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FGS Data Handbook
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Fine Guidance SensorData Handbook > Chapter 1: Instrument Overview > 1.4 FGS Guiding

1.4 FGS Guiding
When an FGS is used for guiding HST, it acquires a guide star in FineLock. The HST pointing control system then corrects telescope’s pointing to bring this guide star (by slewing the telescope) to a pre-determined (x,y) position within the pickle of the guiding FGS. Once HST is properly pointed, the FGS continues to track its target in FineLock under control of both the FGE and the 486 while the pointing control system monitors the (x,y) position of the guide star and averages that data to determine the current pointing of the telescope. The pointing control system uses these data to eliminate translational and rotational drift of the observatory and to repoint the telescope properly. It also compensates for differential velocity aberration, which is the field-dependent change in the apparent positions of stars owing to the telescope’s motion transverse to the direction it is pointed. Because pointing requires two dimensional control, two FGSs are usually used simultaneously to track guide stars. The FGS that controls translational attitude is the dominant guider, and the FGS that controls roll is the roll or subdominant guider.
Sometimes only one FGS actively guides the telescope, maintaining only translational control. Then HST is “free” to roll about the dominant guide star, restrained only by gyroscopic feedback. This situation arises more frequently during astrometry observations than for the other observations because guide star candidates, which can be difficult to find in the first place, are limited to those which appear in two FGSs instead of three.
The FGS astrometry pipeline uses data from the guiding FGSs to provide a high-resolution spatial and temporal HST pointing history over the course of an astrometry observation. These guide star data are useful in both Position Mode and Transfer Mode data reductions.
 

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