Guide Star Availability in Two 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 rare occasions when we have used single FGS guiding, rather than requiring a pair of guide stars. This will not be possible in Two Gyro mode.

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. Unconstrained observations (no orientation or timing requirements) will have sufficient flexibility that we expect no additional trouble finding guide stars for them while in Two Gyro mode. We will have a little more difficulty with highly constrained observations, since the schedulers will have fewer work-arounds available to them in Two Gyro mode. In Three Gyro mode we could sometimes use orientations that differed by 90 or 180 degrees from what the observer specified, and then find guide stars for these programs. In Two Gyro mode we likely will have only one legal 90 degree alternate orientation, and generally will not be able to use the 180 degree alternate. In some cases this reduced availability of orientation ranges may preclude finding suitable guide stars. 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 (as well as to schedule the observations).