3.3 HST Observing Efficiency

HST’s “observing efficiency” is defined as the fraction of total time devoted to acquiring guide stars, acquiring astronomical targets, and obtaining the exposure for a target. The main factors that limit the observing efficiency are

•	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,

•	a relatively slow slew rate.

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%).