From: STORRS@avion.stsci.edu
Date: 12-JUN-1997 15:02:13.79
To: ssd_nicmos@stsci.edu
CC:
Subj: NICMOS and single guidestars
The question of how long one can use single guidestars in NICMOS observations has arisen and in the absence of guidance I propose the following:
I figure if the linear motion due to gyro drift around the star as X = D sin(at). I let X equal the linear motion, D the distance from the guidestar to the aperture, a the angular gyro drift rate, and t the time since the last FHST update. If D = 20 arcmin (worst case) = 1200 arcsec and a = 0.001 arcsec/sec, for one visibility period t = 50 min = 3000 sec we get X = 0.0175 arcsec or less than half a pixel in camera 1. For two orbits t = 146 min = 8760 sec, X = 0.051 arcsec or a little over a pixel in camera 1.
The only time this drift will get noticable in one orbit is when doing coronography-- to be conservative one should only do one orbit on single
GS when aperture NIC2-CORON is used. You can probably get away with two orbits in NIC1, three in NIC2, and six in NIC3. Let me know if I've missed something. Maybe we can make some policy here.
Cheers,
Alex