HST Two-Gyro Handbook Update for Cycle 14 Phase II Proposals

1.3 Scheduling and Target Visibility Issues

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Fixed Targets

The Observation Planning portion of the HST Two-Gyro Handbook (Chapter 6) describes the scheduling of observations in two-gyro mode. The information contained therein has not changed. Observers filling out their Cycle 14 Phase II proposals should consult the Handbook as well as the Astronomers Proposal Tools (APT) Phase II software documentation to assess the schedulability and visibility periods of fixed targets.

Moving Targets

Development of a two-gyro capability for observations of moving targets is ongoing and expected to be available in Cycle 14. The attitude control software required to track moving targets will be tested when HST enters two-gyro mode. Several types of slews bounding those used to track moving targets were performed in the recent two-gyro test. The jitter and pointing control during those slews was similar to the performance expected in three-gyro mode, thus providing preliminary evidence that moving target observations should be feasible. Proposers wanting to observe moving targets should assume that two-gyro observations will work exactly as three-gyro observations, with the caveats that gyro-only tracking and guide star handoffs for moving target observations will not be available in two-gyro mode. Moving targets are subject to the same general scheduling and visibility constraints as fixed targets (see Chapter 6 of the HST Two-Gyro Handbook). See the Cycle 14 Call for Proposals (Section 4.1.3) for additional restrictions on moving target observations.

Scheduling Efficiency

The HST Scheduling Group has constructed a long range two-gyro scheduling plan using the Cycle 13 observation pool as a test case to check the scheduling efficiency expected in two-gyro mode. All of the proposals in that cycle were designed for three-gyro mode, so it was necessary to change some of the constraints to make the test proposal pool consistent with implementation under two-gyro mode. The results of the study are an approximation to the scheduling efficiency expected for a fully qualified two-gyro proposal pool. We expect to be able to schedule ~71-73 two-gyro prime orbits per week compared to ~80 prime orbits per week in three-gyro mode. Thus, the scheduling efficiency of HST should remain high. If HST enters two-gyro mode deliberately near the start of Cycle 14, there will be an initial deficit of available orbits in two-gyro mode compared to three-gyro mode. However, over the expected lifetime of the gyros, a net increase of ~2000 orbits could be achieved if HST enters two-gyro mode deliberately in the summer of 2005.

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