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.
• High-angular resolution observing:

- detect duplicity or structure down to 8 mas

-derive visual orbits for binaries as close as 12 mas.

• Absolute masses and luminosities:

-The absolute masses and luminosities of the components of a multiple-star system can be determined by measuring the system’s parallax while deriving visual orbits and the brightnesses of the stars.

• Measurement of the angular diameters of non-point source objects down to about 8 mas.
  
  
• 40Hz 1–2% long-term relative photometry:

-Long-term studies or detection of variable stars.

• 40Hz milli-magnitude relative photometry over orbital timescales.

-Light curves for stellar occultations, flare stars, etc.