Space Telescope Science Institute   10.1 Overview  10.3 Orbit Use Determination

10.2 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 13 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.

Overheads can be subdivided into two main categories:


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 < 1-2 arcmin, same guide stars)
(x + 10) seconds
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)
19 seconds
Filter change
15 seconds
(shortest 10 seconds, longest 15 seconds)
Exposure overheads:
ACCUM readout
MULTIACCUM
BRIGHTOBJ

(4.5+ NREAD * 0.6) seconds
4 seconds
(exptime x 16384 + 10) seconds
Target acquisition (for coronagraphy)
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 Cameras.

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

 10.1 Overview  10.3 Orbit Use Determination
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