
radius contained only 15-18% of the light from a star instead of the expected
70%. The principal effect of the spherical aberration was a loss in
sensitivity, because most of the light in the halo of a faint star is
effectively lost in the background noise. COSTAR has restored much of the OTA
capability, in that the COSTAR-cor rected PSF contains more than 75% of the
light within a radius of 0.1 arcsecond at visible wave lengths while only
losing less than 20% of the light to the two reflections at the two extra
mirror surfaces. The net increase in sensitivity is a factor of approximately
3-4 at visible wavelengths. The correction COSTAR made to the PSF is
illustrated in Figure 23, which shows the radial pro file of an aberrated PSF
image and a COSTAR-corrected image.

In Table 8, the encircled energy fraction *(*) is tabulated for various
circular apertures against the number of pixels in the aperture and the
effective radius (defined as , with the def inition that the encircled energy
is 1.0 at a radius of 1 arcsecond (70 pixels)). A more thorough discussion of
the definition of the encircled energy calculation and the rationale behind it
is given in the Detective Quantum Efficiency section, 6.3. These energy
fractions are to be used to predict

Figure 23. Radial profiles of pre-COSTAR aberrated PSF (dotted) and
COSTAR-corrected PSF (solid) at 4860A*.



FOC Instrument Handbook Version 6.0 49

how many counts one can expect to measure given the total count rate from
FOCSIM or SYN PHOT, or from using the formulation in Chapter 7. For the most
part, these numbers are believed to be good to approximately 10% or so, but
they are subject to variation due to the changes in the effective focus of the
OTA (`breathing'). This typically causes the fraction of the energy in small
apertures (with  radius 3  pixels  or so)  to  vary  by ~10%,  but  with  worst
case variations of  up  to 50%, and is most severe in the 2000-4000 A* range.
Taking PSF observations just before or after one's science data will not help
to improve the measurement of encircled energy, since the varia tion is orbital
in nature. The  improvement  in  performance  over  the  aberrated  PSF  is
shown  in  Figure  24,  where  the encircled energy curve is compared to that
of the aberrated OTA and with a perfect diffraction limited image from a 2.4m
circular aperture with a 0.33 central obstruction. It can be seen that the
COSTAR-corrected

Table 8: Measured Energy Fraction *(*) for the F/96 Relay

r Filter

n (arcseconds) F120M F140M F170M F210M F278M F346M F410M F486N F550M

1 0.000 0.039 0.061 0.055 0.094 0.093 0.087 0.071 0.064 0.054

9 0.024 0.214 0.294 0.319 0.415 0.452 0.409 0.385 0.380 0.345

21 0.037 0.315 0.421 0.463 0.555 0.621 0.528 0.543 0.548 0.532

37 0.049 0.393 0.499 0.558 0.626 0.712 0.601 0.627 0.613 0.611

69 0.067 0.491 0.577 0.652 0.688 0.774 0.722 0.744 0.697 0.670

97 0.080 0.536 0.619 0.696 0.722 0.802 0.773 0.799 0.769 0.727

137 0.095 0.578 0.657 0.739 0.768 0.832 0.809 0.830 0.831 0.807

177 0.108 0.599 0.682 0.763 0.798 0.858 0.833 0.844 0.855 0.851

225 0.121 0.626 0.703 0.781 0.821 0.886 0.856 0.858 0.870 0.870

293 0.139 0.653 0.724 0.798 0.841 0.909 0.877 0.877 0.886 0.881

349 0.151 0.664 0.736 0.808 0.851 0.919 0.896 0.888 0.897 0.888

421 0.166 0.682 0.749 0.819 0.859 0.928 0.916 0.905 0.909 0.898

489 0.179 0.696 0.762 0.829 0.867 0.934 0.927 0.922 0.917 0.906

577 0.194 0.712 0.777 0.841 0.873 0.942 0.938 0.934 0.928 0.915

665 0.209 0.723 0.790 0.850 0.877 0.948 0.944 0.941 0.941 0.921

749 0.222 0.734 0.799 0.855 0.883 0.951 0.948 0.945 0.950 0.929

861 0.238 0.747 0.810 0.863 0.889 0.954 0.952 0.950 0.955 0.942

973 0.253 0.750 0.821 0.871 0.896 0.958 0.956 0.953 0.958 0.949

1085 0.260 0.765 0.830 0.879 0.902 0.961 0.958 0.957 0.961 0.952

1201 0.281 0.774 0.839 0.886 0.909 0.964 0.959 0.961 0.964 0.955

1313 0.293 0.790 0.847 0.892 0.913 0.967 0.961 0.964 0.966 0.958

1457 0.309 0.803 0.856 0.900 0.917 0.970 0.964 0.967 0.969 0.961

1597 0.324 0.813 0.865 0.907 0.923 0.972 0.966 0.970 0.972 0.963

1741 0.338 0.822 0.875 0.914 0.929 0.974 0.968 0.973 0.975 0.964

50 FOC Instrument Handbook Version 6.0

taken for the DQE measurement program typically allows measurement of the PSF
profile to 25- 50% accuracy when azimuthally averaged over a 1-pixel wide
annulus. Variations of the profile will occur in the core as a result of OTA
orbital variations, but outside 0.15 arcsec radius the PSF profile is dominated
by scattering from small-scale irregularities in the OTA & COSTAR mirrors and
FOC optics. It is recommended that if observers need accurate characterization
of the PSF for their data (for example, if they are trying to detect
low-surface brightness features in the vicinity of bright point-like sources),
then they should explicitly ask for such calibration time in their pro posal.
Again, it is stressed that characterization of the PSF interior to 0.1 arcsec
is not possible, because of its dependence on orbital variations in the focus.
The resolution provided by the HST+FOC combination is characterized in a
simplistic way by specification of the FWHM of the PSF as a function of
wavelength. This was measured

Figure 24. The encircled energy fraction and PSF profile for the
COSTAR-corrected F/96 and pre-COSTAR F/96 relays compared to those expected
from a perfect diffraction lim ited OTA.

FOC Instrument Handbook Version 6.0 51

sures 1.6-1.8 pixels in the visible (~0.08 arcsec). The discrepant point comes
from the PSF using the F320W filter, which is significantly degraded in
resolution to a FWHM of 0.09 by 0.035 arc sec. Despite the outstanding
performance of the OTA+COSTAR+FOC imaging system in terms of encircled energy
within small radii, the PSF appearance does not quite match a true diffraction
limited simulation perfectly at all wavelengths. The PSF obtained using the
F486N filter shows a non-uniform azimuthal intensity distribution in the first
diffraction ring (Figure 26a). There is also a small amount of residual coma
that possibly varies with time, possibly due to some slack in the M1 tilt
mechanism. This was removed for the most part by a small tilt of the COSTAR M1
mirror in early August 1995. In the ultraviolet, the PSF shows a fairly strong
jet-like feature pointing approximately in the - V3  direction  (Figure  26b).
The  strength  decreases  with  increasing  wavelength  but  is  still  quite
noticeable at 4000A*.


Figure 25. Variation of FWHM with wavelength for the F/96 relay of the FOC

52 FOC Instrument Handbook Version 6.0

total light in the PSF, compared to 10% for the F342W filter. A selection of
PSF images is available on STEIS. The PSF images, and other FOC informa tion on
STEIS, is accessible via anonymous FTP, gopher, or WWW's Mosaic interface at
stsci.edu (see Chapter 9 for more details).

6.1.1 Image quality and Field Dependence of the PSF

The FOC was designed to image the HST focal plane in an off-axis position, 6.56
arcminutes from the optical axis. At this distance, the focal plane is tilted
with respect to the V1 axis by 10*. It is this plane that the FOC cameras image
onto their photocathodes. However, the focal surface produced by COSTAR is
tilted with respect to the plane that the FOC images. This results in a
field-dependent focus variation of approximately 0.7mm over the full field of
the F/96 relay. Sim ilarly, the tangential and sagittal focal surfaces are
tilted with respect to each other, and this intro duces field-dependent
astigmatism. Both of these effects increase linearly with distance from the
fully-corrected field point. The field-dependence of the PSF was investigated
during SMOV, but the limited observations do not allow detailed
characterization of the performance as a function of field position. It is
clear from calibration observations  that the PSF is visibly different away
from the central field point across  the  la

6.2 DYNAMIC RANGE

 If two or more photon events overlap during a given frame, the VPU detection
 logic will only count one detected photon. This `undercounting', or
 non-linearity, sets a hard limit on the maxi mum allowable photon rate for the
 FOC. This limit depends on the frame scan time, which is pro portional to the
 area in pixels of the selected format. Users can improve the linearity
 performance by choosing a smaller imaging format (but at the cost of field of
 view). The linearity performance also depends on the image structure. If the
 illumination is nearly

Figure 26.  Images of PSFs taken with the COSTAR-corrected F/96 camera. a.
F486N filter b. F120M filter c. F372M filter.



FOC Instrument Handbook Version 6.0 53

uniform, then the non-linearity depends on the frame scan time and the photon
event size (typi cally 3 * 3 pixels). However, if the illumination comes from a
perfect point source, then the pho ton event size does not matter, since there
are no neighboring photon events, only those that arrive in the illuminated
pixel. For FOC images, no source is truly pointlike, but the linearity
character istics of astronomical point sources and flat fields are sufficiently
different that they are discussed separately.  The  situation  for  more
complex  image  structure  (i.e.  just  resolved  or  linear)  will  of course
be intermediate between these two limiting cases.

6.2.1 Uniform Illumination

Here `uniform illumination' refers to the case where the intensity varies by
less than 20% over scales of 20 pixels. The frame scan time is given by

where z = 1 for normal and z = 2 for zoomed pixels. For the most widely-used
format (512 * 512, normal pixels), this comes to 30 milliseconds. If, during a
frame, more than half of the for mat area is occupied by photon events, a
further event will overlap one or more existing events and will not be counted
as a detected photon. This would predict a maximum count rate for the 512 * 512
format of 0.05 counts/pixel in 30 milliseconds, or about 1.7 counts/sec/pixel.
In prac tice the saturation level is reached at a lower level, because most of
the overlapping events are much larger than a single photon event and are
classified as ion events and rejected.

Table 9: Calculated Flat-Field Linearity Parameters

Camera Format (F*L) Linearity Parameter a N MAX (cs -1 )

F/96 512z*1024 0.11 0.04

512*1024 0.37 0.08

512*512 0.73 0.15

256*256 2.93 0.60

128*128 11.7 2.40

F/48 512z*1024 0.065 0.03

512*1024 0.26 0.05

512*512 0.52 0.10

256*256 2.09 0.40

128*128 8.37 1.60

F/48 SPEC 256z*1024 0.13 0.03

256*1024 0.52 0.10

T f z S L*( )

8.8 6 *10 ---------------------- sec=

54 FOC Instrument Handbook Version 6.0

The flatfield nonlinearity was measured on-orbit during OV and SV using
observations of the internal  LED  calibration  lamps.  It  was  found  that
the  intensity  of  the  light  from  these  lamps  is directly proportional to
the commanded intensity level. The linearity curve was measured for sev eral
different formats to verify the format dependence of the saturation, and the
dependence on zoom mode was also investigated. A plot of the measured linearity
relation for the 512 * 512 for mats of the pre-COSTAR F/96 and pre-COSTAR F/48
detectors is shown in Figures 27a and 27b, respectively. Superposed is a curve
that describes the behavior of the linearity relation for inten sity  values
up  to  approximately  80%  of  the  saturation  value,  originally  suggested
by  Jenkins (M.N.R.A.S., 226, 341 (1987)):

where r is the measured count rate, * is the `true' count rate and a is a
fitting parameter that is identified as the asymptotic measured count rate.
When the FOC is configured for zoomed pixels, the linearity performance is
slightly different from what would be expected from scaling the results for
normal pixels by the ratio of frame scan times. This is because the event sizes
and the VPU detector logic are different for zoomed pixels. However,  once
the  linearity  performance  of  one  format  is  calibrated  for  each
camera  in  zoom mode, the performance at other formats in zoom mode can be
derived by scaling by the ratio of format areas, as is the case for normal
pixels. The validity of the scaling assumption for both nor mal and zoomed
pixels was checked and found to be true. Values  of a  for  the  most
commonly-used  formats  are  given  in  Table  9.  The  values  refer  to
dezoomed  data  in  the  case  of  formats  that  were  originally  zoomed.
In  practice,  the  value  of a depends somew

6.2.2 Non-Uniform Illumination

  When  the  illumination  comes  from  a  star,  the  FOC  is  able  to
  count  at  a  much  higher  rate before  saturation  occurs.  This  is
  because  photon  events  centered  on  pixels  close  to  the  central pixel
  of a star are much less probable than in the flatfield case. However, because
  it is difficult and time-consuming to obtain stellar images at a large number
  of intensity levels, it was not possible to calibrate the point-source
  non-linearity relation for the pre-COSTAR PSF to the same accuracy as could
  be achieved in the flatfield case. The dependence of the core structure of
  the PSF on such factors  as  secondary  mirror  position,  jitter  and
  wavelength  also  made  such  an  investigation impractical. During OV and SV
  it became clear that for the FOC pre-COSTAR F/96 camera when used in the 512
  * 512 format, the maximum obtainable count rate in the core of a star was
  approximately 3 counts/second/pixel. Variations on this level occur because
  of jitter, focus, etc. At count levels higher

r a 1 e *- a/( ) -( )=

FOC Instrument Handbook Version 6.0 55

Figure 27. Flat-field linearity plots for the detectors in the pre-COSTAR F/48
and pre-COSTAR F/96 relays, a and b respectively, based on 512 * 512  pixel
flat-field images of  the internal LEDs at different intensities. The solid
line in each plot is the best-fit solution for the linearity function given in
the text.


Figure 28. Linearity relation for point sources based on 512 * 512 pixel images
taken with the pre-COSTAR F/96 relay through the F342W filter.

56 FOC Instrument Handbook Version 6.0

brightest pixel. This produces the simple guideline for observers: keep the
count rate in the cen tral pixel below 1 count/sec, and then any photometry
method chosen will give results that are not biased by non-linearity effects.
If the central count rate is above 1 count/sec, one will need to use other
methods to determine the brightness of the star (such as measuring the
intensity in the bright halo). An example of the non-linearity relation for
point sources is shown in Figure 28. This is a plot of the brightest pixel
intensity for stars in two FOC pre-COSTAR F/96 512 * 512 images of the center
of the globular cluster  M15.  One image  was  taken with  F342W+F2ND
filters,  while  the other was taken with F342W+F2ND+F1ND filter. Apart from
the large scatter, which is under stood, one can see that there is no
measurable deviation from a linear relation for central count rates up to 1.8
counts/sec. Clearly, any photometry method that includes the brightness of
pixels other than the brightest pixel will be

6.3 ABSOLUTE QUANTUM EFFICIENCY

Spectrophotometric  standard  stars  were  observed  during  SMOV  and  Cycle
4  Calibration using  a variety of  filters to allow measurement  of both  the
PSF  characteristics  and the  detector quantum  efficiency.  These
measurements  were  made  after  the  COSTAR  mirror  tilts  and  DOB focus
were set to optimize the imaging performance at 4860 A* wavelength for the F/96
camera. To date, no attempt has been made to characterize the F/48 camera, so
this section refers to the F/96 camera only. The  encircled  energy  and
detector  quantum  efficiency  are  somewhat  coupled  since  the  PSF does not
have a well-defined edge; instead the flux drops steadily with distance from
the star cen ter until it gets lost in the background noise. The flux in the
wings is due to scattering by dust and small imperfections in the
OTA+COSTAR+FOC optical train, and is more pronounced at shorter wavelengths.
When constructing an encircled energy curve, which is the curve of the fraction
of light enclosed within a circ

FOC Instrument Handbook Version 6.0 57

Table 10: Overall (OTA+FOC+COSTAR) Absolute Quantum Efficiency Q(*) in 10 -3
counts photon -1

*(A*) Q(F/48) Q(F/96) Q(SPI) Q(SPII) Q(SPIII) Q(SPIV)

1150 1.45 1.00 0.20

1160 4.04 1.39 0.32

1170 6.52 2.30 0.40

1180 9.28 3.39 0.47

1190 11.47 4.62 0.61

1200 13.14 5.88 0.46 0.70

1250 19.95 9.11 0.63 0.95

1300 22.74 10.93 0.70 1.08

1400 22.68 12.37 0.88 1.26

1500 22.30 13.47 1.05

1600 20.13 12.92 1.15

1700 18.72 12.32 1.34

1800 17.84 11.72 2.04 1.55

1900 20.26 13.12 2.69

2000 26.41 16.86 3.32

2200 40.80 26.20 4.60

2400 51.59 35.44 5.33

2600 53.59 43.06 5.55

2800 55.43 51.36 5.47

3000 55.87 60.66

3400 56.62 67.65 10.48

3800 51.70 67.73 10.64

4000 47.28 61.51 10.58

4500 35.45 38.66 10.24

5000 21.27 22.06 8.45

5500 8.97 10.88 4.84

6000 3.95 4.90

6500 0.38 0.46

58 FOC Instrument Handbook Version 6.0

Figure  29. Baseline  overall  (OTA  +  COSTAR  +  FOC)  absolute  quantum
efficiency  in  counts photon -1  as a function of wavelength for the three
imaging modes and the four long slit spectrograph orders, including the
obscuration of the OTA.

FOC Instrument Handbook Version 6.0 59

With COSTAR, the magnified plate scale means that such a large aperture size
cannot be used for DQE measurements, particularly since most of the
measurements of spectrophotometric stan dards were made using the 256X256
imaging format to improve the linearity performance. For this reason, it was
decided to define the encircled energy to be 1.0 at a radius of 1.0 arcsecond
(70 pixels) and to define the background as that value which minimizes the
scatter of the points in the encircled  energy  curve  with  0.9"  <  r  <
1.1".  In  practice,  this  is  equivalent  to  setting  the  back ground  to
the  value  measured  at  approximately  1.0"  radius,  and  it  does  give
encircled  energy curves that are qualitatively in agreement with what such a
curve would look like: the encircled energy asymptotically approaches a
constant value at the last measured points. Users should be aware that there is
some flux outside 1 arcsecond radius, especially in the ultraviolet, but this
flux is not considered "useful" an

60 FOC Instrument Handbook Version 6.0

The spectrograph efficiency is shown for the four orders of the grating (I, II,
III and IV) with no order sorting filters in the beam. These measurements were
made before launch; no on-orbit calibration of the spectrograph sensitivity has
been attempted, although the observations that have been made show that the
ground-based calibrations are consistent at the 50% level. For the F/96 relay,
uncertainties in the DQE curve are approximately +-10% (1*), while for F/48
errors  in  the  2000-6500A*  range  for  the  imaging  modes  should  not
exceed +-  20%  and  for wavelengths  below  2000A*  they  are  expected  to
be  of  the  order +-  50%.  This  latter  uncertainty should be applied to all
the spectrograph data especially in the orders III and IV.

6.3.1 Format-dependent Effects

It has been found that the DQE is a function of detector format (see Instrument
Science Report FOC-075). The cause of this is not known. The relative
sensitivities for each format are given in Table 11, where the 512X512 format
is set to 1.0 by definition. The DQE values given in Table 10 and Figure 29
refer to the 512X512 format. Typical uncertainties in these numbers are approxi
mately 5% Table 11: Format-Dependent Sensitivity Ratios.

6.4 DETECTOR BACKGROUND

The detector background arises primarily from thermal electrons at the first
photocathode and high energy particles. In the 600~km altitude, 28* inclination
orbit of HST, substantial fluxes of magnetospheric electrons and protons are
encountered in the South Atlantic Anomaly (SAA). The more energetic of these
particles are capable of generating intense flashes of C ^ erenkov radiation in
the MgF 2  faceplate of the FOC intensifiers. Since this noise source
originates as photons at the very  front  end  of  the  detector,  the  Video
Processing  Unit  of  the  FOC  is  not  able  to  distinguish between real
celestial photons striking the cathode and C ^ erenkov generated photons. The
threshold  energy  for C ^ erenkov  radiation  in  MgF 2   is  E>220~keV  for
electrons  and E>400~MeV for protons. Shielding of 4 mm aluminum or more was
built into the design of the FOC in order to prevent electrons of energies E <
3-5~MeV from reaching the detectors from any

Camera Format (F*L) Relative Sensitivity

F/96 512z*1024 1.25

512z*512 1.45

512*512 *1.0

256*256 1.20

128*128 1.23

F/48 512z*1024 1.44

256z*1024 1.28

512*1024 1.02

512*512 *1.0

256*256 0.85

FOC Instrument Handbook Version 6.0 61

direction. The  effects  of  the  SAA  on  the  FOC  were  extensively  mapped
during  the  commissioning phase.  The  FOC  turned  out  to  be  considerably
less  sensitive  to  SAA  electrons  than  had  been feared. This is presumably
due to the additional shielding to electrons provided by the rest of the HST
spacecraft. The response of the FOC to SAA protons on the other hand is in good
qualitative agreement with the expectations-although the sensitivity of the two
FOC detectors differs some what. The  highest  background  rates  (0.2~counts
pixel -1   s -1   in  the  F/48  during  nominal  operations and 0.02 counts
pixel -1  s -1  in the F/96) are encountered over South America within the peak
of the SAA  proton  density  distribution.  Since  these  rates  are  not
high  enough  to  cause  damage  to  the FOC detectors, the FOC is kept fully
operational during SAA passages. However, such high back ground  rates  do
exclude  useful  scientific  observations.  A  ground-track  contour
delineating

6.5 STRAY LIGHT

Normally, the FOC background is dominated by the detector, by zodiacal light in
the visible and by geocoronal Lyman alpha and diffuse galactic light in the far
UV (see Section 7 for detailed calculations  of  these  components).  When  a
bright  object  such  as  the  Sun,  Moon  or  the  bright Earth limb is
nearby, however, it may be dominated by stray light reaching the OTA focal
plane due  to  scattering  from  the  baffle  system,  the  OTA  tube  and
dust  on  the  mirror.  The  observed brightness of stray radiation at the OTA
focal plane due to the proximity of the Moon or bright Earth  limb  in  the
daytime  part  of  the  orbit  in  V  magnitudes  arcsec -2   as  a  function
of  the  angle between the Moon or the limb and the OTA axis is shown in Figure
30. These in-orbit calibrations of the baffle attenuation were performed by P.
Bely for angles above 30*, while the attenuation for angles less than 30* were
derived by D. Elkin. The spectral shape of the stray radiation in the case of
the Earth

62 FOC Instrument Handbook Version 6.0

This means that observations in the visible will be limited mainly by this
source of background. Below 3400A*, however, this effect will be negligible, as
shown in Figure 31, providing an oppor tunity to use the CVZ for the detection
of faint sources with the minimum level of background light. It  is  of  more
than  passing  interest  to  the  observers  to  pay  some  attention  to  the
maximum allowable background they can tolerate for their specific observation.
The most heavily affected observations  are  those  of  faint  extended
sources,  where  the  background  can  have  comparable count rates. For
example, a spiral galaxy with B=20.5 magnitudes arcsec -2 could be observed
with the F430W filter and achieve a S/N of 10 over a 0.1"x0.1" region in 57
minutes under average observing conditions. Generally this would require a CVZ
observation since most orbit viewing periods are only about 50 minutes long (on
average). However, in the CVZ, the stray light can be as bright as V=21
magnitudes arcsecond -

Figure 30. Stray light illumination in V magnitudes arcseconds -2  at the OTA
focal plane due to  the  Moon  and  daylit  Earth  as a  function  of
off-axis  angle  determined  from  in orbit observations. Limits on zodiacal
light contribution are also given.



FOC Instrument Handbook Version 6.0 63

background  from  stray  light,  and  included  in  the  Phase  II  proposal
if  deemed  necessary.  These considerations can dramatically improve the
detection of faint targets through the best use of the CVZ.

6.6 DETECTOR OVERLOAD

The FOC detectors described in Section 4.6 may be damaged by illumination
levels exceeding 10 7   photons  s -1   pix -1   at  the  photocathode  due
to  point  sources  and  by  an  average  illumination from a diffuse source
over the whole photocathode exceeding 10 4  photons s -1  pix -1 . Because of
this danger, the 36 kV HV power supply on the 3 stage image intensifier is set
to trip off when the point source illumination exceeds the value given above or
if the average illumination from a dif fuse  source  exceeds  200  photons  s
-1   pix -1 .  Thus,  for  safety  reasons,  no  point  source  delivering more
than 10 6  photons s -1  pix -1  at the photocathode or a diffuse source
delivering more than an average rate of 100 photons s -1  pix -1  over the
whole photocathode will be allowed to be imaged by the FOC. These values
correspond to an 9th magnitude blue star or a diffuse source of surface
brightness 10 magnitudes arcsec -2  viewed through the F430W filter with the
F/96 relay. Targets brighter t

Figure 31. The Earth's daylight radiance in Rayleighs A* -1 i.e. 10 6
/4*photonscm -2  sec -1 A* -1 sr -1

as  a  function  of  wavelength  averaged  over  one  orbit  given  for
different  Earth  limb angles. The sun is assumed to be at the zenith providing
the most stray light.



64 FOC Instrument Handbook Version 6.0

6.7 OVERHEAD TIMES AND MULTIPLE EXPOSURES

Assuming that the standard science data dump operations at the 32 kHz rate
apply, it will take a constant 3.9 minutes plus a variable component to
transition from the absolute time tag to stop an exposure to the absolute time
tag to start the next one. The variable component depends on the mode change
required and can be up to another 3.5 minutes for the worst case with the F/96
relay (4 filter wheels) to 1.9 minutes for the worst case with the F/48 relay
(2 filter wheels). Thus, it could take up to a total of 7.4 and 5.8 minutes of
time between successive exposures with an aver age of approximately, 6 and 5
minutes for the F/96 and the F/48 relays, respectively. In some spe cific
situations, it may be advantageous, in order to save time, to take multiple
exposures without closing shutter or dumping science data. Up to 11 consecutive
exposures of this type can be made. If  no  changes  to  FOC  mechanisms  are
required,  the  time  interval  between  exposures  can  be reduced to 23
seconds total. This is t

6.8 GUIDING MODES WITH THE FOC

The  FOC  will  default  to  fine  lock  (estimated  RMS  jitter  0.005
arcseconds  out  of  day/night transitions) for all configurations. Users may
not specify coarse track for their observations since it has been determined
that guiding in this mode has a detrimental effect on the FGSs. Similarly, gyro
hold may not be used because the large pointing and stability uncertainties are
not conducive to optimization of the science. Occasionally, it will be found
that two guide stars are not available for an observation. In that case, STScI
will  notify the  PI  and  offer the possibility of  single  guide star
observing.  For rela tively short exposure times (<2000s), the impact on
performance is very small. The observation will not be executed in this mode
unless the PI gives approval. Sometimes the FGSs fail to achieve fine lock for
an observation. In this case, the Take Data Flag comes down, the shutter is not
opened, and the users are notified of what happened.

6.9 UNIFORMITY OF RESPONSE (FLAT FIELDING)

The  extended  format  (512z *  1024)  geometrically  corrected  flat  fields
for  both  of  the  new relays are shown in Figures A1-A2. The F/48 image shows
the approximate location of the new default 512 * 512 format, which is no
longer at the center, but close to the upper right quadrant. The flat fields
were obtained from overlapped observations of the inner region of the Orion Neb
ula and are at 3727 and 1360A* respectively. The flat fields show a number of
various types of fea tures, some more subtle than others. The more evident
features are the occulting fingers for the F/96 relay, the slit finger for the
F/48 relay (used as a fiducial reference to the spectrograph slit) and the
reseau marks. Because of the geometric correction, the edges of the original
raw images can be seen as curved edges in these images, mainly on the left and
right sides. Because of the large amount of time necessary to obtain external
flat fields for the FOC, these two  UV  flat  fields  (one  for  F/96  and
one  for  F/48)

FOC Instrument Handbook Version 6.0 65

ing this has not been analyzed. In all images, regardless of format, a number
of pixels at the beginning of the scan line (i.e., starting  at  S=1)  are
corrupted  by  defects  in  the  beginning  of  the  scanning  sawtooth
waveform. The number of pixels corrupted depends on the detector and format.
Generally it is about 5% of the scan line for the F/96 relay and relatively
independent of format, whereas for the F/48 relay it gets progressively worse
with smaller formats (for the 128 * 128 format it is as much as 25% of the scan
line). The faint horizontal stripes seen at small L values are due to a ripple
instability of the coil drivers at the beginning of a frame. The narrow line
running from the bottom left corner to the upper right corner is due to the
read beam not being completely blanked when it is forced to fly back to S=0,
L=0 at the end of the frame. This feature is more noticeable with the smaller
formats. The narrow horizontal features at the  right  edge,  especially  at
L=256,  512,  768,

66 FOC Instrument Handbook Version 6.0

decline in sensitivity in the far UV. Another source of fine scale
nonuniformity is the presence of patterns--unfortunately not fixed. Although
not always easily seen in low count extended areas or flat fields, there are
two different patterns always present. The more noticeable one is an approxi
mately sinusoidal pattern with the peaks and troughs oriented at an
approximately 45 degree posi tion angle and a period of 3.35 pixels for the
F/96 relay. It is believed to originate from a moire' effect between a TV tube
grid and the diode array on the target. The RMS amplitude of this pat

Figure 32. a. Contour plot of the vignetting function for the F/48 relay across
the entire photo cathode,  with  the  location  of the  primary  512 *  512
imaging  format shown (dotted line). b. Plot of the vignetting function along
the spectrographic slit.

Figure 33. a. Contour plot of the smoothed flat field for the F/48 relay,
including the effects of vignetting. b. Contour plot of the smoothed flat field
for the F/96 relay.



FOC Instrument Handbook Version 6.0 67

tern is approximately 5% for the F/96 relay and 2.5% for the F/48 relay (the
peak deviations from a flat response due to this pattern are at least twice
these values). This pattern becomes intensified when count rates are in the
nonlinear regime and thus is much more easily seen. In fact, it is a quick way
of recognizing serious nonlinearity in an image. The pattern noise disappears
at very low count rates. A second pattern arises from some form of interference
with an FOC digital timing waveform that has a 4 pixel period. It shows up as
vertically striped patterns on the flat fields. Although very


Figure 34. Plots across row 300 of the UV flat field for the F/48 relay (a) and
the F/96 relay (b). The effect of vignetting has not been included in plot
(a).

a. b. Figure 35.  Contour plot of the ratio between external UV flat field and
internal LED flat field for the F/48 relay (a) and the F/96 relay (b) based on
pre-COSTAR data. The expected effects  of  vignetting  on  the  ratio  for
the  F/48  relay  are  not  included.  The  center  of each plot has been
normalized to 1 with the contours at intervals of 2.5%.

68 FOC Instrument Handbook Version 6.0

coherent in nature with regard to orientation and frequency, the details of the
modulation do not appear to remain constant from image to image. The RMS
amplitude of this pattern is approxi mately 2.5% for both relays. There also
appears to be an intrinsic granularity in the fine scale response, i.e.,
effectively ran dom pixel-to-pixel variations in response which has not yet
been well characterized. Some  comments  on  flat  field  calibration  are  in
order,  especially  with  regard  to  the  routine pipeline calibration. Small
drifts in distortion of the order of a pixel result in misregistration of fine
scale features such as scratches between the flat field and the science image.
Flat fielding the data in this case actually worsens the effects of the fine
scale features by correcting the wrong pix els. For this reason and because FOC
flat fields are of relatively low signal to noise (typically 300- 500 counts
per pixel), the flat fields used in RSDP are heavily smoothed to eliminate most
of the fine s

6.10 VISIBLE LEAKS

Although  the  FOC  narrow and  medium  band  filters are as  good as  the
technology  of 15-20 years ago could provide, they do exhibit a residual
transmission of * 10 -3  - 10 -4  between 5000 and

FOC Instrument Handbook Version 6.0 69

6000A* where the detectors are still relatively sensitive. Consequently,
indiscriminate use of these filters to isolate faint UV features from a bright
visible background can lead to serious errors. The magnitude  of  the  error
is,  of  course,  very  sensitive  to  the  precise  shape  of  the  spectrum
of  the source to be observed throughout the sensitive range of the FOC. Thus,
it is not always sufficient to  know  only  the  expected  flux  of  the
source  in  the  range * 0 +-**/2  in  order  to  estimate  the expected count
rate.

A  striking  example  of  a  possible  observing  scenario  that  can  be
expected  when  imaging  a bright visible source in the UV is shown in Figure
36. In the example shown in this figure, the source  spectrum  is  assumed  to
increase  sharply  with  increasing  wavelength  in  the  manner expected from
an M supergiant star. If this source is fed into the F/96 relay with the F231M
filter on  FW#3  in  the  beam,  the  resulting  monochromatic  count  rate
as  a  function  of  wavelength through  the  entire  OTA+COSTAR+FOC  system
is  shown  by  the  curve  marked  F231M.  The actual  observed  count  rate
in  this  configuration,  of  course,  corresponds  to  the  integral  of  this
curve. If the F231M filter alone is used in this endeavour, the contribution of
the flux within the band 2330 +- 115A* is only * 18% of the total of 39 counts
sec -1 . The counts originating from the region *  >  2580A*  represent,  in
contrast,  71%  of  the  total.  In  this  admittedly  extreme  case,  the
thus d

Figure  36. The  expected  monochromatic  count  rate  as  a  function  of
wavelength  for  the  F/96 relay and the F231M filter or the F231M+F220W
filters in the beam for an extended source  whose  spectrum  varies  as  the
curve  marked  SOURCE  SPECTRUM.  The source flux units are photons cm -2 sec
-1 A* -1 .

70 FOC Instrument Handbook Version 6.0

towards maximizing the suppression of the visible leak while minimizing
absorption in the UV bandpass of interest. In the case worked out in Figure 36,
for example, the F220W filter on FW#2 is ideal as shown by the curve marked
F231M+F220W. Now, the in-band fraction of counts amounts to 69% while the
visible leak is only 5% or less of the total. The exposure time required to
reach a S/N=10 in this  case  increases  by  a  factor  of  six  mainly
because  of  the  effective  suppression  of  the  visible counts.
Unfortunately, the F/48 relay with its much smaller filter complement has far
less flexibility in this regard than the F/96 relay. In this case, another
possible solution to the problem is to use the objective  prisms  to
physically  separate  the  UV  from  the  visible.  This  technique  works
best  for point or, at least, compact sources where spatial and spectral
overlap is minimized. But even for extended sources, appropriate positioning of
the target with respect to the dispersion axis of the

6.11 GEOMETRIC DISTORTION AND STABILITY

Because  of  the  nature  of  the  detectors,  and  the  off-axis  location
of  the  instrument,  the  raw FOC data suffers from geometric distortion,
i.e., the spatial relations between objects on the sky are not preserved in the
raw images produced by the FOC cameras. This geometric distortion can be viewed
as originating from two distinct sources. The first of these, optical
distortion, is exter nal to the detectors and derives from the off-axis nature
of the instrument apertures. The second, and much more significant source of
distortion is the detector itself. Geometric distortion is a fact of life when
dealing with detectors containing image intensifi- ers, primarily because
intensifiers rely on an electric field for accelerating, and a magnetic field
for focusing the photoelectrons. Any variation in the uniformity of either
results in image distor tion within the intensifier. Photon positions are then
further distorted in the process of "reading out" the TV tube's target, firstly
because the read-out

FOC Instrument Handbook Version 6.0 71

new method of determining distortion was developed which is based on
overlapping observations of crowded starfields to determined the net distortion
(the optical distortion is naturally included in this method of determining the
distortion). These observations were then used to determine a two-dimensional
spline distortion model which in turn was used to generate the new geometric
correction files. The improvement in quality is most apparent for smaller
formats where the small number of visible reseaux prevented the determination
of a good model. The new geometric cor rection files have been used in the
calibration pipeline for F/96 data since 19 March 1995 (F/48 geometric
correction files are still based on reseau marks). As an aside, only those who
desire sub pixel accuracy in position measurements or those who have used the
256X256 format should even consider  reprocessing  their  old  data  with  the
new  geometric  correction  files.  For  most,  the improvements will not have
a significant effect on t

72 FOC Instrument Handbook Version 6.0


Figure 37. The 512z * 1024 format distortion field for the F/48 relay.


Figure 38. The 512z * 1024 format distortion field for the F/96 relay.

FOC Instrument Handbook Version 6.0 73

c. There are as much as 0.5% changes in effective plate scale between different
high voltage turn-ons. d. The situation with the F/48 relay is somewhat less
certain since monitoring of the pre-COS TAR F/48 relay has shown it to be
considerably less stable, with several large, unexplained changes in the
geometric configuration. Any future F/48 data must be considered to be poorly
characterized with respect to geometric distortion.

6.12 PLATE SCALE

The plate scale (i.e., the pixel size in arcseconds) had been determined for
both of the FOC relays prior to the deployment of COSTAR. This was done by
taking a series of images of a pair of astrometric stars, moving the telescope
between exposures by a predetermined angular offset. The measured distances (in
pixels) between the astrometric stars, combined with the known sepa ration (in
arc seconds) then give us the plate scale. The effects of COSTAR on the focal
ratio of the image was then applied to these values to obtain preliminary plate
scales. Due to problems observing with the F/48 relay since the deployment of
COSTAR, the prelim inary  plate  scale  of  0.02825  arcsec  pixel -1 remains
the  only  value  available,  with  an  error  of +-0.0002  arcsec  pixel -1 .
Observations  of  a  field  of  stars  already  observed  prior  to  COSTAR
deployment were used to determine the new plate scale for the F/96 relay. The
derived plate scale for the F/96 relay is 0.01435 arcsec pixel -1  (+-0.0002).
This

6.13 CURRENT F/48 PERFORMANCE

The first failure of the F/48 camera to turn on occurred in September 1992. The
high voltage tripped during its ramping up, at the beginning of an observation
sequence. The F/48 camera was switched on again successfully in October 1992,
and a number of darks and flat fields were taken which confirmed background
values more than ten times higher than nominal. The next switch on attempt
(January  1993)  failed,  but  the  F/48  was  again  successfully  switched
on  December  22, 1993, before the deployment of the COSTAR corrective optics.
The camera remained on for the duration of the observation. While the
acquisition image showed a background level consistently high with the previous
dark count images, a preliminary analysis of the following frames showed that
the  background  increased  dramatically  with  time,  eventually  reaching
saturation  levels approximately two hours after HV switch-on. As a
consequence, the F/48 was not made available to GOs during Cycle 4 and 5, while
extensive tests were ca

74 FOC Instrument Handbook Version 6.0

first time after the COSTAR deployment. Images and spectra of an extended
target were success fully obtained, and showed a completely different behavior
from the earlier tests. A direct comparison of all nine spectra taken during
the November test was made to deter mine the characteristics of the background
as a function of time after switch-on. The high back ground was still localized
in the central area of the image (flare), and in the upper arc across the top
of the images. The locations of these features can be seen in Figure 39, where
all the images are displayed with the same intensity contrast to allow direct
visual comparison. But, it was clear from  a  cursory  comparison  that
instead  of  increasing  in  intensity  over  time,  the  flare  actually
tended to subside over time since switch-on.

This finding was confirmed in the course of three additional successful
turn-ons, on March 5 on March 30, and on April 18, 1995. In all cases the
background decreased significantly as a func tion of time, to  reach  a value
lower than 10 -2  cts/sec/pix after approximately  250 minutes after
switch-on.  In  Figure  40,  the  background  is  measured  in  three  sample
regions  (flare,  arc,  back ground) in the images taken during the final run,
and is plotted as a function of time. This test had a  duration  of
approximately  600  minutes,  compared  to  only  300  minutes  for  the
first  two. Although  we  notice  a  similarly  decreasing  trend,  the
latest  test  shows  somewhat  lower  back ground values. Furthermore, the
measurements taken in the final 300 minutes show that the inten sity of the
flare and arc tend to decrease with time after turn-on. The background measured
is still higher than nominal in the regions of the flare and arc (see Section
11 for comparison), but the camera  is  demonst

Figure 39.  Mosaic of F/48 Images taken ordered by time after switch-on during
the Novem ber  1994  test.  Earliest  image  is  on  the  left.  All  are
displayed  with  respect  to  the same count rate scale, to show how the flare
actually tended to subside over time since switch-on. The regions of the flare
and arc are marked in the center image.



FOC Instrument Handbook Version 6.0 75

After the refurbishment mission planned for 1997, STIS will be able to
replicate, more efficiently, all  the  features  of  the  F/48  long  slit
spectroscopy  (see  STIS  mini-Handbook  for  additional details). However,
given the current performances of the F/48 camera, it was felt advisable to pro
vide  the  users  with  the  choice  to  take  advantage  of  these  presently
unique  capabilities.  Since some limited risk is still associated to the
success of the future F/48 observations, the users who will choose to apply for
F/48 time will be allocated F/48 time only, with no possibility to switch to a
different instrument in case of failure.

Figure 40. F/48 camera background measured in three areas (A: arc - solid line;
B: flare - dotted line; C: background -dashed line) during the latest test, and
plotted as a function of time after camera switch-on. The relay showed this
same basic behavior during each observation run.



76 FOC Instrument Handbook Version 6.0

7.0 OBSERVER'S GUIDE (PRESCRIPTION FOR ESTIMATING EXPOSURE TIMES)

The first step consists in specifying the required signal to noise ratio S/N or
the relative accu racy   of  the  measurement.  Then,  the  exposure  time
required  to  attain  that accuracy is given, in general, for Poisson
statistics, by:

(1)

where R S  is the source rate and R B  the background rate in an appropriate
resolution element in counts sec -1 . The problem then simply reduces to
properly estimating R S  and R B . For  a  point  source  in  the  FOC  field
of  view  and  for  a  count  rate  per  pixel  much  less  than N MAX
calculated from Table 9, the source rate is given by:

(2)

where: A = area of the HST primary = 45238.9 cm 2

*(*) = fraction of energy intercepted by the appropriate resolution element
F(*) = source flux at ST in photons cm -2 sec -1 A* -1

Q(*) = FOC+OTA response function for T(*) =1 in counts photon -1

T(*) = transmission of filters or efficiency of dispersing elements The ratio
of obscured area to total area of the primary mirror, equal to 0.138, has been
inte grated  into  the  values  of  Q(*)  for  consistency  with  SYNPHOT.
The  terms  in  eq.  (2)  can  be assumed to be appropriate averages over the
pixel to pixel variations in the instrument response function. Q(*) and T(*)
are plotted in Figures 29 and 11 through 15. The background rate, on the other
hand, can be expressed, in general, as:

(3)

where: n = number of normal (z=1) or zoomed (z=2) pixels in appropriate
resolution element B p  = inherent detector background count rate per normal
pixel I B (*) = specific intensity of diffuse background at ST in photons cm -2
sec -1 sr -1 A* -1

* p  = solid angle subtended by a normal FOC pixel in steradians. Equations (2)
and (3) can be evaluated numerically or by approximating them by assuming that
the spectral passband is sufficiently narrow. This permits the following
simplifications:

(4)

(5)

where all the relevant functions are evaluated at wavelength * 0  of peak
response and **  is the FWHM bandpass of the instrument in Angstroms. The
latter two parameters are listed in Table 3. K takes on the numerical values 1
and 4 for the F/48 and the F/96 relays, respectively. For an extended source,
the size of the resolution element nz is determined by the user accord ing to
his application. For a point source, the encircled energy tabulated in Table 8
should be used

*N N/ S N/( ) 1- =

t S N/( ) 2 R S 2R B +( ) R S 2- =

R S A **( ) F *( ) Q *( ) T *( ) *d 0

* * =

R B nz B P A* p I B *( ) Q *( ) T *( ) *d 0

* * +=