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FGS Instrument Handbook
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Fine Guidance Sensor Instrument Handbook for Cycle 22 > Chapter 1: Introduction > 1.3 The FGS as a Science Instrument

1.3
The FGS has two modes of operation: Position mode and Transfer mode. In Position mode the FGS locks onto and tracks a star’s interferometric fringes to precisely determine its location in the FGS FOV. By sequentially observing other stars in a similar fashion, the relative angular positions of luminous objects are measured with a per-observation precision of about 1 mas over a magnitude range of 3.0 < V < 16.8. This mode is used for relative astrometry, i.e., for measuring parallax, proper motion, and reflex motion. Multi-epoch programs have achieved accuracies of 0.2 mas or better (1-sigma).
In Transfer mode an object is scanned to obtain its interferogram with sub-mas sampling. Using the fringes of a point source as a reference, the composite fringe pattern of a non-point source is deconvolved to determine the angular separation, position angle, and relative brightness of the components of multiple-star systems or the angular diameters of resolved targets (Mira variables, asteroids, etc.).
As a science instrument, the FGS is a sub-milliarcsecond astrometer and a high angular resolution interferometer. Some of the investigations well suited for the FGS are listed here and discussed in detail in Chapter 3:
Relative astrometry (position, parallax, proper motion, reflex motion) with single-measurement accuracies of about 1 milliarcsecond (mas). Multi-epoch observing programs have determine parallaxes with accuracies of 0.2 mas and better.
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Measurement of the angular diameters of non-point source objects down to about 8 mas.
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Fine Guidance Sensor Instrument Handbook for Cycle 22 > Chapter 1: Introduction > 1.3 The FGS as a Science Instrument

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