Abstract
During the second servicing mission of the Hubble Space Telescope (HST), a newly refurbished fine guidance sensor (FGS-1R) was installed into the telescope's Radial Bay No. 1. The successful replacement of the existing FGS-1, whose degraded Star Selector Servo bearings were affecting the scheduling and acquisition of science data, was critical to continued success of the observatory. In addition to solving the bearing problem, the refurbished FGS-1R also provided an innovative approach to minimize the effects of spherical aberration on the interferometric signal generate by the FGS, hereafter referred to as an s-curve. Rather than try to remove the aberration from the wavefront over a very large field of view, FGS-1R was given the capability to realign the beam to the Koester's prism. A symmetric error, such as spherical aberration, which is divided perfectly at the beam center and folded onto itself, will have the effects of the aberration canceled. To this end, FGS-1R's FOld Flat No. 3 was retrofit with an Actuated Mechanisms Assembly (AMA), which allows on orbit correction of the beam alignment. This paper gives an overview of the theory of operation of the FGS, and characterizes the effects of spherical aberration on s-curve modulation. It discusses the theory of operation of the AMA, and how it is used to optimize the optical alignment. It describes the analysis tools and methods used to transform on orbit data into required adjustments of the AMA. Finally, it presents the result of the on orbit optimization of s-curve modulation, and briefly discusses some of the challenges faced in refurbishing the next FGS.
