9.7 Examples of Orbit Estimates

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.

Action	Phase `I` (s)	Chapter 9 (s)	APT Time (s)¹	Comment
Initial guide star acquisition	360	360	333	Required at start of a new visit.
NUV ACQ/IMAGE with 2 s exposure	180	119 + 120 + 2 × 2 + 56 = 299	236 + 56 = 292	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.
First exposure overhead adjustment	N/A	−119	−160	OSM1 movement may be hidden in guide-star acquisition.
NUV G185M at 1850 Å, `TIME-TAG`, `FP-POS=3`, 1175 s exposure	300 + 1175 = 1475	35 + 175 + 1175 + 114 = 1499	204 + 1175 + 114 = 1493	Generic NUV `TIME-TAG` setup; OSM2 change from `MIRRORA` to G185M (175 s); exposure time; `TIME-TAG` memory readout.
NUV G185M at 1817 Å, `TIME-TAG`, `FP-POS=3`, 1175 s exposure	120 + 60 + 1175 = 1355	35 + 75 + 1175 + 114 = 1399	102 + 1175 + 114 = 1391	Generic NUV `TIME-TAG` setup; change central wavelength (75 s); exposure time; `TIME-TAG` memory readout.
Total science time	2350	2350	2350
Total time used in orbit	3370	3438	3349

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.

Action	Phase `I` (s)	Chapter 9 (s)	APT Time (s)¹	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	119 + 306 + 9 × 10 + 37 = 552	515	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.
First exposure overhead adjustment	N/A	−119	−129	OSM1 movement may be hidden in guide-star acquisition.
NUV `ACQ/IMAGE` with 10 s exposure	180	120 + 2 × 10 + 56 = 196	123 + 56 = 179	No OSM movement, `ACQ/IMAGE` setup, twice the exposure time, and memory readout.
NUV G225M at 2250 Å, `TIME-TAG`, `FP-POS=3`, 1200 s exposure	300 + 1200 = 1500	35 + 106 + 1200 + 114 = 1455	135 + 1200 + 114 = 1449	Generic NUV `TIME-TAG` setup; OSM2 change from `MIRRORA` to G225M (106 s); exposure time; `TIME-TAG` memory readout.
FUV G130M at 1309 Å, `TIME-TAG`, `FP-POS=2`, 300 s exposure²	120 + 60 + 300 = 480	66 + 121 + 300 + 114 = 601	190 + 300 + 114 = 604	Generic FUV `TIME-TAG` setup; OSM1 change from NCM1 to G130M (121 s); exposure time; `TIME-TAG` memory readout.
FUV G130M at 1309 Å, `TIME-TAG`, `FP-POS=4`, 300 s exposure²	120 + 300 = 420	66 + 3 + 300 + 114 = 483	67 + 300 + 114 = 481	Generic FUV `TIME-TAG` setup; increment `FP-POS` (3 s); exposure time; `TIME-TAG` memory readout.
Total science time	1800	1800	1800
Total time used in orbit	3360	3528	3432

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.

Action	Phase `I` (s)	Chapter 9 (s)	APT Time (s)¹	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	80 + 306 + 9 × 15 + 37 = 558	516	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.
First exposure overhead adjustment	N/A	−80	−85	OSM1 movement may be hidden in guide-star acquisition.
FUV `ACQ/PEAKXD`, G130M at 1309 Å, 25 s exposure	420	115 + 3 × 25 + 37 = 227	184	Setup (115 s); 3 times the exposure time; memory readout.
FUV `ACQ/PEAKD`, G130M at 1309 Å, 5 steps, 25 s exposure	420	168 + 5 × 25 + 37 = 330	300 + 37	Setup (168 s); 5 times the exposure time; memory readout.
FUV G140L at 1280 Å, `TIME-TAG`, `FP-POS=ALL`, `SEGMENT=A`, 268 s exposure	300 + 268 = 568	66 + 172 + 268 + 114 = 620	233 + 268 + 114	Generic FUV `TIME-TAG` setup; OSM1 change from G130M to G140L (172 s); exposure time; memory readout (note: `FP-POS=1`).
	120 + 268 = 388	66 + 3 + 268 + 114 = 451	67 + 268 + 114	Generic FUV `TIME-TAG` setup; change to `FP-POS=2` (3 s); exposure time; memory readout.
	120 + 268 = 388	66 + 3 + 268 + 114 = 451	67 + 268 + 114	Same as above, but with `FP-POS=3.`
	120 + 268 = 388	66 + 3 + 268 + 114 = 451	67 + 268 + 114	Same as above, but with `FP-POS=4.`
Total science time	1072	1072	1072
Total time used in orbit	2932	3368	3247

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.

Action	Phase `I` (s)	Chapter 9 (s)	APT Time (s)¹	Comment
Initial guide-star acquisition	360	360	333	Required at start of a new visit.
NUV `ACQ/IMAGE` with 2 s exposure	180	119 + 120 + 2 × 2 + 56 = 299	236 + 56 = 292	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.
First exposure overhead adjustment	N/A	−119	−160	OSM1 movement 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 = 1400	66 + 159 + 8 + 1100 + 114 = 1447	229 + 1100 + 114 = 1443	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 Å, `TIME-TAG`, `AUTO WAVECAL`, WCA, `FP-POS=1`, 12 s exposure	120 + 12 = 132	66 + 10 + 12 + 114 = 202	75 + 12 + 38 = 125	`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 Å, `TIME-TAG`, BOA, `FLASH=NO`, `FP-POS=2`, 1100 s exposure	120 + 1100 = 1220	66 + 3 + 10 + 1100 + 114 = 1293	73 + 1100 + 114 = 1287	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 Å, `TIME-TAG`, `AUTO WAVECAL`, WCA, `FP-POS=2`, 12 s exposure	120 + 12 = 132	66 + 10 + 12 + 114 = 202	75 + 12 + 38 = 125	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.
Total science time in orbit 1	2200	2200	2200
Total time used in orbit 1	3424	3684	3445
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.

Action	Phase `I` (s)	Chapter 9 (s)	APT Time (s)¹	Comment
Initial guide star acquisition	360	360	333	Required at start of a new visit.
NUV `ACQ/IMAGE` with 10 s exposure	180	119 + 120 + 2 × 10 + 56 = 315	252 + 56 = 308	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.
First exposure overhead adjustment	N/A	−119	−160	OSM1 movement may be hidden in guide star acquisition.
FUV G130M at 1327 Å, `TIME-TAG`, `FP-POS=1`, `BUFFER-TIME=1210`, 1310 s exposure²	300 + 1310 = 1610	66 + 121 + 1310 + 38 = 1535	197 + 1310 + 38 = 1545	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`, 1310 s exposure²	120 + 1310 = 1430	66 + 3 + 1310 + 114 = 1493	67 + 1310 + 114 = 1491	Generic FUV `TIME-TAG` set-up; move to `FP-POS=2` (3 s); exposure time; `TIME-TAG` memory readout.
Total science time in orbit 1	2620	2620	2620
Total time used in orbit 1	3580	3584	3517
Guide star re-acquisition	240	240	222	Required at start of next orbit.
First exposure overhead adjustment	N/A	−3	−3	`FP-POS` movement may be hidden in guide star re-acquisition.
`TIME-TAG`, `FP-POS=3`, `BUFFER-TIME=1398`, 1498 s exposure²	120 + 1498 = 1618	66 + 3 + 1498 + 38 = 1605	67 + 1498 + 38 = 1603	As for `FP-POS=2`, but with short `TIME-TAG` memory readout.
`TIME-TAG`, `FP-POS=4`, `BUFFER-TIME=1498`, 1498 s exposure²	120 + 1498 = 1618	66 + 3 + 1498 + 114 = 1681	67 + 1498 + 114 = 1679	As for `FP-POS=2.`
Total science time in orbit²	2996	2996	2996
Total time used in orbit²	3476	3523	3501

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.

Action	Phase `I` (s)	Chapter 9 (s)	APT Time (s)¹	Comment
Initial guide star acquisition	360	360	333	Required at start of a new visit.
NUV `ACQ/IMAGE` with 30 s exposure	180	119 + 71 + 8 + 120 + 2 × 30 + 56 = 434	394 + 56 = 450	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.
First exposure overhead adjustment	N/A	−(119 + 71) = −190	−262	OSM1 and OSM2 movements may be hidden in guide-star acquisition.
FUV G130M at 1055 Å, `TIME-TAG`, `FP-POS=ALL`, `BUFFER-TIME=500`, 500 s exposure	500 + 300 = 800	66 + 154 + 8 + 500 + 114 = 842	228 + 500 + 114 = 842	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.
	500 + 120 = 620	66 + 3 + 500 + 114 = 683	67 + 500 + 114 = 681	Generic FUV `TIME-TAG` setup; move to `FP-POS=2` (3 s); exposure time; `TIME-TAG` memory readout.
	500 + 120 = 620	66 + 3 + 500 + 114 = 683	67 + 500 + 114 = 681	Generic FUV `TIME-TAG` setup; move to `FP-POS=3` (3 s); exposure time; `TIME-TAG` memory readout.
	500 + 120 = 620	66 + 3 + 500 + 114 = 683	67 + 500 + 114 = 681	Generic FUV `TIME-TAG` setup; move to `FP-POS=4` (3 s); exposure time; `TIME-TAG` memory readout.
Total science time	2000	2000	2000
Total time used in orbit	3200	3495	3406

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.

Action	Phase `I` (s)	Chapter 9 (s)	APT Time (s)¹	Comment
Initial guide star acquisition	360	360	333	Required at start of a new visit.
NUV `ACQ/IMAGE` with 30 s exposure	180	119 + 71 + 8 + 120 + 2 × 30 + 56 = 434	394 + 56 = 450	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.
First exposure overhead adjustment	N/A	−(119 + 71) = −190	−262	OSM1 and OSM2 movements may be hidden in guide-star acquisition.
FUV G130M at 1055 Å, `TIME-TAG`, `FP-POS=1`, `BUFFER-TIME=1075`, 1175 s exposure	1175 + 300 = 1475	66 + 154 + 8 + 1175 + 38 = 1441	228 + 1175 + 38 = 1441	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 Å, `TIME-TAG`, `FP-POS=2`, `BUFFER-TIME=1075`, 1175 s exposure	1175 + 120 = 1295	66 + 3 + 1175 + 38 = 1282	67 + 1175 + 38 = 1280	Same as above but with `FP-POS=2.`
Total Science Time in Orbit 1	2350	2350	2350
Total Time Used in Orbit 1	3310	3327	3242
Guide star re-acquisition	240	240	222
First exposure overhead adjustment	N/A	−3	−3	`FP-POS` movement may be hidden in guide star re-acquisition.
FUV G130M at 1055 Å, `TIME-TAG`, `FP-POS=3`, `BUFFER-TIME=1275`, 1375 s exposure	1375 + 120 = 1495	66 + 3 + 1375 + 38 = 1482	67 + 1375 + 38 = 1480	Generic FUV `TIME-TAG` setup; move to `FP-POS=3` (3 s); exposure time; short `TIME-TAG` memory readout.
FUV G130M at 1055 Å, `TIME-TAG`, `FP-POS=4`, `BUFFER-TIME=1275`, 1375 s exposure	1375 + 120 = 1495	66 + 3 + 1375 + 38 = 1482	67 + 1375 + 38 = 1480	Same as above but with `FP-POS=4.`
Total science time	2750	2750	2750
Total time used in orbit	3230	3201	3179

¹ Periodic updates to APT may result in small discrepancies from the overheads shown here.

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COS Instrument Handbook

9.7 Examples of Orbit Estimates

9.7.1 Target Acquisition Using ACQ/IMAGE

9.7.2 Target Acquisition Using ACQ/SEARCH plus ACQ/IMAGE

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

9.7.6 Single CENWAVE Example for Non-CVZ Targets with 4 FP-POS in 1 Orbit

9.7.7 Single CENWAVE Example for Non-CVZ Targets with 4 FP-POS in 2 Orbits

On this Page

9.7.1 Target Acquisition Using `ACQ/IMAGE`

9.7.2 Target Acquisition Using `ACQ/SEARCH` plus `ACQ/IMAGE`

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`

9.7.6 Single CENWAVE Example for Non-CVZ Targets with 4 `FP-POS` in 1 Orbit

9.7.7 Single CENWAVE Example for Non-CVZ Targets with 4 `FP-POS` in 2 Orbits