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35.4 Assessing GHRS Data

To help you assess GHRS data we will walk through actual proposals containing examples of many of the observing modes available. A short description of a program being used in the science assessment example is provided and a description and picture of the exposure logsheet, followed by assessment descriptions for the different observing modes represented in the example program.

Every observation in the Archive resulted from a detailed Phase II proposal created by the original observer. It is not usually necessary to examine the Phase II proposal to make sense of the data, but it is sometimes helpful to understand the structure of the observations.

35.4.1 An Example: R136a in the LMC

Program 5297 was executed in the spring of 1994 as an Early Release Observation (ERO), following the 1993 servicing mission. During that mission, COSTAR mirrors were deployed so that the full optical quality of HST could be realized by the GHRS. Also, a repair kit was installed that allowed Side 1 of the GHRS to be used once again (Side 1 capabilities were lost in 1991 due to the failure of a low-voltage power supply).

Program 5297 was designed to take advantage of and demonstrate these capabilities by obtaining low-resolution spectra of closely-spaced hot stars in a young cluster (R136a) in the Large Magellanic Cloud (LMC). Two of the stars to be observed were only about 0.12 arcsec apart, less than the size of the GHRS Small Science Aperture (SSA), which is 0.22 arcsec square. Thus precise positioning was needed. To achieve this, the telescope was first to acquire and center on the brightest object in the R136a field. That object was itself the convolution of two stars, so that some allowance had to be made for the apparent centroid of what was acquired. This was done using information from a WF/PC-1 image of R136a obtained before the servicing mission. Once centered on R136a1, the SSA could then be moved with high relative precision, but to ensure understanding of what was observed, IMAGEs were taken at each position.

35.4.2 The Exposure Logsheet

An abridged version of the Phase II Proposal for 5297 is shown in Figure 35.5. The first portion of the figure is the Target List and the bottom half is the Exposure Logsheet. Here are some notes to go with individual lines of the Exposure Logsheet:

Line 1: R136a1 was acquired with the Large Science Aperture (LSA). The LSA is denoted as "2.0" but has a true size of 1.74 arcsec square with the COSTAR mirrors in place. SEARCH-SIZE=5 is specified to ensure a good acquisition by providing a wider search area. (The default SEARCH-SIZE=3 was adequate in nearly all cases, but the crowding of this field led to using a more liberal value.) The specified exposure time of 25 seconds implied a STEP-TIME of 1.0 second; STEP-TIME is the dwell per point in the spiral search pattern. MIRROR-N2 was used because these stars were faint. Using MIRROR-N1 would have been preferable if these stars were bright enough because then it would have been unnecessary to switch from using detector D2 on Side 2 (implied by MIRROR-N2) to detector D1 on Side 1 (implied by using Spectral Element G140L on line 20). Switching from one Side of the GHRS to the other took about 40 minutes because of the need to let the electronics stabilize. However, MIRROR-N1 reflected light only from about 1200 to 1700 Å, and the STEP-TIME needed to acquire these stars would have exceeded the maximum permitted value of 12.75 seconds. Note the comment explaining how the exposure time was estimated and the Special Requirements that tie this acquisition to the subsequent exposures. Acquisitions are explained in detail in the GHRS Instrument Handbook. The comment about BRIGHT=RETURN means that the onboard acquisition algorithm will determine which dwell point had the most counts and will center the LSA at that point.

Figure 35.5: Exposure Logsheet for Program 5297

PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ID 5297

1.Proposal Title: FIRST SPECTRA OF O STARS IN R136A

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TARGET LIST a) Fixed Targets ID = 5297 [ 3]

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1 | 2 | 3 | 4 | 5 | 6 | 7 | 8

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| | | | | | | |

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1 R136a1 B,103,D,203 RA = 05H 38M 42.438S +/- 2000 1 V=12.77

0.1S, 2 B-V=+0.43

DEC = -69D 06' 02.294" +/-

0.5"

Comments: V,B-V from Malumuth & Heap (1993)

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2 R136a2 B,103,D,203 RA = 05H 38M 42.4576S +/- 2000 1 V=13.38

0.1S, 2 B-V=+0.43

DEC = -69D 06' 02.242" +/-

0.5"

Comments: WN star in R136a; coordinates and V,B-V

from Malumuth & Heap (1993)

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5 R136a5 B,105,D,203 RA = 05H 38M 42.4785S +/- 2000 1 V=13.73

0.1S, 2 B-V=+0.12

DEC = -69D 06' 02.096" +/-

0.5"

Comments: O star in R136a; coordinates and V,B-V

from Malumuth & Heap (1993)

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EXPOSURE LOGSHEET ID = 5297 [ 4]

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1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |10 | 11 | 12 |13 |14| 15

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1 R136a1 HRS ACQ 2.0 MIRROR-N2 SEARCH-SIZE=5 1 25S 1 1 CYCLE 4/1-101

ONBOARD ACQ FOR 2-

SEQ 1-101 NO GAP

Comments: Time=(128*Nmap +SS2)*StepT =(0

+5x5)*1.0S = 25S GHRS HDBK/Fig

7.3 gives N1 c/s=2500 for

V=13.0, (B-V)o=-.3, E(B-V)=.4

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2 R136a1 HRS ACQ/ 2.0 MIRROR-N2 1 102S 1 1 CYCLE 4

PEAKUP

Comments: Time=Fapt*StepT =102 * 1S

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3 R136a1 HRS ACQ/ 0.25 MIRROR-N2 SEARCH-SIZE=5 1 25S 1 1 ONBOARD ACQ FOR 4-

PEAKUP 52

Comments: Time=SSIZE^2 * StepT =25 * 1S

BRIGHT=RETURN assumed

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4 R136a1 HRS IMAGE 0.25 MIRROR-N2 NX=13 1 169S 1 1

NY=13

DELTA-X=1

DELTA-Y=1

Comments: Time=13*13*1S=169S

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5 R136a2 HRS IMAGE 0.25 MIRROR-N2 NX=13 1 169S 1 1

NY=13

DELTA-X=1

DELTA-Y=1

Comments: Time=13*13*1S=169S

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20 R136a2 HRS ACCUM 0.25 G140L 1300 STEP-PATT=3 1 10M 1

FP-SPLIT=NO

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21 R136a2 HRS ACCUM 0.25 G140L 1610 STEP-PATT=3 4 10M 1

FP-SPLIT=NO

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50 R136a5 HRS ACCUM 0.25 G140L 1610 STEP-PATT=3 4 30M 1

FP-SPLIT=FOUR

Comments: total texp=120M

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51 R136a5 HRS ACCUM 0.25 G140L 1300 STEP-PATT=3 1 30M 1

FP-SPLIT=FOUR

Comments: total texp=30M

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52 WAVE HRS ACCUM SC2 G140L 1300 STEP-PATT=3 1 25.6S 1

FP-SPLIT=NO

Comments: WAVECAL FOR SPECTRA

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99 R136a5 HRS IMAGE 0.25 MIRROR-N2 NX=13 1 169S 1 1

NY=13

DELTA-X=1

DELTA-Y=1

Comments: Time=13*13*1S=169S

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100 R136a5 HRS IMAGE 2.0 MIRROR-N2 NX=22 1 968S 1 1

NY=22

DELTA-X=2

DELTA-Y=2

Comments: Time=22*22*2S=968S

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101 R136a5 HRS ACQ 2.0 MIRROR-N2 SEARCH-SIZE=3 1 1161S 1 1

LOCATE=NO

MAP=ALL-POINTS

Comments: Time=(128*Nmap +SS2)*StepT

=(128*9 +3X3)*1.0S=1161S

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Line 2: Here the object R136a1 was centered in the LSA. The exposure time of 102 seconds implied a STEP-TIME of 1.0 seconds, as for the initial acquisition. As we noted, what is called "R136a1" is not a single object but is a convolution of R136a1 with R136a2 because of the finite resolution of HST and the size of the LSA. An estimate of the centroid of a1+a2 was made from a pre-COSTAR WF/PC-1 image.
Line 3: Here R136a1 was centered in the SSA, with SEARCH-SIZE=5 and an implied STEP-TIME of 1.0 second. The true post-COSTAR size of the SSA is 0.22 arcsec square. As for line 1, the acquisition algorithm automatically determined which dwell point had the most counts and moved the SSA to that point. (The SSA ACQ/PEAKUP algorithm is the same as the LSA BRIGHT=RETURN acquisition algorithm.)
Line 4: An IMAGE of R136a1 in the SSA was obtained. The values of NX, NY, DELTA-X, and DELTA-Y were chosen to fully cover the SSA with the minimum spacing between points. The time spent at each point was 1.0 seconds, as implied by the 169 second total exposure time.
Line 5: This is the same as line 4 except that the object was R136a2. This implies that the telescope has been moved 0.052 arcsec north and 0.105 arcsec east, for a net motion of 0.117 arcsec, about half the size of the SSA. An IMAGE was taken to confirm the positioning.
Line 20: This line obtained a ten-minute exposure on R136a2 with grating G140L centered at 1300 Å. The STEP-PATT value chosen (3) was not the default value of 5, but was selected to maximize the time on target at the cost of some uncertainty in the background. Because the object was faint and low signal-to-noise was anticipated, FP-SPLIT=NO has been chosen. Using a Side 1 spectrum element (G140L) implied a wait of about 40 minutes between lines 5 and 20 to allow one detector to be turned off and the other to be turned on and brought to a stable configuration.
Line 21: Here a second spectrum centered at 1610 Å was obtained. Lines 20 and 21 together provided as much spectrum as G140L was capable of delivering.
Line 50: Now the target was R136a5, implying a small motion of the telescope. The central wavelength was the same as for line 21 to eliminate a grating movement. A longer exposure of 30 minutes was needed for this fainter target. In this case the FP-SPLIT feature has been used.
Line 51: Same as line 20, but for R136a5. Again, FP-SPLIT was used.
Line 52: A wavelength calibration exposure obtained at the previous position for G140L.
Line 99: An IMAGE of R136a5, analogous to line 5. As before, the use of MIRROR-N2 implied waiting about 40 minutes to switch Sides.
Line 100: An IMAGE of R136a5 in the LSA with MIRROR-N2.

Line 101: This "acquisition" at the end of the sequence of operations may seem odd, but what it achieved was a coarse image over a broader area (SEARCH-SIZE=3) than an IMAGE itself was capable of. Each of these lines was an operation specified by the observer, and they corresponded closely to the operations performed by the instrument and the data files that were generated, as shown in Table 35.5. For example, each of the ACQuisitions, ACQ/PEAKUPs, and IMAGEs resulted in a data file, but each is also preceded by a DEFCAL. This is a DEFlection CALibration, in which an internal lamp illuminated an aperture (SC2) which the acquisition mirror then imaged onto the photocathode. Software on the spacecraft then determined where the actual image of the aperture was falling on the diodes so that the image could be properly centered. The image moved slightly due to thermal effects and the earth's magnetic field. These DEFCALs were essential for proper operation of the GHRS, but the information they contain is rarely of use to the observer.

You will note that many, but not all, of the ACCUMs were preceded by a SPYBAL exposure. SPYBAL stands for SPectrum Y BALance, and it, like a DEFCAL, was performed to ensure proper alignment of the spectrum on the science diodes. However, a SPYBAL was an actual spectrum recorded at a selected wavelength for each grating (chosen to provide a uniform distribution of comparison lines) and it was performed each time a different grating was used for the first time. In longer programs, SPYBALs will appear about every 90 minutes of alignment time as well, in order to compensate for thermal drifts and the like. An exposure was not interrupted to insert a SPYBAL, however.

Relationship Between Proposal Lines and Files for Program 5297

Exposure Logsheet Line #

OBSMODE

Aperture

Mirror or Grating

Targname

FP_SPLIT

Central Wavelength (Å)

ROOTNAME
for output file

1

DEFCAL

z2bd0101t

ACQ

LSA

R136a1

z2bd0102t

2

DEFCAL

z2bd0103t

ACQ/PEAKUP

LSA

R136a1

z2bd0104t

3

DEFCAL

z2bd0105t

ACQ/PEAKUP

SSA

R136a1

z2bd0106t

4

DEFCAL

z2bd0107t

IMAGE

SSA

N2

R136a1

z2bd0108t

5

DEFCAL

z2bd0109t

IMAGE

SSA

N2

R136a2

z2bd010at

20

SPYBAL

SC2

G140L

WAVE

NO

1414.899

z2bd010bt

ACCUM

SSA

G140L

R136a2

NO

1304.579

z2bd010ct

21

ACCUM

SSA

G140L

R136a2

NO

1608.027

z2bd010dt

50

SPYBAL

SC2

G140L

WAVE

NO

1414.903

z2bd010et

ACCUM

SSA

G140L

R136a5

FOUR

1598.548

z2bd010ft

51

SPYBAL

SC2

G140L

WAVE

NO

1414.728

z2bd010gt

ACCUM

SSA

G140L

R136a5

FOUR

1294.922

z2bd010ht

52

ACCUM

SC2

G140L

WAVE

NO

1313.492

z2bd010it

99

DEFCAL

z2bd010jt

IMAGE

SSA

N2

R136a5

z2bd010kt

100

DEFCAL

z2bd010lt

IMAGE

SSA

N2

R136a5

z2bd010mt

101

DEFCAL

z2bd010nt

ACQ

LSA

N2

R136a5

z2bd010ot

Also, in some cases the PI suppressed SPYBALs in order to keep the carrousel from moving during a sequence of observations.

**Relationship Between Proposal Lines and Files for Program 5297**
Exposure Logsheet Line #	OBSMODE	Aperture	Mirror or Grating	Targname	FP_SPLIT	Central Wavelength (Å)	ROOTNAME for output file
1	DEFCAL						z2bd0101t
ACQ	LSA		R136a1			z2bd0102t
2	DEFCAL						z2bd0103t
ACQ/PEAKUP	LSA		R136a1			z2bd0104t
3	DEFCAL						z2bd0105t
ACQ/PEAKUP	SSA		R136a1			z2bd0106t
4	DEFCAL						z2bd0107t
IMAGE	SSA	N2	R136a1			z2bd0108t
5	DEFCAL						z2bd0109t
IMAGE	SSA	N2	R136a2			z2bd010at
20	SPYBAL	SC2	G140L	WAVE	NO	1414.899	z2bd010bt
ACCUM	SSA	G140L	R136a2	NO	1304.579	z2bd010ct
21	ACCUM	SSA	G140L	R136a2	NO	1608.027	z2bd010dt
50	SPYBAL	SC2	G140L	WAVE	NO	1414.903	z2bd010et
ACCUM	SSA	G140L	R136a5	FOUR	1598.548	z2bd010ft
51	SPYBAL	SC2	G140L	WAVE	NO	1414.728	z2bd010gt
ACCUM	SSA	G140L	R136a5	FOUR	1294.922	z2bd010ht
52	ACCUM	SC2	G140L	WAVE	NO	1313.492	z2bd010it
99	DEFCAL						z2bd010jt
IMAGE	SSA	N2	R136a5			z2bd010kt
100	DEFCAL						z2bd010lt
IMAGE	SSA	N2	R136a5			z2bd010mt
101	DEFCAL						z2bd010nt
ACQ	LSA	N2	R136a5			z2bd010ot

Table 35.6 shows some of the different kinds of files generated in this program for the different operations that resulted from the Exposure Logsheet. The entries are Exposure Logsheet line numbers. The lines at the bottom of the table show what kinds of files are generated for the different kinds of operations. For example, all observations generate log files with suffixes .shh, .ulh, and .trl. The "*" next to the acquisition files for lines 1 and 3 indicate that these observations also generate .d1h files and the corresponding .q1h data quality files. .d1h files are generated for Return-to-Brightest (RTB) acquisitions and SSA peakups (which use the RTB algorithm). Notice that LSA peakups do not generate .d1h files; nor would an LSA acquisition that does not use the RTB algorithm.

Breakdown of Files by Proposal Line Number for Program 5297

Target

DEFCAL

ACQ

ACQ/PEAKUP

IMAGE

( MAP)

SPYBAL

WAVECAL

ACCUM

FP-SPLIT

R136a1

1

1*

2

2

3

3*

4

4

R136a2

5

5

20

20

21

R136a5

50

50

51

51

52

99

99

100

100

101

101

Header File Suffixes

Log files: .shh, .ulh, .trl.

All log files, plus raw data files: .d0h, .q0h, .x0h, .xqh

All log files, all raw data files, plus calibrated files: .c0h,.c1h,.c2h, .c3h, .c4h, .c5h, .cqh

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stevens@stsci.edu

35.4 Assessing GHRS Data

35.4.1 An Example: R136a in the LMC

35.4.2 The Exposure Logsheet

Figure 35.5: Exposure Logsheet for Program 5297

Exposure Logsheet Line #

OBSMODE

Aperture

Mirror or Grating

Targname

FP_SPLIT

Central Wavelength (Å)

ROOTNAMEfor output file

Target

DEFCAL

ACQ

ACQ/PEAKUP

IMAGE

( MAP)

SPYBAL

WAVECAL

ACCUM

FP-SPLIT

ROOTNAME
for output file