9.7	Examples of Orbit Estimates

9.7.1 Target Acquisition Using ACQ/IMAGE

9.7.2 ACQ/SEARCH plus ACQ/IMAGE Acquisition

9.7.3 FUV Acquisition plus TIME-TAG

9.7.4 FUV TIME-TAG with BOA and Multiple FP-POS

9.7.5 FUV TIME-TAG with Modified BUFFER-TIME

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

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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.

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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 seconds 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 seconds. 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 21 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

Action	Phase I (s)	Chapter 10 (s)	APT Time (s)	Comment
Initial guide star acquisition	360	360	333	Required at start of a new visit
NUV ACQ/IMAGE with 2 s exposure	180	116 + 169 + 120 + 4 + 58 = 467	419 + 56 (dump)	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.
First exposure overhead adjustment	N/A	−(116 + 169) = −285	−333	OSM1 and OSM2 movements may be hidden in guide-star acquisition.
NUV G185M at 1850 Å, TIME-TAG, FP-POS=3, 1175 s exposure	300 + 1175	36 + 174+ 1175 + 116 = 1501	204 + 1175 + 116	Generic NUV TIME-TAG setup; change from MIRRORA to G185M (174 s); exp time; TIME-TAG memory readout
NUV G185M at 1817 Å, TIME-TAG, FP-POS=3, 1175 s exposure	120 + 60 + 1175	36 + 75 + 1175 + 116 = 1402	102 + 1175 + 116	Generic NUV TIME-TAG exposure setup; central-wavelength change (75 s); exp time; TIME-TAG memory readout
Total science time	2350	2350	2350
Total time used in orbit	3370	3445	3363

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

Action	Phase I (s)	Chapter 10 (s)	APT Time (s)	Comment
Initial guide star acquisition	360	360	333	Required at start of a new visit
NUV ACQ/SEARCH, MIRRORA, 3 × 3 pattern, 10 s exposure	420	116 + 169 + 9×(20+10) = 555	696	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)
First exposure overhead adjustment	N/A	−(116 + 169) = −285	−291	OSM1 and OSM2 movements may be hidden in guide-star acquisition.
NUV ACQ/IMAGE with 10 s exposure	180	120 + 2×10 + 58 = 198	131 + 56 (dump)	No OSM2 movement; ACQ/IMAGE setup, twice exp time, and memory readout
NUV G225M at 2250 Å, TIME-TAG, FP-POS=3, 1200 s exposure	300 + 1200	36 + 105 + 1200 + 116 = 1457	135 + 1200 + 114	Generic NUV TIME-TAG setup; change from MIRRORA to G225M (105 s); exp time; TIME-TAG memory readout
FUV G130M at 1309 Å, TIME-TAG, FP-POS=2, 300 s exposure	120 + 60 + 300	71 + 109 + 300 + 116 = 596	197 + 300 + 114	Generic FUV TIME-TAG setup; OSM1 move from NCM1 to G130M (109 s); exp time; TIME-TAG memory readout
FUV G130M at 1309 Å, TIME-TAG, FP-POS=4, 300 s exposure	120 + 300	71 + 6 + 300 + 116 = 493	64 + 300 + 114	Generic FUV TIME-TAG setup; increment FP-POS by 2 settings (6 s); exp time; TIME-TAG memory readout
Total science time	1800	1800	1800
Total time used in orbit	3360	3374	3463

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

Action	Phase I (s)	Chapter 10 (s)	APT Time (s)	Comment
Initial guide-star acquisition	360	360	333	Required at start of a new visit
FUV ACQ/SEARCH, G130M at 1309 Å, 3 × 3 pattern, 15 s exposure	420	9 × (20 + 15) = 315	441	COS starts from G130M 1309 Å 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)
First exposure overhead adjustment	N/A	0	−12	No instrument movements prior to first exposure in this example.
FUV ACQ/PEAKXD, G130M at 1309 Å, 25 s exposure	420	80 + 25 = 105	103 + 25	PEAKXD overhead; exp time
FUV ACQ/PEAKD, G130M at 1309 Å, 5 steps, 25 s exposure		5 × (20 + 25) + 39 = 264	293+ 37	5 slews plus 5 exposures; memory readout
FUV G140L at 1280 Å, TIME-TAG, FP-POS=ALL, SEGMENT=A, 268 s exposure	3301 + 300 + 268	3301+ 71 + 164 + 268+ 116 = 949	300 (reconfig) + 243 + 268 + 114	SEGMENT reconfiguration change; generic FUV TIME-TAG setup; OSM1 grating change (164 s); exposure time; TIME-TAG memory readout (note: FP-POS=1)
	120 + 268	71 + 3 + 268 + 116 = 458	67 + 268 + 114	Generic FUV TIME-TAG setup; change to FP-POS=2 (3 s); exp time; TIME-TAG memory readout
	120 + 268	71 + 3 + 268 + 116 = 458	67 + 268 + 114	Same as above, but with FP-POS=3
	120 + 268	71 + 3 + 268 + 116 = 458	67 + 268 + 114	Same as above, but with FP-POS=4
Total science time	1472	1472	1472
Total time used in orbit	3262	3367	3492

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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-second 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

Action	Phase I (s)	Chapter 10 (s)	APT Time (s)	Comment
Initial guide-star acquisition	360	360	333	Required at start of a new visit
NUV ACQ/IMAGE with 2 s exposure	180	116 + 169 + 120 + 4 + 58 = 467	421 + 56 (dump)	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.
First exposure overhead adjustment	N/A	−(116 + 169) = −285	−333	OSM1 and OSM2 movements may be hidden in guide star acquisition.
FUV G160M at 1600 Å, TIME-TAG, BOA, FLASH=NO, FP-POS=1, 1100 s exposure	300 + 1100	71 + 154 + 8 + 116 + 1100 + 116 = 1449	233 + 1100 + 114	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 Å, TIME-TAG, AUTO WAVECAL, WCA, FP-POS=1, 12 s exposure	120 + 12	71 + 10 + 12 + 116 = 209	75 + 12 + 38	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 Å, TIME-TAG, BOA, FLASH=NO, FP-POS=2, 1100 s exposure	120 + 1100	71 + 3+ 10 + 1100 + 116 = 1300	73 + 1100 + 114	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 Å, TIME-TAG, AUTO WAVECAL, WCA, FP-POS=2, 12 s exposure	120 + 12	71 + 10 + 12 + 116 = 209	75 + 12 + 38	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
Total science time in orbit 1	2200	2200	2200
Total time used in orbit 1	3424	3709	3463
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

Action	Phase I (s)	Chapter 10 (s)	APT Time (s)	Comment
Initial guide star acquisition	360	360	333	Required at start of a new visit
NUV ACQ/IMAGE with 10 s exposure	180	116 + 169 + 120 + 20 + 58 = 483	437 + 56	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.
First exposure overhead adjustment	N/A	−(116 + 169) = −285	−333	OSM1 and OSM2 movements may be hidden in guide star acquisition.
FUV G130M at 1327 Å, TIME-TAG, FP-POS=1, BUFFER-TIME=1210, 1310 s exposure	300 + 1310	109 + 71 + 1310 + 40 = 1530	197 + 1310 + 38	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, 1310 s exposure	120 + 1310	71 + 3 + 1310 + 116 = 1500	67 + 1310 + 114	Generic TIME-TAG set-up; move to FP-POS=2 (3 s); exp time; TIME-TAG memory readout
Total science time in orbit 1	2620	2620	2620
Total time used in orbit 1	3580	3588	3529
Guide star re-acquisition	240	240	222	Required at start of next orbit
TIME-TAG, FP-POS=3, BUFFER-TIME=1398, 1498 s exposure	120 + 1498	71 + 3 + 1498 + 40 = 1612	67 + 1498 + 38	As for FP-POS=2, but with short TIME-TAG memory read-out
TIME-TAG, FP-POS=4, BUFFER-TIME=1498, 1498 s exposure	120 + 1498	71 + 3 + 1498 + 116 = 1688	67 + 1498 + 114	As for FP-POS=2
Total science time in orbit 2	2996	2996	2996
Total time used in orbit 2	3476	3540	3504