The photometric properties of the HST astrometer Fine Guidance Sensor

This paper presents the results of the photometric calibration of the F538W filter in astrometer Fine Guidance Sensor on the Hubble Space Telescope. Dozens of observations of the 99.m58 Fine Guidance Sensor TRANSfer Mode reference star Upgren 69 (in the cluster NGC 188) have been utilized to verify the consistency and demonstrate the temporal stability of the photomultipliers. The measurements which provided the material for a transformation from the FGS instrumental system to the Johnson V magnitude consisted of the extensive POSition Mode observations performed during the Optical Field Angle Distortion calibration. 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.m05 is available with an accuracy (i.e., systematic errors) of 0.m05 over two years.