HST Two-Gyro Handbook

1.4 Scheduling and Target Visibility

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Observations with either orientation or timing constraints are more difficult to implement in two-gyro mode than in three-gyro mode because of the additional pointing restrictions necessary for attitude control and observatory safety. Roughly half the sky is visible at any point in time in two-gyro mode, compared to >80% of the sky in three-gyro mode.

Whenever possible, observers should try to minimize the number of scheduling requirements placed on their observations. This will result in improved schedulability and long range planning, and more efficient use of observing time. Descriptions and examples of how orientation and timing constraints can affect the scheduling of an object and its orbital visibility period can be found in Chapter 2.

Fixed Targets

Observers planning their Cycle 16 Phase I and Phase II proposals should consult Chapter 2, which describes the scheduling of observations in two-gyro mode. The information contained therein has not changed significantly since Cycle 15. The scheduling information should be used in conjunction with the visibility tools available on the Two-Gyro Science Mode web site and the Visit Planner in the Astronomers Proposal Tools (APT).

Moving Targets

Proposers wanting to observe moving targets should assume that observations in two-gyro mode will work exactly as they did for three-gyro mode, with the caveats that gyro-only tracking and guide star handoffs are not available in two-gyro mode. Moving targets are subject to the same general scheduling and visibility constraints as fixed targets (see Chapter 2). See the Cycle 16 Call for Proposals for additional restrictions on moving target observations.

Scheduling Efficiency

The scheduling efficiency of HST has remained high in two-gyro mode. During the past cycle, the first full cycle of two-gyro observations, the HST Scheduling Group was able to schedule ~71-73 two-gyro prime orbits per week compared to ~80 prime orbits per week in three-gyro mode. The scheduling efficiency of HST in Cycle 16 is expected to be similar.

The need to schedule prime science observations as efficiently as possible in two-gyro mode has resulted in a reduction in the completion rate of snapshot (SNAP) observations. This issue is being addressed through the introduction of the new Survey Proposal category described in the Cycle 16 Call for Proposals.