4.3 Fine Guidance Sensor (FGS1R)
As a scientific instrument, the FGS1R offers accurate relative astrometry and high spatial resolution.
In Position (POS) mode it measures the relative positions of objects in its 69 square arc minute FOV with a per observation precision of about 1 mas. Position mode observing is used to determine the relative parallax, proper motion and reflex motion of single stars and binary systems. Multi-epoch programs have resulted in measurements accurate to about 0.5 mas or less.
In Transfer (TRANS) mode the FGS 5" x 5" instantaneous field of view (IFOV) is scanned across an object to obtain an interferogram with high spatial resolution (conceptually equivalent to an imaging device that samples an object's point spread function with 1 mas pixels).
Transfer mode observing is used to measure the angular size of extended objects or to resolve binary systems and measure the separation, position angle and relative brightness of its components. FGS1R can resolve close binary systems with angular separations of only 8 mas and magnitude differences of less than 1.0. Systems with magnitude differences as large as 4 can be resolved provided the separation of the stars is larger than about 30 mas.
In either mode the FGS yields 40 Hz photometry with a relative precision of about 1 milli-magnitude. Objects over a dynamic range of 3 < V < 17 can be observed.
