The HST Fine Guidance Sensor Transfer Function and its Impact on Telescope Alignment and Guidance

Abstract

The Hubble Space Telescope (HST) line of sight was required to be stabilized to better than 0.007 arcseconds rms in order to realize the resolution inherent in the width of the image core. Despite the redistribution of energy resulting from the presence of spherical aberration in the primary mirror, the image core remains small although with significantly less energy than that estimated for the as-built telescope. Therefore the pointing requirement for the current state of the telescope remains unchanged and, of course, would be imposed in any case when wavefront correctors are fitted to the telescope instruments. The line of sight is stabilized by a closed loop pointing control system based on a pointing error signal generated by the HST fine guidance sensors (FGS). An overview of this system as well as a description of the FGS’s can be found in references 1-3. Each of the three FGS work in a ninety degree annular segment (called a pickle) of the HST field of view and can interferometrically lock on to a star anywhere in the pickle. Two such locks are required simultaneously in order to point the telescope to the top level requirement while the third FGS is used for astrometry1-3. The system thus has redundancy and can perform science if all three sensors are working.