Optical Calibration of the Hubble Space Telescope Fine Guidance Sensors

Important objectives of the Hubble Space Telescope (HST) mission are to place targets within a science instrument aperture to an accuracy of 10 milliarcseconds and to perform astrometric measurements to an accuracy of 2.7 milliarcseconds, rms. In order to achieve these goals, ground-to-orbit changes in magnification and optical distortion must be characterized for each erf the three Fine Guidance Sensors (FGS). An additional step, not presented in this paper, aligns the three FGSs to a common reference frame via the FGS-to-FGS alignment calibration. This paper identifies the sources of distortion, describes the calibration philosophy and pre-launch feasibility studies and finally, discusses on-orbit performance pertaining to the 10 milliarcsecond pointing requirement.