3.2 Position Mode Precision Astrometry

A Position mode visit consists of sequentially measuring the positions of stars in the FGS FOV while maintaining a fixed HST pointing. This is accomplished by slewing the FGS Instantaneous Field of View (IFOV, see Figure 1.3) from star to star in the reference field. acquiring each in FineLock (fringe tracking) for a short time (2 to 100 sec.). This yields the relative positions of the observed stars to a precision of ~1 milli-arcsecond (mas).

With only three epochs of observations at times of maximum parallax factor (a total of six HST orbits), the FGS can measure an object’s relative parallax and proper motion with an accuracy of about 0.5 mas. Several multi-epoch observing programs have resulted in measurements accurate to ~0.2 mas. Unlike techniques which rely upon photometric centroids, the accuracy of FGS measurements are not degraded when observing variable stars or binary systems. And techniques which must accumulate data over several parallactic epochs would have greater difficulty detecting comparably high frequency reflex motions (if present).