Moving Target Observations in Two Gyro Mode

Operations in Three Gyro mode provided for a 2 degree radius cap around the anti-sun direction in which observations at any roll orientation can take place. This had occasionally been useful for observations of planets at opposition. The radius of this cap is reduced to 1 degree in Two Gyro mode.

There are several pointing control options previously available that will not be available in Two Gyro mode. These have been used very rarely in recent cycles. It will not be possible to observe in Gyro-only mode, as the pointing errors in the equivalent mode will be degrees, rather than arc-seconds. It will not be possible to start observations under FGS control and transfer to Gyro-only mode as the guide stars leave the FGS field-of-view. It will not be possible to start observations under FGS control and "hand-off" to a second pair of guide stars as the observations proceed. We expect that these restrictions will affect very few programs.