In this situation, the telescope is controlled in roll by gyroscopes, which may allow a slow drift around the axis defined by the direction toward the single guide star. As a member of the ST ScI's OPUS team, I examined the data and provided the following assessment. As a result there may be some motion around a point centered on the guide star position in FGS3.
Preliminary analysis indicates that the failed guide star was binary.
These observations were implemented as observation set U3LY60 in Science Mission Specification 972097C2. The exposure logsheet line that defined them are listed in the following table. The intention of this report is to provide you with information on problems with the execution of your observations. If you have questions about what went wrong, you should get in touch with your contact scientist. If the observations are repeated, proprietary rights to the existing data will be released.