9.7 Examples of Orbit Estimates

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In this section we present eight 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 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. An exception to this policy are FP-POSs restricted by COS 2025 rules and users of G160M at LP6 (Section 9.5.1).
  • 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 9.7.6. (This is the same strategy outlined in Section 5.4.)

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 NUV ACQ/IMAGE | Single FUV G130M cenwave TIME-TAG  observation | 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 9.6: Overhead Values for NUV FUV TIME-TAG: 4 FP-POS in 1 Orbit.

ActionPhase I (s)Chapter 9 (s)APT Time (s)1Comment

Initial guide star acquisition

390

390

393

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.

9.7.2 NUV ACQ/IMAGE | Single FUV G130M cenwave TIME-TAG  observation | 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 9.7: Overhead Values for FUV TIME-TAG: 4 FP-POS in 2 Orbits.

ActionPhase I (s)Chapter 9 (s)APT Time (s)1Comment

Initial guide star acquisition

390

390

393

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.

9.7.3 NUV ACQ/IMAGE | Single FUV G160M cenwave TIME-TAG observation | 2 FP-POS in 1 Orbit 

This example is a single orbit TIME-TAG observation using the G160M grating and the 1533 CENWAVE with 2 FP-POS. It uses a 30 s ACQ/IMAGE target acquisition with MIRRORB and the BOA. The PSA is used for science exposures. With 2 FP-POS in a single orbit, it will execute at the current LP for G160M (see Figure 9.5.1) which includes additional WAVECAL overheads (Section 5.7.6 and Table 9.5).


Table 9.8: Overhead Values for FUV TIME-TAG: 2 FP-POS in 1 Orbit.

ActionPhase I (s)Chapter 9 (s)APT Time (s)1Comment

Initial guide star acquisition

390

390

393

Required at start of a new visit.

NUV ACQ/IMAGE with 30 s exposure

180

119 + 71 + 8 + 120 + 2 × 30 + 56 = 434

411 + 56 = 467

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 G160M2 at 1533 Å, TIME-TAG, FP-POS=1, BUFFER-TIME=500,
500 s exposure

500 + 300 + 240 = 1040

66 + 159 + 8 + 500 + 114+ 236 + 76 = 1159

418 + 62 + 500 + 114 = 1094

Generic FUV TIME-TAG setup; change from MIRRORB to G160M (159 s); aperture change from BOA to PSA (8 s); exposure time; TIME-TAG memory readout; G160M WAVECAL overheads 1st exposure (236 + 76 s).

FUV G160M2  at 1533 Å, TIME-TAG, FP-POS=4, BUFFER-TIME=500,
500 s exposure

500 + 120 +240 = 860

66 + 3 + 500 + 114 + 236 = 919

236 + 62 + 500 + 114 = 912

Generic FUV TIME-TAG setup; move to FP-POS=4 (3 s); exposure time; G160M WAVECAL overheads (236 s); TIME-TAG memory readout.

Total science time

1000

1000

1000


Total time used in orbit

2470

2712

2604

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

2 Starting in Cycle 30 G160M observations have additional WAVECAL overheads (see Section 9.5.1). 


9.7.4 NUV ACQ/IMAGE | Single FUV G160M cenwave TIME-TAG observation | 4 FP-POS in 1 Orbit


This example is a single orbit TIME-TAG observation using the G160M grating and the 1533 CENWAVE with 4 FP-POS. It uses a 30 s ACQ/IMAGE target acquisition with MIRRORB and the BOA. The PSA is used for the science exposures. With more than 2 FP-POS in a single orbit, it is eligible to execute at LP4 (see Figure 9.5.1), and this example assumes this to be the case.


Table 9.9: Overhead Values for FUV TIME-TAG: 4 FP-POS in 1 Orbit.

ActionPhase I (s)Chapter 9 (s)APT Time (s)1Comment

Initial guide star acquisition

390

390

393

Required at start of a new visit.

NUV ACQ/IMAGE with 30 s exposure

180

119 + 71 + 8 + 120 + 2 × 30 + 56 = 434

388 + 56 = 444

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

−255

OSM1 and OSM2 movements may be hidden in guide-star acquisition.

FUV G160M at 1533 Å, TIME-TAG, FP-POS=1, BUFFER-TIME=380,
490 s exposure

490 + 300 = 790

66 + 159 + 8 + 490 + 57 = 780

242 + 490 + 57 = 789

Generic FUV TIME-TAG setup; change from MIRRORB to G160M (159 s); aperture change from BOA to PSA (8 s); exposure time; short TIME-TAG memory readout (57 s).

FUV G160M at 1533 Å, TIME-TAG, FP-POS=2, BUFFER-TIME=380,
490 s exposure

490 + 120 = 610

66 + 3 + 490 + 57 = 616

67 + 490 + 57 = 614

Same as above but with FP-POS=2.

FUV G160M at 1533 Å, TIME-TAG, FP-POS=3, BUFFER-TIME=380,
490 s exposure		490 + 120 = 61066 + 3 + 490 + 57 = 61667 + 490 + 57 = 614Same as above but with FP-POS=3.
FUV G160M at 1533 Å, TIME-TAG, FP-POS=4, BUFFER-TIME=380,
490 s exposure		490 + 120 = 61066 + 3 + 490 + 57 = 61667 + 490 + 57 = 614Same as above but with FP-POS=4.

Total Science Time 

1960

1960

1960


Total Time Used in Orbit 

3190

3262

3213


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



9.7.5 FUV ACQ/PEAKXD +  ACQ/PEAKD | Single FUV G140L cenwave TIME-TAG observation | 4 FP-POS in 1  Orbit 

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 9.10: Overhead Values for FUV Acquisition and FP-POS=ALL.

ActionPhase I (s)Chapter 9 (s)APT Time (s)1Comment

Initial guide-star acquisition

390

390

393

Required at start of a new visit.

FUV ACQ/SEARCH, G160M at 1577 Å, 3 × 3 pattern, 15 s exposure

420

117 + 306 + 9 × 15 + 37 = 595

516

OSM1 home position is G130M, so move to G160M requires 117 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

−117

−85

OSM1 movement may be hidden in guide-star acquisition.

FUV ACQ/PEAKXD, G160M at 1577 Å, 25 s exposure

420

115 + 3 × 25 + 37 = 227

184

Setup (115 s); 3 times the exposure time; memory readout.

FUV ACQ/PEAKD, G160M at 1577 Å, 5 steps, 25 s exposure

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

3288

3247


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


9.7.6 NUV ACQ/IMAGE | Single FUV G130M cenwave TIME-TAG observation with modified BUFFER-TIME | 4 FP-POS in 2 Orbits

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 9.11: Overhead Values for FUV TIME-TAG with Modified BUFFER-TIME.

ActionPhase I (s)Chapter 9 (s)APT Time (s)1Comment

Initial guide star acquisition

390

390

393

Required at start of a new visit.

NUV ACQ/IMAGE with 10 s exposure

180

119 + 120 + 2 × 10 + 56 = 315

272 + 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

−179

OSM1 movement may be hidden in guide star acquisition.

FUV G130M at 1291 Å
TIME-TAG, FP-POS=1, BUFFER-TIME=1210,
1310 s exposure2

300 + 1310 = 1610

66 + 121 + 1310 + 38 = 1535

204 + 1310 + 38 = 1552

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

3585


Guide star re-acquisition

240

240

392

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 exposure

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 exposure

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

3671


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

9.7.7 NUV ACQ/SEARCH + ACQ/IMAGE | NUV G225M and FUV G160M TIME-TAG observations  | 1 and 2 FP-POS in 1 Orbit

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 9.12: Overhead Values for ACQ/SEARCH plus ACQ/IMAGE.

ActionPhase I (s)Chapter 9 (s)APT Time (s)1Comment

Initial guide star acquisition

390

390

393

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.


9.7.8 NUV  ACQ/IMAGE | Single FUV G160M cenwave TIME-TAG observation with BOA aperture  | 4 FP-POS in 2 Orbits 

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 9.13: Overhead Values for FUV TIME-TAG Using the BOA.

ActionPhase I (s)Chapter 9 (s)APT Time (s)1Comment

Initial guide-star acquisition

390

390

393

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.

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