WFPC2 Instrument Handbook for Cycle 10

SAA and Scheduling System Changes

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Changes in the WFPC2 observation scheduling system were introduced early in 1999 primarily in order to increase the scheduling efficiency of HST observations starting with Cycle 8.

First, the South Atlantic Anomaly (SAA) contours used to limit WFPC2 observations have been modified slightly. The SAA is a region where irregularities in the Earth's magnetic field cause very high cosmic ray rates. WFPC2 imaging is generally not scheduled near the SAA, so as to avoid excessive cosmic ray hits which degrade images by obliterating data in numerous pixels. These adverse effects are usually minimized by operating each instrument only when HST is outside a designated "SAA avoidance contour." (WFPC2 observations of time-critical phenomena can be taken inside the SAA avoidance contour, if necessary.) Biretta and Baggett (1998) have analyzed available WFPC2 data, together with data from Air Force satellites flying in similar orbits, and have redefined the WFPC2 SAA avoidance contour. This change results in a 3% increase in designated SAA-free orbits, which allows better scheduling efficiency, and is expected to impact negatively less than 0.1% of WFPC2 science observations. The new contour is given by the M26 area in Figure 4.11.

Figure 4.11: SAA Avoidance Contours.
 

Second, WFPC2 visits are now limited to a maximum length of 5 orbits. Very long visits (up to the previous maximum of 8 orbits) have very limited opportunities for scheduling, reduce the efficiency of telescope use, and can cause long delays in execution, with long GO wait times. The transition to shorter visits improves the scheduling opportunities for large proposals. One possible drawback is the lower pointing repeatability across visits; this is likely to be significant only for programs with special dithering requirements.

A third change for Cycle 8 is that an extra minute of overhead has been added to each orbit in RPS2, which allows splitting an orbit in the Phase 2 proposal into two separate spacecraft alignments. This one-minute "efficiency adjustment" allows much more efficient scheduling of HST orbits impacted by the SAA.