|Space Telescope Science Institute|
D.5 Types of MotionsThe Phase II optional parameter OFFSET defines which type of telescope motion will be performed during a pattern. Telescope motions fall into three categories:
• Small angle maneuvers (SAMs) 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.
• SAMs without FGS guiding (i.e. GYRO pointing control). These are necessary for larger motions (> 2 arcmin). The telescope will drift at a rate of 1 to 2 milliarcsec per second of time (elapsed time since dropping to gyro—not exposure time).
• SAMs with RE-ACQuisitions of guide stars at each return to the target position. 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-acquired. This incurs an overhead of ~3 minutes for each re-acquisition.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.