|Space Telescope Science Institute|
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 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 overheads in the Phase I proposal is important; additional time to cover unplanned overhead will not be granted later.
- 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 peakups (method to recenter the star behind the coronagraphic hole) are not needed following a re-acquisition.
- Time must be allowed for each deliberate movement of the telescope. For example, time must be allowed if a target acquisition exposure is being performed on a nearby star and then offset to the target. Also, time for moves must be allowed in a series of exposures in which the target is moved relative to the camera (dithers or chops).
- The 19 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 15 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 burdensome, 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.
- Overhead times for changing cameras are given in Table 10.2. The values in Table 10.2 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 order to refocus the Cameras). In addition, the observer must include 19 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.
- 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. Spacecraft POS-TARG 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 0–1 degree ~2 minutes ACCUM readout (4.5+ NREAD × 0.6) seconds(exptime × 16384 + 10) seconds 157 seconds + 2 × exptime seconds (includes slew) Table 10.2: Overheads (in seconds) for camera change. These include times for telescope slews and for refocus of the NICMOS Cameras.