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9.7 FGS Astrometry

The two astrometry modes of the FGS are POSITION mode and TRANSFER mode. An FGS in POSITION mode acquires an object in FineLock and tracks it for an extended period of time. An FGS in TRANSFER mode acquires an object and scans the IFOV back and forth along a 45° diagonal path in detector space to sample the interference pattern fully enough to construct an S-curve.

POSITION Mode Observing

POSITION mode observing with the FGS can determine the parallax, proper motion, and/or reflex motion of a given object. A typical POSITION mode observing program, regardless of the scientific goal, measures the (x,y) detector space positions of several objects concurrently observable in FGS's total FOV, or pickle. An FGS can observe only one object at a time, so each of the objects are visited and tracked in FineLock in a sequence specified by the proposal. The DF 224 slews the IFOV of the astrometer to the expected location of the first star in the sequence and relinquishes control to the FGE. The FGE then commands the FGS to acquire and track the target via the Search, CoarseTrack, and FIneLock sequences. Later, at a specific spacecraft clock time, the DF 224 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 observed all stars in the observation set belonging to that visit. (The time an FGS tracks a given object in FineLock is usually some 20 seconds longer than the exposure time specified in the proposal because of unused overheads.)

TRANSFER Mode Observing

TRANSFER mode observing with the FGS can resolve the components of multi-star systems and measure the angular dimensions of extended objects. In a TRANSFER mode observation the FGS acquires the object as described above, but instead of attempting to keep the FGS's IFOV at or near interferometric null as in a POSITION observation, the FGS, under control of the DF 224, steps its IFOV is back and forth across the object along a 45 deg diagonal path in detector space, sampling the entire S-curve and its immediate vicinity. Each sweep across the object is referred to as a scan. The number of scans, size of each step, and length of each scan are specified in the proposal. Typically, a TRANSFER mode observation will consist of 20 or more scans, each 1.4" long, with a step size of 1 mas.

Mixed Mode Observing

It is sometimes possible to determine the parallax, proper motion, and reflex motion of a multiple star system resolvable by the FGS in TRANSFER mode. If the FGS can both resolve a binary system and measure its parallax, then the absolute masses of its components would be determined. To accomplish such tasks, the FGS observes the target in TRANSFER mode and other nearby stars in POSITION mode. A mixed mode observing strategy would include a series of POSITION mode observations of the reference stars and a TRANSFER mode observation embedded somewhere in the sequence. Although the post-observation analysis of mixed mode observing data can be challenging, the potential scientific returns have made it an increasingly popular way to use the FGS as an astrometer.


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