Instrument Science Report STIS 96-029
Statistics from the STIS GTO Science
Program
R. Downes
August 5, 1996
ABSTRACT
The submitted STIS GTO Exposure Catalog has been used to estimate the type of science
to be obtained with STIS, as well as the statistics of that usage. We find that most of the
prime science will be long-exposure UV (MAMA) and optical (CCD) observations. There
will be roughly 1340 MSM positions per year, with 1150 images and 3000 spectra of prime
science. The raw prime science data rates are determined to be 158,000 Mbits/year (433
Mbits/day) for the MAMAs, and 38,280 Mbits/year (105 Mbits/day) for the CCD. Most
targets will have three or fewer configurations, and most objects will be observed in four
or fewer orbits. The bulk of the exposures are less than 2500s (roughly an orbit visibility
period). About 40% of the program (by exposure) will be CCD or FUV-MAMA observations, with NUV-MAMA comprising the remaining 20%; 70% of the observations will be
spectroscopic. Comparison with the initial estimates reveal good agreement, although the
number of exposures is slightly smaller and the amount of MAMA data (after correction
for the larger WAVECAL size) slightly larger than in the Design Reference Mission report.
1. Introduction
An estimate of the expected usage for STIS was present by Baum, Hodge, and Kutina
(STIS ISR 95-002). The IDT has now submitted its official GTO Exposure catalog, so a
more accurate determination of the type of science to be performed with STIS, as well as
information useful for determining expected mechanism usage, can now be obtained.
As the Exposure Catalog does not provide detailed (i.e., Phase II) information, the following assumptions were used in the determination of the statistics:
1. All observations assumed to be full array (for the MAMAs, 2048x2048; for the
CCD 1064x1044), and only prime science has been analyzed.
2. The numbers are all approximate; no detailed study of the catalog was performed.
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3. The total number of orbits per year is 5400, with STIS prime science obtained in
33% (1800) of the orbits. This means that the GTO program (555 orbits over 3
years) is the equivalent of 31% the total STIS science program in one year.
4. The GTO science program is typical of the science GOs will be performing.
2. Prime Science breakdown
Category 1: Faint target, UV and Βright target, UV, Echelle. There are 656 exposures
(exposure time greater than 1000s) totaling 973782 seconds which is 61/51% of the time/
exposures in the STIS GTO program; 56 exposures (9%) and 92800 seconds (10%) of the
observations are imaging.
Category 2: Bright target, UV. There are 43 exposures totaling 23160 seconds which is
1/3% of the time/exposures in the STIS GTO program; 24 exposures (56%) and 9820
seconds (42%) of the observations are imaging.
Category 3: Faint target, visible. There are 220 exposures totaling 361635 seconds
which is 23/17% of the time/exposures in the STIS GTO program; 57 exposures (26%)
and 105600 seconds (29%) of the observations are imaging.
Category 4: Bright target, visible. There are 298 exposures totaling 133250 seconds
which is 8/23% of the time/exposures in the STIS GTO program; 204 exposures (68%)
and 80180 seconds (60%) of the observations are imaging.
Category 5: Time-tag, UV. There are 68 exposures totaling 109780 seconds which is 7/
5% of the time/exposures in the STIS GTO program; 6 exposures (9%) and 10500 seconds
(10%) of the observations are imaging.
The table below gives the breakdown on the science exposures by instrumental setup.
3. Target breakdown
The total number of GTO MAMA targets is 83, and the total number of GTO CCD targets is 59. This would result, assuming acquisitions only for spectroscopic observations, in
a total number of target acquisitions of 124; note that these number do not take multi-cycle
targets into account. A total of 400 target acquisition per year should be expected.
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Table 1. Breakdown of Science Exposures
Instrumental
Setup
Exposure Time
% of Total
Program by
Time
Number of
Exposures
% of Total
Program by
Exposure
CCD imaging
185580
12
272
21
MAMA imaging
113120
7
86
7
CCD spectra
309305
19
246
19
MAMA first
order spectra
373522
23
238
19
MAMA echelle
spectra
620079
39
443
34
CCD
494885
31
518
40
NUV-MAMA
354181
22
247
20
FUV-MAMA
752540
47
520
40
4. MSM positions breakdown
The number of unique wavelength settings (MSM positions) per target used in the
GTO prime science program is 291 (78 MAMA echelle, 58 MAMA first-order, 98 CCD,
12 slitless spectra (6 MAMA and 6 CCD), and 45 imaging (MIRROR) settings); there are
80 repeat exposures at the same setting. Since target acquisition exposures were not
included in the Exposure Catalog, and most acquisitions will not be performed with the
science element, the corrected number of settings is 291+124 = 415, which results in 1340
settings expected per year.
5. Data volume
The number of images in the GTO prime science program is 415 (86 MAMA, 329
CCD), while the number of spectra is 1090 (681 MAMA, 409 CCD); note that CCD
observations with exposure times greater than 1000 seconds are assumed to be CR-SPLIT
into two exposures. This translates to 1338 (277 MAMA, 1061 CCD) images and 3516
(2200 MAMA, 1319 CCD) spectra expected per year; note that an equivalent number of
WAVECALs (for unique settings), as well as 400 target acquisitions, will accompany the
spectroscopic observations. A full-array MAMA exposure is 67 Mbits, a MAMA exposure with a WAVECAL is 84 Mbits, a CCD (both science and WAVECAL) exposure is 18
Mbits, and a target acquisition exposure is 0.5 Mbits. Note that these values are much dif-
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ferent than those in STIS ISR 95-002, which assumed sub-arrays would be used for the
WAVECALs. We therefore expect 203,400 Mbits/year of raw MAMA data, and 66780
Mbits/year of raw CCD data; this corresponds to roughly 560 Mbits/day and 185 Mbits/
day of raw MAMA and CCD data, respectively.
6. Configurations per target
The table below gives the number of configurations per target in the GTO prime science program, where a configuration is defined as a unique selection of detector, mode,
aperture, spectral element, and central wavelength.
Table 2. Summary of Configurations per Target
# of configs
# of targets
1
65
2
39
3
11
4
12
5
2
6
4
7
1
8
1
11
1
25
1
7. Orbits per target (total orbits 555)
The table below gives the number of orbits per target in the GTO prime science
program.
Table 3. Summary of # of Orbits per Target
# of orbits
# of objects
# of orbits
# of objects
1
36
11
1
2
30
12
1
3
11
16
1
4
# of orbits
# of objects
# of orbits
# of objects
4
16
20
1
5
8
21
1
6
11
23
1
7
2
24
1
8
1
25
2
9
1
28
1
10
2
8. Exposure Time Histogram
The attached plots show histograms (at two scales) of the number of exposures in a
given exposure time bin. Most exposures (97%) are less than 2500s (at typical orbital visibility is 3200 seconds), with a strong peak (50%) between 1000 and 1500 seconds. This
means we could anticipate roughly 2 science exposures per orbit, for a total of 3900 science exposures per year. This is in good agreement with the number of (non CR-SPLIT)
science exposures predicted (4140) above (see Data Volume).
Figure 1: Distribution of exposure times for the STIS GTO program.
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Figure 2: Expanded scale.
9. Comparison with ISR 95-002 (Design Reference Mission)
Although the science categories used here are somewhat different than those used in
the preliminary analysis, a comparison of the two analyses can be performed. The initial
determination was that 60% of the prime science program would be long-exposure UV
observations, and 30% would long-exposure optical observations. The results presented
here (60% UV and 25% visible) are quite consistent with the preliminary values.
The total number of orbits per year used in ISR 95-002 is 1834, consistent with the
value 1800 used here. The number of prime exposures per year of long-exposure UV and
optical observations is 9600, which means the total program would have 10,600 exposures. From the GTO program, I determined that there would be 4850 prime science
exposures, 3000 WAVECALs associated with those exposures, and 400 target acquisitions, for a total of 8250 exposures, about 78% of what was initially predicted.
The raw data volume for the two main categories of science was originally estimated
to be 386 Mbits/day of MAMA data and 102 Mbits/day of CCD data. The GTO program
results in 560 Mbits/day of MAMA data and 185 Mbits/day of CCD data (as the CCD
observations are a smaller percentage of the GTO program, the scaled number is 260
Mbits/day). Taking into account the difference in assumptions (the ISR assumed WAVECALs would be obtained in a sub-array), there is good agreement with the preliminary
numbers; using the ISR data sizes, there would be 433 and 105 Mbits/day for the MAMAs
and CCD, respectively. Although it is not possible to analyze the parallel or calibration
observations from the GTO catalog, the close agreement between the Design Reference
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Mission predictions and the prime GTO science program lends credence to the values
determine for the parallel and calibration programs.
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