NICMOS Instrument Handbook for Cycle 11

NICMOS Exposure Overheads

---

The overheads on exposures are summarized in Table 10.1. All numbers are approximate and, for the observatory level overheads, rounded up to the nearest half minute. These overhead times are to be used (in conjunction with the actual exposure time and the Cycle 11 Phase I Proposal Instructions) to estimate the total time in orbits for NICMOS proposal time requests. After an HST proposal is accepted, the observer will be asked to submit a Phase II proposal to allow scheduling of the approved observations. At that time the observer will be presented with actual, up to date overheads by the scheduling software. Allowing sufficient time for overhead in the Phase I proposal is important; additional time to cover unplanned overhead will not be granted later.

Overheads can be subdivided into two main categories:

• Generic (Observatory Level) Overheads:
  • The first time an object is acquired, the overhead time for the guide star acquisition must be included.
  • In subsequent contiguous orbits the overhead for the guide star re-acquisition must be included; if the observations are occurring in the continuous viewing zone (CVZ, see the CP/Phase I Proposal Instructions), no guide star re-acquisitions are required.
  • The re-acquisitions can be assumed to be accurate to < 10 milli-arcsecs; thus additional target acquisitions or pick-ups are not needed following a re-acquisition. However, if doing coronagraphy on a single guide star, a target re-acquisition is strongly encouraged (see Chapter 5).
  • Time must be allowed for each deliberate movement of the telescope; e.g., if a target acquisition exposure is being performed on a nearby star and then offsetting to the target or if a series of exposures in which the target is moved relative to the camera (dithers or chops) are being performed, time for the moves must be allowed
• NICMOS Specific Overheads:
  • The 18 second set-up time at the beginning of each orbit or at each different telescope pointing is inclusive of the filter selection.
  • For each pattern position, a 16 second overhead is scheduled for filter wheel motion, even if no filter change is executed.
  • Overheads are operating-mode dependent. The overhead for the BRIGHTOBJ mode is particularly onerous, since this mode resets and reads each pixel, one pixel at a time.
  • The target acquisition overhead of (157 + 2*exptime) seconds for coronagraphy needs to be accounted for the first time an object is acquired under the coronagraphic spot. Here, `exptime' is the exposure time needed to observe the target outside the coronagraphic spot for centroiding. No target re-acquisition is required after a filter change or from one orbit to the next, if the same two guide stars are re-acquired after occultation (see Chapter 5 if using a single guide star).
  • Overhead times for changing cameras are given in Table 10.2. The values in the Table include the time to perform the Small Angle Maneuver (to change from one camera to the other) and the time for Instrument reconfiguration (to change PAM position). In addition, the observer must include 18 seconds for set-up which includes filter selection.
  • The amount of time required to chop depends on the chop throw, and whether an on-target guide star re-acquisition is desired. The telescope can maintain lock on the guide stars if the chop throw is smaller than 1-2 arcminutes. If it is larger, then the observer can choose to maintain pointing through the gyros (DROP-TO-GYRO) or re-acquire the guide stars (3 minute overhead per re-acquisition-note that this is not the 5.3-minute orbit re-acquisition) every time the telescope goes back to the target; with the first option the pointing uncertainty is about 1 milliarcsec/second due to telescope drift. The drop-to-gyro option can be adopted for background pointings, where telescope drift is not a concern.
  • In most cases, the data management overhead of 3 minutes will be hidden inside the orbit occultation time or placed in parallel with exposures. The latter, however, does not always happen as the software may not find a good location to place the data management (buffer dump) in parallel. Proposers whose observations require them to obtain multiple sets of 94 read-outs are advised to include the data management overhead for at least half of the times in their orbit computation.

Table 10.1: NICMOS Overheads 

Action	Overhead
Generic (Observatory Level)
Guide star acquisition	Initial acquisition 6 minutes re-acquisitions on subsequent orbits = 5 minutes per orbit
Spacecraft POSTARG moves	for offsets less than 1 arcminute and more than 10 arcsecs = 1 minute, for offsets between 10 arcsecs and 1 arcsec = 0.5 minute; for offsets less than 1 arcsec in size = 10 seconds
Slew of x arcsecs to new target within an orbit (slew < 2 arcmin, same guide stars)	(x + 20) seconds
Type 2 Slew to new target within an orbit (slew > 2 arcmin, new guide stars)	<1 degree slew: 2.5 min. + 7.1 min. guide star acq. 1 degree < slew <10 degrees: 5.0 min. + 7.1 min. guide star acq. 10 degrees < slew <100 degrees: 13 min. + 7.1 min. guide star acq.
Spacecraft Roll	0-1 degrees ~2 minutes 1-10 degrees ~6 minutes 10-22 degrees ~8 minutes 22-30 degrees ~ 9 minutes
NICMOS Specific Overheads
Set-up at beginning of each orbit or at each different telescope pointing - always required. (other than dither/chop maneuvering)	18 seconds
Filter change	16 seconds (shortest 10 seconds, longest 16 seconds)
Exposure overheads: ACCUM readout MULTIACCUM BRIGHTOBJ	(7 + NREAD * 0.6) seconds 4 seconds (exptime x 16384 + 10) seconds
Target acquisition (for coronography)	157 seconds + 2 * exptime seconds (includes slew)
Dithering/Chopping of x arcsecs (< 1-2 arcmin)	(x + 10) seconds
Chopping of x arcsec (> 2 arcmin, using drop-to-gyro)	(x + 31) seconds
Chopping of x arcsec (> 2 arcmin, with guide star re-acquisition)	(x + 31) seconds + 3 minutes for each guide star re-acq
Data management (for every 94 read-outs within an orbit)	3 minutes

 

Table 10.2: Overheads (in seconds) for camera change. These include times for telescope slews and for refocus of the NICMOS PAM.

Going From:	Going To:
	Coronagraph	NIC 1	Intermediate	NIC 2	NIC 3
Coronagraph	0	40	80	125	620
NIC 1	55	0	40	85	580
Intermediate	95	40	0	45	540
NIC 2	150	95	55	0	285
NIC 3	645	590	550	285	0