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NICMOS Exposure Overheads

Estimates of the overheads on exposures are summarized in Table 9.1. All numbers given are preliminary, approximate, rounded where appropriate to the nearest half minute and do not attempt to differentiate in detail the overheads for different NICMOS modes and configurations. They are subject to change prior to the actual scheduling of Cycle 7-NICMOS proposals (i.e., prior to Phase II). They represent our current best predictions. These overhead times are to be used (in conjunction with the actual exposure time and the Cycle 7-NICMOS Phase I Proposal Instructions) to estimate the total time in orbits for your Cycle 7-NICMOS proposal time request. After your HST proposal is accepted, you will be asked to submit a Phase II proposal to allow scheduling of your approved observations. At that time you will be presented with actual, up to date overheads by the scheduling software. Allowing sufficient time for overhead in your Phase I proposal is important; additional time to cover unplanned overhead will not be granted later.

We note the following important points.

Generic (Observatory Level) Overheads:

NICMOS Specific Overheads:

The 12 second set-up time at the beginning of each orbit or at each different telescope pointing is inclusive of the filter selection. There is no additional set-up time for new telescope pointings due to dithering or chopping with the same instrument configuration.
Overheads are operating-mode dependent. In particular, 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 98 seconds for coronography needs to be accounted for the first time an object is acquired under the coronographic spot. No re-acquisition is required for observing the same target in different filters, or from one orbit to the next.
Overhead times for changing cameras are equal to the slew time for the physical distance between the centers of the cameras in arcsecs + 10 seconds. In addition, the observer must include 12 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 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 6-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. However, if a total of (summed over the three cameras) 100 or more read-outs occur within the visibility period of one orbit, the observer must include 3 minutes overhead every 100 read-outs.

Table 9.1: NICMOS Overheads
Action	Overhead
Generic (Observatory Level)
Guide star acquisition	Initial acquisition 7.1 minutes re-acquisitions on subsequent orbits = 5.3 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 = 20 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. + 9 min. guide star acq. 1 degree < slew <10 degrees: 5.0 min. + 9 min. guide star acq. 10 degrees < slew <100 degrees: 13 min. + 9 min. guide star acq.
Provisional 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	18 seconds
Exposure overheads: ACCUM MULTIACCUM BRIGHTOBJ	(28 + NREAD x 0.705) seconds in FAST readout 27 seconds (exptime x 16384 + 10) seconds
Target acquisition (for coronography)	110 + 2 * exptime seconds (includes slew)
Change of camera: from 2 to 3 or vice versa from 1 to 2 or vice versa from 1 to 3 or vice versa	64 seconds + 216 seconds for set-up 38 seconds + 52 seconds for set-up 59 seconds + 221 seconds for set-up
Dithering/Chopping of x arcsecs (< 2 arcmin)	(x + 20) 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 + 6.4 minutes for each guide star re-acq
Data management (for every 100 read-outs within an orbit)	3 minutes

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stevens@stsci.edu

NICMOS Exposure Overheads

Table 9.1: NICMOS Overheads

Action

Overhead