Hubble Space Telescope fine-guidance-sensor transfer function and its impact on telescope alignment and guidance

We describe the use of Hubble Space Telescope Fine Guidance Sensor transfer scans to characterize telescope alignment. To accomplish this we developed a software system to extract the aberration content from the observed transfer scans. The transfer scans show large aberration levels that do not originate in the telescope. The appearance of significant coma in the transfer scans has been identified as resulting from the shearing of spherical aberration caused by a beam misalignment within the Fine Guidance Sensors themselves.