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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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Tar| Target | Target | Target |Coord | Radial |Acqui|FLX| Flux data

No | Name | Description | Position |Eqnx | Vel. |Prblm|REF|

| | | | | | | |

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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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Line | Seq | Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | S/N |Flx|Pr| Special

Number | Name | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| |Rel. Time|Ref| | Requirements

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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-

3

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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