Hubble Space Telescope Primer for Cycle 11

3.3 HST Observing Efficiency

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HST's "observing efficiency" is defined as the fraction of the total time that is devoted to acquiring guide stars, acquiring astronomical targets, and exposing on them. The main factors that limit the observing efficiency are:

• The low spacecraft orbit, resulting in frequent Earth occultation of most targets.
• Interruptions by passages through the South Atlantic Anomaly.
• The number of user constrained visits.
• The relatively slow slew rate.

In recent cycles, the observing efficiency has been around 50%. Of the usable spacecraft time, about 80% is allocated to science observations, with the remainder devoted to calibration and engineering observations (10%), DD programs and repeats of failed observations (also 10%).