COS Instrument Handbook for Cycle 26
help@stsci.edu
Table of Contents Previous Next Index PDF


Cosmic Origins SpectrographInstrument Handbook for Cycle 26 > Chapter 9: Scheduling Observations > 9.7 Examples of Orbit Estimates

9.7
In this section we present seven 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, proposers using the FUV channel of COS, but who do not intend to use all four FP-POS settings for each central wavelength setting, must justify this choice in the observing strategy section of their Phase I proposal. A modest reduction in observational overheads will not normally be considered sufficient justification for not using all four FP-POS settings.
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 114 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 38 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. (This is the same strategy outlined in Section 5.4.4.)
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 25.4.0.1. The version of APT available for constructing future Phase II proposals may return values that differ slightly from those given below. An up-to-date version of 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 is useful to reduce flat-field noise).
Table 9.6: Overhead Values for ACQ/IMAGE Acquisition.
APT Time (s)1
NUV ACQ/IMAGE with 2 s exposure
119 + 120 + 2  2 + 56 = 299
COS starts at G130M on OSM1, so move to NCM1 requires 119 s. OSM2 home position is MIRRORA, so no move is needed. Add 120 s ACQ/IMAGE setup, twice the exposure time, and memory readout.
119
160
NUV G185M at 1850 , TIME-TAG, FP-POS=3, 1175 s exposure
Generic NUV TIME-TAG setup; OSM2 change from MIRRORA to G185M (175 s); exposure time; TIME-TAG memory readout
NUV G185M at 1817 , TIME-TAG, FP-POS=3,
Generic NUV TIME-TAG setup; change central wavelength (75 s); exposure time; TIME-TAG memory readout
1
Periodic updates to APT may result in small discrepancies from the overheads shown here.

9.7.2 Target Acquisition Using ACQ/SEARCH plus ACQ/IMAGE
In this example, we begin with an NUV ACQ/SEARCH target acquisition 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.
APT Time (s)1
NUV ACQ/SEARCH, MIRRORA, 3 3 pattern, 10 s exposure
119 + 306 + 9  10 + 37= 552
COS starts at G130M on OSM1, so move to NCM1 requires 119 s. OSM2 home position is MIRRORA, so no move is needed. Add 306 s SCAN-SIZE=3 setup, 9 times the exposure time, and memory readout.
119
129
NUV ACQ/IMAGE with 10 s exposure
120 + 2  10 + 56 =
196
No OSM movement, ACQ/IMAGE setup, twice the exposure time, and memory readout
NUV G225M at 2250 , TIME-TAG, FP-POS=3, 1200 s exposure
Generic NUV TIME-TAG setup; OSM2 change from MIRRORA to G225M (106 s); exposure time; TIME-TAG memory readout
FUV G130M at 1309 , TIME-TAG, FP-POS=2,
300 s exposure2
Generic FUV TIME-TAG setup; OSM1 change from NCM1 to G130M (121 s); exposure time; TIME-TAG memory readout
FUV G130M at 1309 , TIME-TAG, FP-POS=4,
Generic FUV TIME-TAG setup; increment FP-POS (3 s); exposure time; TIME-TAG memory readout
1
Periodic updates to APT may result in small discrepancies from the overheads shown here.
2
Starting in Cycle 25, only segment A spectroscopy is permitted at cenwave 1309.

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 for a set of FUV TIME-TAG exposures using FP-POS=ALL and SEGMENT=A. Since the COS 2025 policy requires SEGMENT=A for acquisition with CENWAVE=1309, there is no reconfiguration penalty for the G140L spectroscopy.
Table 9.8: Overhead Values for FUV Acquisition and FP-POS=ALL.
APT Time (s)1
FUV ACQ/SEARCH, G130M at 1309 , 3 3 pattern, 15 s exposure
80 + 306 + 9  15 + 37 = 558
OSM1 home position is cenwave 1291, so move to 1309 requires 80 s. OSM2 home position is MIRRORA, so no move is needed. Add 306 s SCAN-SIZE=3 setup, 9 times the exposure time, and memory readout.
80
85
FUV ACQ/PEAKXD, G130M at 1309 , 25 s exposure
115 + 3  25 + 37 = 227
FUV ACQ/PEAKD, G130M at 1309 , 5 steps, 25 s exposure
168 + 5  25 + 37 = 330
FUV G140L at 1280 , TIME-TAG, FP-POS=ALL, SEGMENT=A, 268 s exposure
Generic FUV TIME-TAG setup; OSM1 change from G130M to G140L (172 s); exposure time; memory readout (note: FP-POS=1)
Generic FUV TIME-TAG setup; change to FP-POS=2 (3 s); exposure time; memory readout
1
Periodic updates to APT may result in small discrepancies from the overheads shown here.

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.
APT Time (s)1
NUV ACQ/IMAGE with 2 s exposure
119 + 120 + 2  2 + 56 = 299
COS starts at G130M on OSM1, so move to NCM1 requires 119 s. OSM2 home position is MIRRORA, so no move is needed. Add 120 s ACQ/IMAGE setup, twice the exposure time, and memory readout.
119
160
FUV G160M at 1600 , TIME-TAG, BOA, FLASH=NO, FP-POS=1, 1100 s exposure
Generic FUV TIME-TAG setup; OSM1 change from NCM1 to G160M (159 s); aperture change from PSA to BOA (8 s); exposure 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); exposure 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); exposure 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; generic FUV TIME-TAG setup; aperture change from BOA to WCA (10 s); exposure time; TIME-TAG memory readout
Note: Two additional exposures, using FP-POS=3 and 4 in a second orbit, are not shown.
1
Periodic updates to APT may result in small discrepancies from the overheads shown here.

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 114 to 38 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.
APT Time (s)1
NUV ACQ/IMAGE with 10 s exposure
119 + 120 + 2  10 + 56 = 315
COS starts at G130M on OSM1, so move to NCM1 requires 119 s. OSM2 home position is MIRRORA, so no move is needed. Add 120 s ACQ/IMAGE setup, twice the exposure time, and memory readout.
119
160
FUV G130M at 1327 , TIME-TAG, FP-POS=1, BUFFER-TIME=1210,
1310 s exposure2
Generic FUV TIME-TAG set-up; OSM1 change from NCM1 to G130M (121 s);exposure time; short TIME-TAG memory readout (38 s)
TIME-TAG, FP-POS=2, BUFFER-TIME=1310,
Generic FUV TIME-TAG set-up; move to FP-POS=2 (3 s); exposure time; TIME-TAG memory readout
FP-POS movement may be hidden in guide star re-acquisition.
TIME-TAG, FP-POS=3, BUFFER-TIME=1398,
As for FP-POS=2, but with short TIME-TAG memory readout
TIME-TAG, FP-POS=4, BUFFER-TIME=1498,
As for FP-POS=2
1
Periodic updates to APT may result in small discrepancies from the overheads shown here.
2
Starting in Cycle 25, only segment A spectroscopy is permitted at cenwave 1327.

9.7.6 Single CENWAVE Example for Non-CVZ Targets with 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: 4 FP-POS in 1 Orbit
(s)1
NUV ACQ/IMAGE with 30 s exposure
COS starts at G130M on OSM1, so move to NCM1 requires 119 s. OSM2 home position is MIRRORA, so move to MIRRORB takes 71 s. PSA to BOA change takes 8 s. Add 120 s ACQ/IMAGE setup, twice the exposure time, and memory readout.
(119 + 71) = −190
262
FUV G130M at 1055 , TIME-TAG, FP-POS=ALL, BUFFER-TIME=500,
Generic FUV TIME-TAG setup; change from MIRRORB to G130M (154 s); aperture change from BOA to PSA (8 s); exposure time; TIME-TAG memory readout
Generic FUV TIME-TAG setup; move to FP-POS=2 (3 s); exposure time; TIME-TAG memory readout
Generic FUV TIME-TAG setup; move to FP-POS=3 (3 s); exposure time; TIME-TAG memory readout
Generic FUV TIME-TAG setup; move to FP-POS=4 (3 s); exposure time; TIME-TAG memory readout
1
Periodic updates to APT may result in small discrepancies from the overheads shown here.

9.7.7 Single CENWAVE Example for Non-CVZ Targets with 4 FP-POS in 2 Orbits
This example is a two-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 9.12: Overhead Values for FUV TIME-TAG: 4 FP-POS in 2 Orbits.
(s)1
NUV ACQ/IMAGE with 30 s exposure
COS starts at G130M on OSM1, so move to NCM1 requires 119 s. OSM2 home position is MIRRORA, so move to MIRRORB takes 71 s. PSA to BOA change takes 8 s. Add 120 s ACQ/IMAGE setup, twice the exposure time, and memory readout.
(119 + 71) = −190
262
FUV G130M at 1055 , TIME-TAG, FP-POS=1, BUFFER-TIME=1075,
Generic FUV TIME-TAG setup; change from MIRRORB to G130M (154 s); aperture change from BOA to PSA (8 s); exposure time; short TIME-TAG memory readout (38 s)
FUV G130M at 1055 , TIME-TAG, FP-POS=2, BUFFER-TIME=1075,
FP-POS movement may be hidden in guide star re-acquisition.
FUV G130M at 1055 , TIME-TAG, FP-POS=3, BUFFER-TIME=1275,
Generic FUV TIME-TAG setup; move to FP-POS=3 (3 s); exposure time; short TIME-TAG memory readout
FUV G130M at 1055 , TIME-TAG, FP-POS=4, BUFFER-TIME=1275,
1
Periodic updates to APT may result in small discrepancies from the overheads shown here.


Cosmic Origins SpectrographInstrument Handbook for Cycle 26 > Chapter 9: Scheduling Observations > 9.7 Examples of Orbit Estimates

Table of Contents Previous Next Index PDF