Calibrating FGS1r's Interferometric Response as a Function of Spectral Color. Once 4 Timed to occur with GO observations. Once 3 Depends upon Target's P.A. 0.15 x,y axis scale for Transfer Mode obs. 1 mas This test measures the roll error of the Koesters Prisms in FGS1r by comparing data from observations of a wide binary system at three HST roll angles. The relative scale along x,y is immediately obtained.
Absolute scale is derived using supplemental data from the binary's orbital elements.
The scale and distortions within an FGS continue to slowly evolve in time. This proposal gathers the data needed to maintain the FGS1r OFAD calibration and relative scale, and hence the instrument's astrometric performance. The standard FGS astrometric star cluster M35 is used as the target field. The relative positions of selected stars are measured by FGS1r in Position Mode. Changes in distortion and plate scale are derived.