Download Document 8087556

FGS Proposals FGS observations can be done in either of 2 modes: POS mode or TRANS mode. • POS mode is used for astrometry by performing a positional measurement of multiple targets in the FGS1R aperture (the “pickle”). Because these targets can cover a large area across the pickle, it is frequently necessary to impose an ORIENT range on the observation to ensure all targets stay in the pickle. In general, the telescope does not move when observing the different targets – at least one prime target and a few or several reference targets. Instead, the FGS instantaneous Field of View (iFoV) moves by adjusting the prisms that generate the interferometry patterns. There are two ways to prevent telescope motion, but only one is necessary. o The telescope pointing is determined by the position of the prime target, and exposures of all other targets have the exposure special requirement SAME POS AS the exposure setting up the pointing. o The telescope pointing is determined by the position of the prime target, and the other targets in the visit all have “-­‐REF” at the ends of their names. This option is not used very often. • TRANS mode uses the iFoV to scan across a single target, usually looking for the interferometry patterns that identify and characterize a multi-­‐object system. For TRANS mode, the PI may impose an ORIENT range on the visit to line up the scan direction relative to an axis of the suspected multi-­‐object system. POS mode and TRANS mode may be used in the same visit. FGS observations must have only one alignment per obset, and usually only one obset per visit. Most FGS visits are only one orbit in duration. Using “-­‐REF” at the end of the target names for the reference targets will automatically set up the single alignment structure. Otherwise, a SEQ NON-­‐INT requirement is needed for all of the exposures in the obset or visit. FGS observations are automatically taken in FN format (engineering telemetry), while other science observations are taken in HN format. FN format is limited to 180 minutes of uninterrupted duration. This is only an issue when a multi-­‐orbit CVZ observation needs to be executed with the FGS, but this has only happened in one program since HST was launched. The most common scheduling problem is the lack of guide stars, since FGS1R is being used as the science aperture and can not be used as a guider, and because most FGS observations are ORIENT restricted. FGS3 is automatically turned on in the guide star search for all FGS observations (this is built into APT and Trans). So, looking for single guide stars is the first step in solving the problem. Other guide star search scenarios and tweaks can be used as well. 25% of all FGS observations are executed with single guide stars, with no ill effect on the data. As with other types of observations, adjusting the primary pointing of the visit (usually with a POS TARG) or changing the ORIENT range can also help with finding guide stars. Aladin in APT and FOV in the PC Toolbox are good tools to use for this.