THE PHOTOMETRIC PROPERTIES OF THE HST ASTROMETER FINE GUIDANCE SENSOR

This paper presents the results of the photometric calibration of the F583W filter in the astrometer Fine Guidance Sensor on the Hubble Spac e Telescope. Dozens of observations of the 9.58 mag Fine Guidance Sens or (FGS) transfer mode reference star Upgren 69 (in the cluster NGC 18 8) have been utilized to verify the consistency and demonstrate the te mporal stability of the photomultipliers. The measurements which provi ded the material for a transformation from the FGS instrumental system to the Johnson V magnitude consisted of the extensive position mode o bservations performed during the Optical Field Angle Distortion calibr ation. A total of 588 measurements of 92 stars in the galactic cluster M35 were performed. Johnson V band photometry with a precision (i.e., random errors) of +/- 0.05 mag is available with an accuracy (i.e., s ystematic errors) of 0.05 mag over 2 yr.