Guide Star Availability

We expect that guide stars as faint as 14.0 magnitude can be used in Three Gyro mode. On occasion (<1% of the time) we use guide stars fainter than 14.0, when no other stars are available. When using these faint guide stars, there is a possibility that loss of lock may occur, resulting in termination of science data collection (either for the remainder of the orbit or possibly the remainder of the visit). It is anticipated that loss of lock will occur infrequently, and perhaps only if multiple (pointing) disturbances occur during guiding. There are also occasions when we have used single FGS guiding when a pair of guide stars is not available. There are often situations, especially for targets at high galactic latitude, when we must take advantage of orientation flexibility in order to find a suitable pair of guide stars. It may be necessary to use orientations that differed by 90 or 180 degrees from what the observer specified to find suitable guide stars for these programs. It is particularly important for observers of high latitude targets to be as generous as possible when adding orientation or timing constraints, in order to maintain the flexibility needed to find guide stars and scheduling opportunities in general.