3.3 HST Observing Efficiency

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
• Additional scheduling restrictions imposed by two-gyro mode

In recent cycles, the average observing efficiency in three-gyro mode had been around 50%. However, that value has dropped to < 45% in two-gyro mode. Of the usable observing time, about 90% is allocated to science observations, with the remainder devoted to calibration and engineering observations (<~10%), and repeats of failed observations (~2%).