The Phase II optional parameter OFFSET defines which type of telescope motion will be performed during a pattern. Telescope motions fall into three categories:
RE-ACQ
uisitions of guide stars at each return to the target position (RESTRICTED: Not allowed under Two Gyro Mode). This can be used to chop between a target and an offset background measurement pointing (which would be observed with GYRO pointing control).
The available options for OFFSET
are:
SAM
, the default, will use guide stars whenever possible. If a motion, or the sequence of motions, moves the telescope sufficiently from the original position that guide stars are no longer available, then exposures will be obtained using GYRO
control. If a subsequent motion returns the telescope to a point where the original guide stars become available then the guide stars will be re-acq
uired. This incurs an overhead of ~3 minutes for each re-acq
uisition. (RE-ACQ
uisition of guide stars will not occur under Two Gyro Mode.)
The use of the Field Offset Mirror (FOM) to perform dithering and mapping is an "available" (not "supported") mode of observation. The FOM is not exactly repeatable, so artifacts on the mirror may appear to move around. Observations requiring small and rapid offsets (or making observations in parallel to another instrument) may benefit from this mode. Note that the astigmatism changes with FOM offsets. Note: flat field calibration reference files are only available for the nominal FOM position.
Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |