Space Telescope Science Institute
COS Instrument Handbook
help@stsci.edu
Table of Contents Previous Next Index Print


Cosmic Origins SpectrographInstrument Handbookfor Cycle 22 > Chapter 9: Scheduling Observations > 9.7 Examples of Orbit Estimates

9.7
In this section we present five example COS observations using both detectors and all of the target-acquisition modes. Besides the topics discussed in the previous sections we include examples of
Multiple FP-POS settings (Section 5.8.2): To minimize the damage to the FUV detector caused by strong Lyman-α airglow lines and to improve the limiting S/N of an observation. Observers are strongly encouraged to use multiple FP-POS settings for each CENWAVE setting. Observers should use at least two and, if possible, all four FP-POS positions of a given CENWAVE.
Adjusting the BUFFER-TIME (Section 5.4): If BUFFER-TIME is greater than the exposure time, one would normally set BUFFER-TIME = EXPTIME. In orbits with a series of long FUV exposures, one can minimize overheads by setting BUFFER-TIME = EXPTIME-100. The full buffer takes 116 s to empty, so most of the data will be read out before the exposure is completed. The post-exposure data dump then requires only 40 s. For the final exposure of an orbit, the buffer dump can occur during the occultation, so adjusting the BUFFER-TIME will not save time. See the example in Section 9.7.5.
While the overhead rules presented in this chapter may appear complex, the actual rules used by the HST scheduling software are even more so. It is thus imperative that you use APT to construct your Phase II proposal. In the examples that follow, we present three sets of overhead estimates: one using the Phase I rules (Section 9.1), one using the rules in this chapter (Sections 9.2 to 9.6), and one computed using APT version 20.2.3. The APT estimates should be considered definitive. COS is a relatively new instrument, and the overhead rules will continue to evolve as we learn how to use it more efficiently. The version of APT available for constructing Cycle 22 Phase II proposals may return values that differ slightly from those given below. An up-to-date version of the APT must be used for the Phase II planning of each visit.
9.7.1 Target Acquisition Using ACQ/IMAGE
In this example, we begin with an NUV ACQ/IMAGE target acquisition, then add two NUV TIME-TAG exposures using the same grating but different central wavelengths. For NUV observations, the use of multiple FP-POS settings is not required (though it useful to reduce flat-field noise).
Table 9.6: Overhead Values for ACQ/IMAGE Acquisition
NUV ACQ/IMAGE with 2 s exposure
COS starts at G130M on OSM1, so move to NCM1 requires 116 s. OSM2 home position is G185M, so move to MIRRORA takes 169 s. Add 2 min ACQ/IMAGE setup, twice the exposure time, and memory readout.
(116 + 169) = −285
333
NUV G185M at 1850 , TIME-TAG, FP-POS=3, 1175 s exposure
Generic NUV TIME-TAG setup; change from MIRRORA to G185M (174 s); exp time; TIME-TAG memory readout
NUV G185M at 1817 , TIME-TAG, FP-POS=3,
Generic NUV TIME-TAG exposure setup; central-wavelength change (75 s); exp time; TIME-TAG memory readout
9.7.2 ACQ/SEARCH plus ACQ/IMAGE Acquisition
In this example, we begin with an NUV ACQ/SEARCH followed by an ACQ/IMAGE target acquisition. We obtain an NUV TIME-TAG exposure, then switch to the FUV channel for a pair of FUV TIME-TAG exposures. To minimize damage to the detector, we employ two FP-POS settings.
Table 9.7: Overhead Values for ACQ/SEARCH plus ACQ/IMAGE
NUV ACQ/SEARCH, MIRRORA, 3 3 pattern, 10 s exposure
116 + 169 + 9(20+10) = 555
COS starts at G130M on OSM1, so move to NCM1 requires 116 s. OSM2 home position is G185M, so move to MIRRORA takes 169 s.
9 ACQ/SEARCH sub-exposures, so overhead includes 9 slews (20 s each) plus 9 exposures (10 s each)
(116 + 169) =
−285
291
NUV ACQ/IMAGE with 10 s exposure
120 + 210 + 58 =
198
No OSM2 movement; ACQ/IMAGE setup, twice exp time, and memory readout
NUV G225M at 2250 , TIME-TAG, FP-POS=3, 1200 s exposure
Generic NUV TIME-TAG setup; change from MIRRORA to G225M (105 s); exp time; TIME-TAG memory readout
FUV G130M at 1309 , TIME-TAG, FP-POS=2,
Generic FUV TIME-TAG setup; OSM1 move from NCM1 to G130M (109 s); exp time; TIME-TAG memory readout
FUV G130M at 1309 , TIME-TAG, FP-POS=4,
Generic FUV TIME-TAG setup; increment FP-POS by 2 settings (6 s); exp time; TIME-TAG memory readout
9.7.3 FUV Acquisition plus TIME-TAG
In this example, we begin with an FUV ACQ/SEARCH, followed by ACQ/PEAKXD and ACQ/PEAKD, all with G130M, then change to G140L and SEGMENT=A for a set of FUV TIME-TAG exposures using FP-POS=ALL.
Table 9.8: Overhead Values for FUV Acquisition and FP-POS=ALL
FUV ACQ/SEARCH, G130M at 1309 , 3 3 pattern, 15 s exposure
9 (20 + 15) = 315
COS starts from G130M 1309 on OSM1, so no initial move. 9 ACQ/SEARCH sub-exposures, so overhead includes 9 slews (20 s each) plus 9 exposures (15 s each)
12
FUV ACQ/PEAKXD, G130M at 1309 , 25 s exposure
PEAKXD overhead; exp time
FUV ACQ/PEAKD, G130M at 1309 , 5 steps, 25 s exposure
5 (20 + 25) + 39 = 264
FUV G140L at 1280 , TIME-TAG, FP-POS=ALL, SEGMENT=A, 268 s exposure
3301 + 300 + 268
330a+ 71 + 164 + 268+ 116 = 949
SEGMENT reconfiguration change; generic FUV TIME-TAG setup; OSM1 grating change (164 s); exposure time; TIME-TAG memory readout (note: FP-POS=1)
Generic FUV TIME-TAG setup; change to FP-POS=2 (3 s); exp time; TIME-TAG memory readout
1
We include the 330 s required to shut off detector Segment B (Table 9.5).

9.7.4 FUV TIME-TAG with BOA and Multiple FP-POS
In this example, we start with an NUV ACQ/IMAGE, followed by a switch to the FUV channel and a TIME-TAG science exposure using G160M, FP-POS=ALL, the BOA, and, as required with the BOA, FLASH=NO. The science exposure will be followed automatically by a 12 s wavecal (see Table 5.2). As required, we obtain two exposures with FP-POS=1 and 2. In the second orbit (not shown), we obtain exposures with FP-POS=3 and 4.
Table 9.9: Overhead Values for FUV TIME-TAG Using the BOA
NUV ACQ/IMAGE with 2 s exposure
OSM1 starts at G130M, so move to NCM1 requires 116 s. OSM2 starts at G185M, so move to MIRRORA takes 169 s. Add 2 min ACQ/IMAGE setup, twice the exposure time, and memory readout.
(116 + 169) = −285
333
FUV G160M at 1600 , TIME-TAG, BOA, FLASH=NO, FP-POS=1, 1100 s exposure
Generic FUV TIME-TAG setup; change from NCM1 to G160M (154 s); aperture change from PSA to BOA (8 s); exp time; TIME-TAG memory readout
FUV G160M at 1600 , TIME-TAG, AUTO WAVECAL, WCA, FP-POS=1, 12 s exposure
AUTO WAVECAL inserted, since FLASH=YES is not allowed with BOA; generic FUV TIME-TAG setup; aperture change from BOA to WCA (10 s); exp time; TIME-TAG memory readout
FUV G160M at 1600 , TIME-TAG, BOA, FLASH=NO, FP-POS=2, 1100 s exposure
Generic FUV TIME-TAG setup; increment FP-POS (3 s); aperture change from WCA to BOA (10 s); exp time; TIME-TAG memory readout
FUV G160M at 1600 , TIME-TAG, AUTO WAVECAL, WCA, FP-POS=2, 12 s exposure
Another AUTO WAVECAL required as FP-POS has changed; again generic FUV TIME-TAG exposure setup; aperture change from BOA to WCA (10 s); exp time; TIME-TAG memory readout
Note: Two additional exposures, using FP-POS=3 and 4 in a second orbit, are not shown.
9.7.5 FUV TIME-TAG with Modified BUFFER-TIME
In this example, we begin with an NUV ACQ/IMAGE exposure, then switch to the FUV channel for four long G130M exposures, one at each FP-POS position. We use a couple of tricks to maximize the exposure time. First, we shorten the BUFFER-TIME for the first exposure of each orbit as described in Section 5.4.2, which reduces the length of the memory read-out following the exposure from 116 to 40 seconds. Second, we extend the exposure times, pushing the final memory read-out of each orbit into the occultation period. Note that we do not use FP-POS=ALL, because that would generate four identical exposures; instead, we increment the FP-POS by hand.
Table 9.10: Overhead Values for FUV TIME-TAG with Modified BUFFER-TIME
NUV ACQ/IMAGE with 10 s exposure
OSM1 starts at G130M, so move to NCM1 requires 116 s. OSM2 starts at G185M, so move to MIRRORA takes 169 s. Add 2 min ACQ/IMAGE setup, twice the exposure time, and memory readout.
(116 + 169) = −285
333
FUV G130M at 1327 , TIME-TAG, FP-POS=1, BUFFER-TIME=1210,
Move OSM1 from NCM1 to G130M (109 s); generic TIME-TAG set-up (71 s); exposure time; short TIME-TAG memory read-out (40 s)
TIME-TAG, FP-POS=2, BUFFER-TIME=1310,
Generic TIME-TAG set-up; move to FP-POS=2 (3 s); exp time; TIME-TAG memory readout
TIME-TAG, FP-POS=3, BUFFER-TIME=1398,
As for FP-POS=2, but with short TIME-TAG memory read-out
TIME-TAG, FP-POS=4, BUFFER-TIME=1498,
As for FP-POS=2
9.7.6 Single CENWAVE Example for Non-CVZ Targets using 4 FP-POS in 1 Orbit
This example is a single orbit TIME-TAG observation using the G130M grating and the 1055 CENWAVE with FP-POS=ALL. It uses a 30 s ACQ/IMAGE target acquisition with MIRRORB and the BOA. The PSA is used for the science exposures.
Table 9.11: Overhead Values for FUV TIME-TAG with 4 FP-POS
NUV ACQ/IMAGE with 30 s exposure
COS starts at G130M on OSM1, so move to NCM1 requires 116 s. OSM2 home position is G185M, so move to MIRRORB takes 175 s. PSA to BOA change takes 8 s. Add 2 min ACQ/IMAGE setup, twice the exposure time, and memory readout.
(116 + 175) = −291
333
FUV G130M at 1055 , TIME-TAG, FP-POS=ALL, BUFFER-TIME=500,
Generic FUV TIME-TAG setup; change from MIRRORB to G130M (109 s); change from BOA to PSA (8 s); exp time; TIME-TAG memory readout
Generic FUV TIME-TAG setup; change to FP-POS=2 (3 s); exp time; TIME-TAG memory readout
Generic FUV TIME-TAG setup; change to FP-POS=3 (3 s); exp time; TIME-TAG memory readout
Generic FUV TIME-TAG setup; change to FP-POS=4 (3 s); exp time; TIME-TAG memory readout
 
9.7.7 Single CENWAVE Example for Non-CVZ Targets with 4 FP-POS in 2 Orbits
This example is a TIME-TAG observation using the G130M grating and the 1055 CENWAVE with FP-POS=ALL spread over two orbits. It uses a 30 s ACQ/IMAGE target acquisition with MIRRORB and the BOA. The PSA is used for the science exposures.
Table 9.12: Overhead Values for FUV TIME-TAG with 4 FP-POS in 2 Orbits
NUV ACQ/IMAGE with 30 s exposure
COS starts at G130M on OSM1, so move to NCM1 requires 116 s. OSM2 home position is G185M, so move to MIRRORB takes 175 s. PSA to BOA change takes 8 s. Add 2 min ACQ/IMAGE setup, twice the exposure time, and memory readout.
(116 + 175) = −291
333
FUV G130M at 1055 , TIME-TAG, FP-POS=1, BUFFER-TIME=1075,
Generic FUV TIME-TAG setup; change from MIRRORB to G130M (109 s); change from BOA to PSA (8 s); exp time; TIME-TAG memory readout
FUV G130M at 1055 , TIME-TAG, FP-POS=2, BUFFER-TIME=1075,
FUV G130M at 1055 , TIME-TAG, FP-POS=3, BUFFER-TIME=1275,
Generic FUV TIME-TAG setup; change to FP-POS=3 (3 s); exp time; TIME-TAG memory readout
FUV G130M at 1055 , TIME-TAG, FP-POS=4, BUFFER-TIME=1275,
 

Cosmic Origins SpectrographInstrument Handbookfor Cycle 22 > Chapter 9: Scheduling Observations > 9.7 Examples of Orbit Estimates

Table of Contents Previous Next Index Print