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Manual Pointing Control

This is the method used for most observations on Astro-1. Here the astronauts use the guide stars identified in the HUT TV camera field of view to keep the target centered in the HUT aperture using the manual pointing controller, just as a ground-based observer would use a button-box or joy stick to guide the telescope during an observation. When guide stars were available, the crew on Astro-1 did an excellent job of maintaining pointing stability. For most observations the rms pointing jitter was typically <2'' in radius, only a factor of $\sim2$ worse than the IPS in optical hold. Several observations, particularly those of bright stars, had no visible guide stars, however, and the only way to guide was a combination of looking for light leaking from the edge of the HUT aperture and guessing pointing corrections that would compensate for the gyro drift rates.