7.7.3 Pointing Repeatability

The Hubble Deep Field (HDF) afforded an opportunity to study the repeatability of pointing over many images and acquisitions of the same field. The pointing appears to have been stable to better than 5 mas accuracy while taking many images of the same field without interruption over several orbits. The accuracy for full-up acquisition of the same field after slewing to other targets appeared to be ~10 mas typically, with occasional 20 mas errors seen. However, a few large errors were seen; in about 1 in 100 acquisitions the FGSs locked-up incorrectly resulting in a ~1" error.

Other programs report similar 3 mas pointing accuracy if simple re-acquisitions are done between orbits. Approximately once per day a "full-up" acquisition is usually required (for engineering reasons) where the dominant FGS is fixed in position, but the sub-dominant FGS performs a spiral search for the guidestar and tracks wherever the star is found. On rare occasion these full-up acquisitions produce position errors of several hundred mas, and field rotations of up to ~0.1°, relative to previous images of the same field. This may impact long sequences of exposures requiring half a day or more to execute.