The Phase II optional parameter OFFSET defines which type of telescope motion will be performed during a pattern. Telescope motions fall into three categories:
SAM
s) where FGS Fine Lock guiding is maintained. Such SAMs are typically limited to < 2 arcmin from the point of the initial guide star acquisition. This is the practical limit of the radial extent of the pattern. Often it will be smaller due to guide star availability.
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-ACQuired. This incurs an overhead of ~3 minutes for each RE-ACQuisition.
Patterns may use the FOM instead of moving the whole telescope, however this 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.
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