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29.3 FOS Instrument Basics

This section provides the basic information needed to understand the geometry and operation of the FOS instrument and the optical components affecting the light path. FOS Instrument Handbook versions 1.1 and 6.0 give more details about the instrument performance and capabilities. The final sections of this chapter are summary discussions of each FOS target acquisition and science observing mode.

29.3.1 FOS History and the Introduction of COSTAR

The FOS operated successfully throughout its six-year mission on HST. There were however a number of major milestones during that period, most notably the installation of COSTAR, which corrected the effects of primary mirror aberration for the FOS. Table 29.1 summarizes the most important HST milestones that affected FOS.

During the summer of 1991 (part of the Science Verification (SV) period) substantial changes in certain FOS photocathode granularity patterns (flatfields) were observed, particularly for FOS/RD. Detailed monitoring during the remainder of the period prior to the First Servicing Mission (January 1, 1992, through December 15, 1993) revealed moderate variations in flatfield combined with overall time- and OTA focus-dependent changes in instrument sensitivity.

The aberration of the primary mirror affected FOS acquisitions and science observations. Modifications to ACQ/BINARY procedures to compensate for the extended PSF wings reduced the likelihood of target acquisition failure by early 1992, but throughout the pre-servicing period aberration-related crowded field acquisition failures occurred. The FOS geomagnetically-induced image motion (GIM) was discovered early in SV. In addition to photometric effects, this motion degraded the accuracy of ACQ/BINARY target acquisitions until the implementation of an onboard compensation algorithm in April, 1993.

Y-bases were monitored and regularly updated throughout the FOS operational lifetime. Temporal trending of some FOS y-bases was established by the summer of 1993 and subsequent updates included predicted values based upon the observed trending.

Installation of the Corrective Optics Space Telescope Axial Replacement (COSTAR) on the spacecraft in December of 1993 corrected the effects of spherical aberration of the HST primary mirror on the FOS. COSTAR deployed two correcting mirrors, M1 and M2, into the optical path of FOS. The COSTAR mirrors introduced a modest anamorphic magnification which differed by approximately two percent between the COSTAR x and y. Aperture and pixel sizes decreased to approximately 0.86 of their pre-COSTAR values. COSTAR restored the design PSF at a modest cost in efficiency attributable to the two corrector reflections. Additional information concerning the impact of COSTAR on FOS is in "Influence of COSTAR on FOS Data" on page 29-19.

Other updates were made to either improve or extend various aspects of instrumental operation and calibration. These included commanding updates to improve the efficiency of ACQ/PEAK, to enhance the brightness dynamic range and reduce the failure rate of ACQ/BINARY, and to enable the use of ACQ/PEAK for moving target acquisitions. Software updates to implement scattered light correction, correctly calibrate OBJ-OBJ observations, and substantially improve flux calibration were also implemented.

Important Dates in FOS History



April 1990

HST launched with FOS onboard

January 1, 1992

SV ends (updated reference files, aperture locations and sizes, and y-bases)

After July 1, 1992

FOS calibration observations commence use of high-precision ACQ/PEAK (0.025" accuracy each coord)

April 5, 1993

Onboard GIM correction implemented

June 1993

ACQ/BINARY 8-steps to success implemented

(11 groups became success, not failure)

December 1993

Servicing Mission (SM) 1

About January 1, 1994

COSTAR FOS M1, M2 mirrors deployed

January 1-26, 1994

Calibration of COSTAR-assisted FOS; no FOS observations in this period are formally calibrated due to many different COSTAR mirror positions

February 1, 1994

FOS COSTAR mirrors properly positioned - photometry now possible

March 21, 1994

Scattered light correction algorithm implemented in pipeline

May 30, 1994

FOS post-COSTAR aperture locations updated; SMOV ends

July 1994

More efficient ACQ/PEAK including moving targets implemented

July-November 1994

No header ACQ/PEAK scan parameter information (FOS paper products will not work for ACQ/PEAK taken in this time period)

November 1994

GHRS/FOS acquisition assists initiated

December 1994

Reuse_target_offset implemented

February 5, 1995

Jitter files routinely provided on GO tapes and placed in archive

January 15, 1996

Improved scattered light correction algorithm implemented

March 17, 1996

AIS flux calibration implemented in pipeline

February 13, 1997

FOS removed from HST

29.3.2 Optical Layout

Figure 29.1 shows an optical layout of the FOS. Light entered the FOS through a pair of entrance ports roughly 490" off the optical axis of HST. The light from the object of interest then passed through one of the two independent optical channels, each of which focused nearly stigmatic spectral images on the photocathode of a photon-counting Digicon detector. These two channels differed only in the wavelength sensitivity of their respective detectors, and were referred to as FOS/RD (or Red Side) and FOS/BL (Blue Side). The FOS aperture wheel, located at the HST focal surface, contained separate sets of single or paired apertures (including one blank for background measurements) for each detector which ranged in size from 0.1" to 4.3" (pre-COSTAR) projected onto the sky. From the aperture wheel the optical beam then passed through the polarization analyzer (which included a clear aperture for non-polarimetric observations) and on to the grazing incidence mirror, a roof prism, which deflected the beam 22 degrees upward. This deflection was required in order to allow the apertures to be placed near the HST optical axis to minimize astigmatism, while meeting the packaging constraints within the FOS. The deflected beam then passed through an order-sorting filter, when required, in the fore part of the filter-grating wheel, and on to an off-axis paraboloidal mirror which collimated the beam and directed it back to the filter-grating wheel. The beam was then dispersed or imaged (for the one imaging position). and focused onto the detector by the selected element on the filter-grating wheel.

29.3.3 Detectors

As described above, the FOS had two Digicon detectors with independent optical paths (Figure 29.1). The Digicons operated by accelerating photoelectrons emitted from a two-dimensional transmissive photocathode onto a linear array of 512 silicon diodes. All 512 FOS diodes were exposed by dispersed light, i.e., none of the diodes were dedicated to background measurement or other purposes. The photoelectron pulses were counted (if a pulse had an amplitude above a programmable threshold) in each of the individual diode channels. A separate microprocessor for each detector controlled the electronics, mechanisms, and Digicons. The counts were summed in microprocessor memory for a mode-dependent preset time and then read from the FOS to the HST computer.

The bi-alkali (Na2 KSb) blue detector (FOS/BL) photocathode on a magnesium fluoride faceplate was sensitive from 1150 Å to 5400 Å, while the tri-alkali (Na2 KSb Cs) red Digicon (FOS/RD) photocathode on a fused silica faceplate covered the wavelength range from 1620 Å to 8500 Å. Figure 29.2 shows the quantum efficiencies of both detectors. Note that the graphs in Figure 29.2 are illustrative only; the plotted values are not accurate enough for quantitative use in data analysis. The general characteristics of FOS/BL and FOS/RD are compiled in Table 29.2.

Both photocathodes had spatial irregularities in response (granularity) and localized blemishes that could limit the S/N achieved. These spatially variable features could be removed with appropriate flatfielding provided adequate care was taken in target centering. The diodes themselves also had small response irregularities that were of little consequence and were also removed by routine flatfielding.

Figure 29.2: Detector Quantum Efficiencies

Detector Characteristics



Wavelength coverage

FOS/BL: 1150Å to 5400Å in several grating settings.

FOS/RD: 1620Å to 8500Å in several grating settings.

Spectral resolution

High: / 1300.

Low: / 250.

Time resolution

t 0.033 seconds.

Acquisition aperture

4.3" x 4.3" (4.3): pre-COSTAR

3.71" x 3.66" (4.3) : post-COSTAR

Science apertures

Largest: 4.3" x 1.43" (pre); 3.71" x 1.29" (post) (4.3).

Smallest: 0.09" square paired (0.1-PAIR).

Diode size

pre-COSTAR: width=0.356"; height=1.43"

post-COSTAR: width=0.305+/-.004"; height=1.291+/-.007"


pre-COSTAR: FOS/BL and FOS/RD x=y=0.0896" pixel-1


FOS/BL x=.0774+/-.001" pixel-1; y=.0786+/-.001" pixel-1

FOS/RD x=.0752+/-.001" pixel-1; y=.0812+/-.002" pixel-1

Mean dark count rate

FOS/BL: 0.0064 counts s-1 diode-1

FOS/RD: 0.0109 counts s-1 diode-1

Example post-COSTAR

exposure times

0.9" (1.0) aperture

F1300=2.5 x 10-13, SNR=20/(1.0Å), t=260s.

F2800=1.3 x 10-13, SNR=20/(2.0Å), t=10s (FOS/BL).

F2800=1.3 x 10-13, SNR=20/(2.0Å), t=6.6s (FOS/RD).

1 Post-COSTAR telescope movement commanding assumed mean of x and y values as the mean plate-scale for each detector.

FOS Apertures

(Header Desig.)




Size (")


Size (")





0.30 dia

0.26 dia





0.50 dia

0.43 dia





1.00 dia

0.86 dia





























0.25 x 2.00

0.21 x 1.71





0.70 x 2.00

0.60 x 1.71

















4.3 x 4.3

3.66 x 3.713




0.50 and 4.3

0.43 and 3.7

1 Pre-COSTAR separation of PAIR apertures was 2.82", whereas post-COSTAR separation was 2.57"; pre-COSTAR aperture positions were assumed symmetric about center of SINGLE apertures. post-COSTAR PAIR "A" apertures were assumed symmetric about center of A-1 and PAIR "C" apertures symmetric about center of C-2.

2 Three of the four A-4 apertures have been consistently measured as one size and the fourth, the FOS/BL LOWER aperture, approximately 25% smaller in each dimension. Due to measurement uncertainties, the FOS/BL LOWER aperture size was determined as ranging from 0.1" (pre-COSTAR) to a size 25% smaller (see FOS ISRs 019 and 138).

3 Post-COSTAR size was not the same in both coordinates due to the COSTAR anamorphism (magnification factor in x different from that in y).

29.3.4 Apertures

The entrance aperture mechanism allowed selection of any one of twelve apertures for each detector. Table 29.3 lists the apertures.

The FOS aperture naming convention was to use the pre-COSTAR aperture size as aperture designation for all observing epochs despite the 16% scale magnification introduced by the COSTAR optics. There was a large aperture for acquiring targets using on-board software (pre-COSTAR: 4.3" x 4.3"; post-COSTAR: 3.7" x 3.7"; designation 4.31). Since the diode array extended only 1.29" (1.43" pre-COSTAR) in the direction perpendicular to the dispersion, this largest aperture had an effective collecting area of 3.7" x 1.29" (pre-COSTAR: 4.3" x 1.43"). Other apertures included several circular apertures with sizes 0.86" (1.0), 0.43" (0.5), and 0.26" (0.3), as well as paired (UPPER and LOWER) square apertures with sizes 0.86" (1.0-PAIR), 0.43" (0.5-PAIR), 0.21" (0.25-PAIR), and 0.09" (0.1-PAIR), for isolating spatially resolved features and for measuring the sky. In addition, a slit and two barred apertures were available (Figure 29.3 and Table 29.3). Separate aperture sets existed for FOS/BL and FOS/RD. Aperture size measurements were performed both pre-launch and on-orbit. Precise aperture sizes were reported in FOS ISRs 019 and 138, for the pre- and post-COSTAR cases, respectively. Aperture positioning repeatability was extremely accurate.

Aperture Center Locations

Before COSTAR was installed, all SINGLE FOS apertures were assumed to be concentric and all spacecraft-centering operations used this assumption. Post-COSTAR SMOV calibrations indicated that the 1.0 and smaller SINGLE apertures were concentric, but that the 4.3 aperture was slightly offset, particularly for FOS/RD, from this center. This offset affected the positional accuracy determined by ACQ/BINARY and required that different flatfields be employed for the 4.3 than for the other SINGLE apertures. The degree of uncertainty in this small offset also affected precise positional determination based upon 4.3 aperture imaging or ACQ/BINARY alone. Pre-COSTAR telescope commanding assumed that the mid-point between each set of paired apertures was concentric with the SINGLE apertures. Post-COSTAR measurements led to the adoption of aperture locations that placed all of the "A" apertures at a common center, all of the "B" apertures (all SINGLE) at a slightly displaced common center, and the "C" apertures at another common center. The 1.0-PAIR apertures were slightly displaced from all of the other paired apertures and required separate flatfields. The individual components of paired apertures were separated by approximately 2.57". All post-COSTAR telescope commanding after May 30, 1994 included these aperture location offsets.

29.3.5 Dispersers

FOS dispersers provided both "high" spectral resolution (1-6 Å-diode-1, / 1300) and "low" spectral resolution (6-25 Å-diode-1, / 250). Their designations and basic properties are presented in Table 29.4. Full optical specification of the FOS dispersers is found in Harms et al., SPIE, 183, 74, 1989 and FOS ISR 127. The actual spectral resolution depended on the point spread function of HST, the dispersion, the aperture, and whether the target was physically extended. Representative line widths are given in Table 29.5.

Unlike the apertures, the same dispersers were used with both FOS/BL and FOS/RD. The dispersers and appropriate blocking filters were mounted on the Filter Grating Wheel (FGW) which always rotated in the same direction to specified locations in the beam. Due to the physical nature of the mechanism involved, the repeatability of FGW positioning in the beam was not as precise as for aperture positioning. For certain types of FOS observations, FGW positional uncertainty is an important "error source" (see Chapter 33 and FOS ISRs 131, 142, and 145.

29.3.6 Detector Geometry

From the photocathode, electrons were deflected magnetically without magnification onto the diode array of the Digicon. In this way the electronic image of the light transmitted by the aperture was projected onto the diode array. The relative size of the different apertures as they were projected onto a section of the diode array is shown in panel (a) of Figure 29.3, which is displayed from the perspective of an observer positioned directly "behind" the detector looking out toward the sky. The individual diodes were 50 x 200 microns in physical dimension which corresponded to spacings of 0".31 (post-COSTAR) or 0".35 (pre-COSTAR) along the dispersion direction and height of 1".29 (1".43) perpendicular to it. In the FOS detector coordinate system (and throughout this handbook) dispersion lies along the x-direction. The x-coordinate is always defined positive to the left in the sense of Figure 29.3. Wavelength always increased with increasing x for FOS/BL gratings, but always decreased with increasing x for FOS/RD gratings and vice versa for the prisms. The y-axis of the FOS coordinate frame was perpendicular to the dispersion and was defined positive toward the "top" of the detector as shown. (As one "looks through the detector toward the sky," if east were aligned toward the +x-axis, then north was aligned with the +y-axis.) The lower paired apertures were closest to the optical axis of HST.

Panel (b) of Figure 29.3 shows three other apertures-two occulting bars and one slit-which were nearly concentric with the 4.3 aperture. Panel (c) shows the orientation of the +y-axis in the (V2,V3) plane for the post-COSTAR era.

COSTAR introduced a 180 degree rotation of the telescope (V2,V3) coordinate frame, not the instrument (x,y) frame. Therefore, before COSTAR, the (V2,V3) coordinate frame was rotated 180 degrees with respect to the orientations shown in Figure 29.3. The angle between FOS/BL and FOS/RD slit orientations (+y-directions) was always 73.6 degrees.

The deflection of the photoelectrons was controlled by an internal magnetic field, which in turn depended on a high-voltage setting. The unit of distance in the y-direction was the so-called Y-base unit. The high voltage was adjusted so that a deflection of 256 Y-base units corresponded to the 200 micron physical height of the diodes (both pre and post-COSTAR). The photocathode coordinate system extended from -2048 to +2048 Y-base units. Each disperser directed incoming light onto a different location on the photocathode as shown in Figure 29.4.

29.3.7 Waveplates and Polarizers

The FOS polarization analyzer positioned one of three elements into the optical path; a clear aperture, a thin magnesium fluroide waveplate (plate "B") plus Wollaston prism assembly, or a thick MgFl waveplate (plate "A") plus Wollaston prism assembly. One waveplate was permanently located in front of each Wollaston, however the waveplate could be rotated so as to alter the position angle of the waveplate fast axis relative to the Wollaston, in increments of 22.5 degrees. The polarimetry appendix of FOS Instrument Handbook version 1.1 presents a technical description of the FOS polarimeter.

The technique used for spectropolarimetry in the FOS was very similar to that developed for ground-based instruments. When introduced into the beam, the Wollaston prism assembly produced twin dispersed images of the aperture with opposite senses of polarization at the detector. In order to determine the linear and circular polarization properties of the incoming beam, usually 8 or 16 observations were taken with the waveplate turned in 45 or 22.5 degree intervals relative to the Wollaston prism. This allowed the polarization effects in the dispersing optics following the analyzing prism to be fully removed from the science observations. Waveplate "A" was recommended for use with the disperser G400H and "B" waveplate was recommended for use with the G130H, G190H, and G270H dispersers. Polarizer position "C" (clear) was the default for all non-spectropolarimetric observations.

FOS Dispersers


Diode No.

at Low



Diode No.

at igh




Blocking Filter

Blue Digicon

G130H (H13)







G190H (H19)







G270H (H27)







G400H (H40)






WG 305

G570H (H57)






WG 375

G160L (L15)







G650L (L65)






WG 375








Red Digiconi

G190H (H19)







G270H (H27)







G400H (H40)






WG 305

G570H (H57)






WG 375

G780H (H78)






OG 530

G160L (L15)







G650L (L65)






WG 375








1 The blue Digicon MgF2 faceplate absorbed light shortward of 1140 Å.

2 The photocathode electron image typically was deflected across 5 diodes, effectively adding 4 diodes to the length of the diode array.

3 The second order overlapped the first order longward of 2300 Å, but its contribution was at a few percent.

4 Quantum efficiency of the blue tube was very low longward of 5500 Å.

5 PRISM wavelength direction was reversed with respect to gratings of the same detector.

6 The sapphire prism absorbed some light shortward of 1650 Å.

7 The red Digicon fused silica faceplate strongly absorbed light shortward of 1650Å.

8 Quantum efficiency of the red detector was very low longward of 8500 Å.

i. Dispersion direction was reversed for FOS/RD relative to FOS/BL.

FOS Line Widths (FWHM) as a Function of Aperture Size


Size (")

Aperture Filled with

Uniform Source


Point Source

at 3400Å



Point Source

at 3400Å


G130H (Blue)


G570H (Red)




1.00+_ .01

0.95+_ .02





1.27+_ .04

1.20+_ .01





2.29+_ .02

2.23+_ .01





0.97+_ .03

0.92+_ .02





0.98+_ .01

0.96+_ .01





1.30+_ .04

1.34+_ .02





2.65+_ .02

2.71+_ .02




0.21 X 1.71(slit)

0.99+_ .01

0.96+_ .01




0.60 X 1.71

1.83+_ .02

1.90+_ .01





5.28+_ .07

5.43+_ .04




3.66 X 3.71

12.2 +_ 0.1

12.2 +_ 0.1



1 The FWHM are given in units of diodes. A pre-COSTAR diode was 0.356" wide and 1.43" high. A post-COSTAR diode was 0.305" wide and 1.29" high.

29.3.8 Internal Wavelength Calibration

Two Pt-Ne hollow-cathode lamps provided emission line spectra for calibrating the FOS wavelength scales. The lamp beams passed through the same optics as an external source, but they did not fill the collimator. As a result, it was possible to have an offset in the photocathode position of any particular wavelength in the dispersed internal beam and the dispersed external beam. This internal to external wavelength offset is discussed in more detail in "Wavelength Calibration" on page 32-49. Due to the effect of the FGW positional uncertainty mentioned in "Dispersers" on page 29-11, highly accurate FOS external wavelengths could be determined only with the aid of an internal WAVECAL exposure taken immediately before or after the science exposure with no intervening movement of the FGW.

Figure 29.3: Post-COSTAR Aperture Sizes Projected On Diode Array Viewed from Behind Detector Looking toward Sky

29.3.9 Data Acquisition Fundamentals

The FOS had several parameters that could be altered to change the way data was accumulated and read out. These parameters are summarized in Table 29.6.

Data Acquisition and Readout Parameters




Number of repetitions of the livetime/deadtime cycle, whereby electrons were counted in the selected diode accumulators with no changes in the magnetic deflection. Changing INTS provided a means for altering the amount of time data were collected at a given deflection value while keeing the livetime short so that as little good data as possible was rejected. The header keyword is INTS.


Number of magnetic field deflections of the photoelectrons in the dispersion direction. The default value was 4 (quarter-stepping). The data went into new memory locations with each substep. The corresponding header keyword is NXSTEPS.


Number of times to repeat the addition of counts into the memory array. The accumulation in the memory array was such that data acquired from an offset of one diode was added at each overscan. Each offset was accompanied by a corresponding magnetic deflection in the dispersion direction so that the data from the photocathode location falls into the same memory location independent of the diode which sampled it. The default value of this parameter is 5. The purpose of overscanning was to average the response of the different diodes, including the dead diodes. The header keyword is OVERSCAN.


Number of magnetic deflections of the electrons in the direction perpendicular to the dispersion. This parameter was used to map the photocathode (IMAGE mode), to switch between the two spectra produced by the polarizer, or to acquire either of the paired apertures. In the standard ACCUM mode this parameter had the default value of 1. The header keyword is YSTEPS.


A slice consists of an entire deflection sequence. Data from each slice went to a new memory location. This was used for time resolved spectrophotometry. The header keyword is SLICE.


A pattern was a complete series of slices. Counts in subsequent patterns were added to the corresponding previous values in the data array. The header keyword NPAT determined the number of times a pattern should be executed to achieve the exposure time.


A readout sent the science data to the CU/SDF computer for either storage on the tape recorder or for telemetry to the ground. FOS readouts were non-destructive and were performed at regular intervals (~4 min for the FOS/BL and ~2 min for the FOS/RD) without memory clears in order to protect against loss of data. The last readout contained all the data accumulated since the previous memory clear. In the standard ACCUM mode each readout is stored as a separate group in the data files.

Memory clears

A memory clear set to zero all locations in the science data array to allow input of new data. The number of times that a memory clear was repeated in a given observation is given in the NMCLEARS keyword.

To maximize the science data output from the FOS, routine data-taking commanding oversampled spectra and shifted the object spectrum with respect to the diode array during several subintegrations. These two procedures were called substepping and overscanning.

Substepping was used to better sample the spectrum in the wavelength direction. This was necessary because a spectral resolution element mapped to a single diode, so critical sampling of the line spread function required substepping. Overscanning was used to assure that each pixel in the final spectrum contained data received from multiple diodes (to smooth out diode-to-diode variations and insure against data loss when a single diode was disabled). Both substepping and overscanning relied on the magnetic focus assembly in the Digicon detector to magnetically deflect the photoelectrons in the dispersion direction so that they fell on slightly different locations on the diode array.

For substepping, the spectrum was deflected by a fraction of a diode in the dispersion direction (where the fraction was given by 1/NXSTEPS and NXSTEPS is a header keyword; see Chapter 30). The diodes were read out into unique memory locations for each substep and the substepping was performed NXSTEPS times.

Figure 29.4: Approximate Location of Spectra on the Photocathode.
Dashed Lines Represent Red Detector, Solid Lines Represent Blue Detector

For overscanning, the process of substepping was continued over more than one diode in the dispersion direction. A complete round of substepping was performed for each overscan step. The number of overscan steps performed was determined by the overscan parameter (header keyword OVERSCAN). Each time a given wavelength position was deflected onto and measured by a new overscan diode, its counts were co-added into the same memory location in the FOS microprocessor. When using the full 512-element diode array, the result was a spectrum with 512 * NXSTEPS plus a small number (NXSTEPS x (OVERSCAN - 1)) of edge pixels. Each pixel (excluding the edge pixels) had data contributed from the number of diodes specified by OVERSCAN. Thus, substepping changed the number of pixels in the final spectrum; and overscanning principally changed the number of diodes that contributed to a single pixel. Although the number of diodes in the diode array was 512, the actual number of diodes read out could be restricted via a wavelength range specification in certain modes. The number of pixels in any observation is given by the equation:

# of pixels = (# of diodes + (OVERSCAN - 1)) x NXSTEPS

Although other values could be specified, all FOS data acquisition modes, except ACQ/PEAK, used by default, NXSTEPS=4 and OVERSCAN=5. ACQ/PEAK observations used NXSTEPS=1 and OVERSCAN=1.

The default values of NXSTEPS=4 and OVERSCAN=5 yielded a typical spectrum of 2064 pixels in which a given diode contributed to the data in NXSTEPS x OVERSCAN, normally 20, consecutive pixels and a given pixel normally contained contributions from 5 adjacent diodes. The exposure time devoted to the first 2048 pixels (that is, all but edge pixels-described below) was 1/NXSTEPS (1/4) of the total exposure time specified for the observation.

Due to the nature of FOS overscanning, (OVERSCAN - 1) groups of NXSTEPS pixels at the +x edge of the diode array (long wavelength end for FOS/BL gratings and vice versa for FOS/RD) had contributions from fewer than OVERSCAN diodes. There were 16 such pixels in the default case, with the first four of these receiving 4/5 of the exposure of the first "normal" 2048 pixels, the next group of four receiving 3/5 of the typical exposure, the next four 2/5, and the last 4 1/5 of the normal pixel exposure. All standard pipeline routines within STSDAS and calfos handle these calculations automatically.

STScI FOS calibrations, which are described in chapter 31, support only OVERSCAN=5 observations. FOS calibrations are also designed for NXSTEPS=4, but may be easily adapted to all allowed values of NXSTEPS by a straightforward automatic resampling of standard NXSTEPS=4 reference files.

Note: Due to this FOS-specific substepping, the typical effective exposure per pixel in any FOS observation was the total observation exposure time divided by NXSTEPS. A few (i.e., NXSTEP*(OVERSCAN-1), typically 16, "edge" pixels had even less effective exposure.

29.3.10 Photon Counting Characteristics

The FOS Digicons were photon counting detectors. The output of the diodes was stored in 16-bit accumulators within the FOS microprocessor. These accumulators were readout at intervals defined by the detector and the data-taking mode, e.g., roughly every two minutes for FOS/RD ACCUM. Sufficiently bright sources could produce more than 65535 counts (216-1) in an interval and thereby suffer one, or more, occurrences of accumulator "wrap around." (see "unwrap" on page 33-11).

29.3.11 Other Characteristics

To minimize external influences on the magnetic deflection of electrons from the photocathode onto the diode array, both Digicons were magnetically shielded. However-especially for FOS/RD-the shielding was inadequate. Thus, telescope orientation relative to the Earth's magnetic field influenced the magnetic deflection characteristics of the FOS. This produced a geomagnetically induced image motion (GIM, occasionally referred to as GIMP). A post-observation pipeline correction algorithm corrected for this effect in the x-direction only prior to April 5, 1993. In order to minimize this effect, on-board software was implemented on April 5, 1993, to compensate for this error in both detector coordinates. Residual uncertainties remained, however, which affected the calibration accuracy of the instrument, as described further in "Geomagnetically Induced Image Motion (GIM)" on page 32-14.

The FOS Y-bases-the amount of magnetic deflection necessary to direct the photoelectrons from the photocathode onto the diode array-did not remain constant over the FOS lifetime due to hysteresis in the repeated magnetization and de-magnetization cycling of the Digicons. The detectors accumulated residual magnetization over time that required progressively larger deflections to steer the photoelectrons onto the diode array. Uncertainties in the Y-base values remain an important limitation on photometric accuracy (see "Y-bases" on page 32-7 and "Image Centering (Image Location) Factors" on page 32-22).

29.3.12 Influence of COSTAR on FOS Data

COSTAR was installed in HST during the First Servicing Mission in December 1993 to correct the spherical aberration of the HST primary mirror. COSTAR restored the point spread function (PSF) to nearly the HST design specifications. Aperture throughputs were also dramatically improved. For example, the throughputs of the 4.3, 1.0, and 0.3 apertures increased by factors of 1.3, 2.0, and 2.5, respectively. The throughput increase for the 4.3 aperture was compensated to a large extent by losses due to the two additional reflections, but small aperture relative throughput improvements allowed much of the original FOS science program goals to be pursued post-COSTAR, particularly in those programs that required excellent small aperture PSFs and LSFs.

The two COSTAR correcting mirrors, M1 and M2, introduced a modest anamorphic magnification which differed by approximately two percent between COSTAR x and y. All aperture and pixel scales decreased to approximately 0.86 of their pre-COSTAR values. For example, the height of the diodes projected on the sky changed from 1.43" pre-COSTAR to 1.29" post-COSTAR.

Between December 28, 1993, and February 1, 1994, numerous adjustments were made to the COSTAR mirrors. No effective instrumental calibration is available for any FOS data taken in that time period. By February 1, 1994, the COSTAR FOS-correction mirrors had been optimally aligned. The post-COSTAR FOS instrumental configuration that is calibrated by STScI began on February 1, 1994.

Throughout most spectral regions COSTAR introduced only moderate modifications to the wavelength dependence of instrumental sensitivity, however, an unanticipated broad decrease in instrumental sensitivity was recorded for FOS/BL in the wavelength range 1600-2400 Å. This feature was roughly Gaussian in shape and at 2000 Å. the post-COSTAR sensitivity was approximately 70% of its expected post-COSTAR value.

Additionally, the narrower post-COSTAR PSF more selectively illuminated fine-scale photocathode granularity rather than smoothing it out as was the case with the aberrated PSF. As a result, very precise target acquisitions became an important requirement for both science and calibration programs that needed to achieve high flatfielding accuracy.

The narrower PSF also led to a narrower line spread function (LSF) for post-COSTAR data compared to the pre-COSTAR era. Polarimetric data were affected, because additional optical elements (mirrors) were introduced into the light path, and this changed the characteristics of the incoming wavefronts. On the other hand, the internal wavelength calibration was not measurably affected, although some indication of grating-dependent internal-to-external wavelength offset changes was seen.

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1 Henceforth, if pre- or post-COSTAR time periods are not specifically stated, we will routinely use post-COSTAR aperture sizes with the pre-COSTAR size aperture designation in parentheses.

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