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 fourFP-POS
settings for each central wavelength setting, must justify this choice in the observing strategy section of their PhaseI
proposal. A modest reduction in observational overheads will not normally be considered sufficient justification for not using all fourFP-POS
settings. An exception to this policy are FP-POSs restricted by COS 2025 rules. - Adjusting the
BUFFER-TIME
(Section 5.4): IfBUFFER-TIME
is greater than the exposure time, one would normally setBUFFER-TIME = EXPTIME
. In orbits with a series of long FUV exposures, one can minimize overheads by settingBUFFER-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 theBUFFER-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 | 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 |
First exposure overhead adjustment | N/A | −119 | −160 | OSM1 movement may be hidden in guide-star acquisition. |
NUV G185M at 1850 Å, | 300 + 1175 = 1475 | 35 + 175 + 1175 + 114 = 1499 | 204 + 1175 + 114 = 1493 | Generic NUV |
NUV G185M at 1817 Å, | 120 + 60 + 1175 = 1355 | 35 + 75 + 1175 + 114 = 1399 | 102 + 1175 + 114 = 1391 | Generic NUV |
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 | 390 | 390 | 393 | Required at start of a new visit. |
NUV | 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 |
First exposure overhead adjustment | N/A | −119 | −129 | OSM1 movement may be hidden in guide-star acquisition. |
NUV | 180 | 120 + 2 × 10 + 56 = | 123 + 56 = 179 | No OSM movement, |
NUV G225M at 2250 Å, | 300 + 1200 = 1500 | 35 + 106 + 1200 + 114 = 1455 | 135 + 1200 + 114 = 1449 | Generic NUV |
FUV G130M at 1309 Å, | 120 + 60 + 300 = 480 | 66 + 121 + 300 + 114 = 601 | 190 + 300 + 114 = 604 | Generic FUV |
FUV G130M at 1309 Å, | 120 + 300 = 420 | 66 + 3 + 300 + 114 = 483 | 67 + 300 + 114 = 481 | Generic FUV |
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 | 390 | 390 | 393 | Required at start of a new visit. |
FUV | 420 | 80 + 306 + 9 × 15 + 37 = 558 | 516 | OSM1 home position is cenwave 1291, so move to 1533 requires 80 s. OSM2 home position is |
First exposure overhead adjustment | N/A | −80 | −85 | OSM1 movement may be hidden in guide-star acquisition. |
FUV | 420 | 115 + 3 × 25 + 37 = 227 | 184 | Setup (115 s); 3 times the exposure time; memory readout. |
FUV | 168 + 5 × 25 + 37 = 330 | 300 + 37 | Setup (168 s); 5 times the exposure time; memory readout. | |
FUV G140L at 1280 Å, | 300 + 268 = 568 | 66 + 172 + 268 + 114 = 620 | 233 + 268 + 114 | Generic FUV |
120 + 268 = 388 | 66 + 3 + 268 + 114 = 451 | 67 + 268 + 114 | Generic FUV | |
120 + 268 = 388 | 66 + 3 + 268 + 114 = 451 | 67 + 268 + 114 | Same as above, but with | |
120 + 268 = 388 | 66 + 3 + 268 + 114 = 451 | 67 + 268 + 114 | Same as above, but with | |
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 | 390 | 390 | 393 | Required at start of a new visit. |
NUV | 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 |
First exposure overhead adjustment | N/A | −119 | −160 | OSM1 movement may be hidden in guide star acquisition. |
FUV G160M at 1600 Å, | 300 + 1100 = 1400 | 66 + 159 + 8 + 1100 + 114 = 1447 | 229 + 1100 + 114 = 1443 | Generic FUV |
FUV G160M at 1600 Å, | 120 + 12 = 132 | 66 + 10 + 12 + 114 = 202 | 75 + 12 + 38 = 125 |
|
FUV G160M at 1600 Å, | 120 + 1100 = 1220 | 66 + 3 + 10 + 1100 + 114 = 1293 | 73 + 1100 + 114 = 1287 | Generic FUV |
FUV G160M at 1600 Å, | 120 + 12 = 132 | 66 + 10 + 12 + 114 = 202 | 75 + 12 + 38 = 125 | Another |
Total science time in orbit 1 | 2200 | 2200 | 2200 | |
Total time used in orbit 1 | 3424 | 3684 | 3445 | |
Note: Two additional exposures, using |
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 | 390 | 390 | 393 | Required at start of a new visit. |
NUV | 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 |
First exposure overhead adjustment | N/A | −119 | −160 | OSM1 movement may be hidden in guide star acquisition. |
FUV G160M at 1577 Å, | 300 + 1310 = 1610 | 66 + 121 + 1310 + 38 = 1535 | 197 + 1310 + 38 = 1545 | Generic FUV |
| 120 + 1310 = 1430 | 66 + 3 + 1310 + 114 = 1493 | 67 + 1310 + 114 = 1491 | Generic FUV |
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 |
|
| 120 + 1498 = 1618 | 66 + 3 + 1498 + 38 = 1605 | 67 + 1498 + 38 = 1603 | As for |
| 120 + 1498 = 1618 | 66 + 3 + 1498 + 114 = 1681 | 67 + 1498 + 114 = 1679 | As for |
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 | 390 | 390 | 393 | Required at start of a new visit. |
NUV | 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 |
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, | 500 + 300 = 800 | 66 + 154 + 8 + 500 + 114 = 842 | 228 + 500 + 114 = 842 | Generic FUV |
500 + 120 = 620 | 66 + 3 + 500 + 114 = 683 | 67 + 500 + 114 = 681 | Generic FUV | |
500 + 120 = 620 | 66 + 3 + 500 + 114 = 683 | 67 + 500 + 114 = 681 | Generic FUV | |
500 + 120 = 620 | 66 + 3 + 500 + 114 = 683 | 67 + 500 + 114 = 681 | Generic FUV | |
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 | 390 | 390 | 393 | Required at start of a new visit. |
NUV | 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 |
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, | 1175 + 300 = 1475 | 66 + 154 + 8 + 1175 + 38 = 1441 | 228 + 1175 + 38 = 1441 | Generic FUV |
FUV G130M at 1055 Å, | 1175 + 120 = 1295 | 66 + 3 + 1175 + 38 = 1282 | 67 + 1175 + 38 = 1280 | Same as above but with |
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 |
|
FUV G130M at 1055 Å, | 1375 + 120 = 1495 | 66 + 3 + 1375 + 38 = 1482 | 67 + 1375 + 38 = 1480 | Generic FUV |
FUV G130M at 1055 Å, | 1375 + 120 = 1495 | 66 + 3 + 1375 + 38 = 1482 | 67 + 1375 + 38 = 1480 | Same as above but with |
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.
-
COS Instrument Handbook
- Acknowledgments
- Chapter 1: An Introduction to COS
-
Chapter 2: Special Considerations for Cycle 29
- • 2.1 COS FUV Detector Lifetime Positions
- • 2.2 Visit Length
- • 2.3 Central Wavelength Settings Added in Cycle 26
- • 2.4 The G285M Grating is Available but Unsupported
- • 2.5 COS Observations Below 1150 Angstroms: Resolution and Wavelength Calibration Issues
- • 2.6 Time-Dependent Sensitivity Changes
- • 2.7 Spectroscopic Use of the Bright Object Aperture
- • 2.8 Non-Optimal Observing Scenarios
- • 2.9 NUV Spectroscopic Acquisitions
- • 2.10 SNAP, TOO, and Unpredictable Source Programs with COS
- • 2.11 Choosing between COS and STIS
- Chapter 3: Description and Performance of the COS Optics
- Chapter 4: Description and Performance of the COS Detectors
-
Chapter 5: Spectroscopy with COS
- • 5.1 The Capabilities of COS
- • 5.2 TIME-TAG vs. ACCUM Mode
- • 5.3 Valid Exposure Times
- • 5.4 Estimating the BUFFER-TIME in TIME-TAG Mode
- • 5.5 Spanning the Gap with Multiple CENWAVE Settings
- • 5.6 FUV Single-Segment Observations
- • 5.7 Internal Wavelength Calibration Exposures
- • 5.8 Fixed-Pattern Noise
- • 5.9 COS Spectroscopy of Extended Sources
- • 5.10 Wavelength Settings and Ranges
- Chapter 6: Imaging with COS
- Chapter 7: Exposure-Time Calculator - ETC
-
Chapter 8: Target Acquisitions
- • 8.1 Introduction
- • 8.2 Target Acquisition Overview
- • 8.3 ACQ SEARCH Acquisition Mode
- • 8.4 ACQ IMAGE Acquisition Mode
- • 8.5 ACQ PEAKXD Acquisition Mode
- • 8.6 ACQ PEAKD Acquisition Mode
- • 8.7 Exposure Times
- • 8.8 Centering Accuracy and Data Quality
- • 8.9 Recommended Parameters for all COS TA Modes
- • 8.10 Special Cases
- Chapter 9: Scheduling Observations
- Chapter 10: Bright-Object Protection
- Chapter 11: Data Products and Data Reduction
-
Chapter 12: The COS Calibration Program
- • 12.1 Introduction
- • 12.2 Ground Testing and Calibration
- • 12.3 SMOV4 Testing and Calibration
- • 12.4 COS Monitoring Programs
- • 12.5 Cycle 17 Calibration Program
- • 12.6 Cycle 18 Calibration Program
- • 12.7 Cycle 19 Calibration Program
- • 12.8 Cycle 20 Calibration Program
- • 12.9 Cycle 21 Calibration Program
- • 12.10 Cycle 22 Calibration Program
- • 12.11 Cycle 23 Calibration Program
- • 12.12 Cycle 24 Calibration Program
- • 12.13 Cycle 25 Calibration Program
- • 12.14 Cycle 26 Calibration Program
- • 12.15 Cycle 27 Calibration Program
- • 12.16 Cycle 28 Calibration Program
- Chapter 13: Spectroscopic Reference Material
- • Glossary