4.1.1 The Position Mode Visit

A Position mode visit yields measurements of the location of objects in the FGS’s total field of view (FOV), and hence their relative angular positions. The objects are observed sequentially according to the sequence of exposure lines in the proposal. The target list of a typical Position mode visit consists of the science object(s) and reference stars used to define the local reference frame. A subset of the targets, referred to as check stars, should be observed several times during the course of the visit to track any spurious motion of the FGS’s FOV on the plane of the sky (e.g., thermally induced drift or OTA focus changes). The changes in the positions of the check stars are used to model the drift as a function of time so that its contaminating effect can be eliminated from the astrometry.

An FGS astrometry visit begins when the HST computer - the 486 - commands the Star Selector Servos to place the IFOV at the predicted location of the first star specified in the visit (as per Phase II proposal). Control is transferred to the Fine Guidance Electronics (FGE) microprocessor, which commands the FGS to acquire and track the target (Search, CoarseTrack, and FineLock). Later, at a specific spacecraft clock time, after the exposure time + overhead has expired, the 486 resumes control of the FGS, terminates the FineLock tracking of the object and slews the IFOV to the expected location of the next star in the sequence. This process repeats until the FGS has completed all exposures in the visit. The spacecraft’s pointing is held fixed on the sky under the control of the guiding FGSs for the entire visit unless otherwise instructed by the Phase II proposal. The status flags, photometry and instantaneous location of the IFOV is recorded every 25 msec (40 Hz).

4.1.2 The Position Mode Exposure

During a Position mode exposure the object is tracked in FineLock (see Appendix A:Target Acquisition and Tracking). After the target is acquired by the Search and CoarseTrack procedures, FineLock begins with a WalkDown, a series of steps of the IFOV toward the CoarseTrack photocenter. At each step, the IFOV is held fixed for a period defined by FESTIME (Fine Error Signal averaging time) while the PMT data are integrated to compute the Fine Error Signal (FES, the instantaneous value of the S-Curve) on each axis. Once the FES on both axes have exceeded a pre-set threshold, FineLock tracking begins. The star selectors are continuously adjusted after every FESTIME to re-position the IFOV in an attempt to zero out the FES during the next integration period. The objective is to present the Koesters prism a wavefront with zero tilt.

The defining parameters of an exposure are the target magnitude, the filter, the FESTIME and the exposure time. These topics are discussed in the following sections.