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

The current average observing efficiency has returned to the previous three-gyro mode levels. About 90% of the usable observing time is allocated to science observations, with the remainder devoted to calibration and engineering observations (≤10%), and repeats of failed observations (~2%).