3.8 Summary of FGS Performance

In Position mode, the FGS offers capabilities not achievable by other HST instrument or by the current generation of ground-based interferometers. These capabilities include:

• a large field of view (69 arcmin²).
• large dynamic range.
• a per-observation precision of ~ 1 mas for V < 16.8.
• multi-epoch astrometry accurate to ~ 0.2 mas.

Similarly, the FGS Transfer mode offers:

• 7 to 10 mas resolution down to V = 14.5, with wider separations observable to V = 16.8.
• the ability to determine relative separation and position angle of a binary system’s components, and hence the apparent relative orbit of the system.

Additionally, mixed-mode observations - employing both Position mode and Transfer mode - allow the user to combine parallax, proper motion, and relative orbit information to derive the true orbit of a multiple-star system and a determination of stellar masses.

The FGS’s two observing modes make it possible for the instrument to resolve structure in objects too faint for other interferometers and on scales too small for any imaging device, while simultaneously measuring the distance to that object. It is anticipated the FGS will be the sole occupant of this niche until the arrival of the long baseline interferometer in space, such as the Space Interferometry Mission (SIM), expected to launch in 2009.