... through round or square apertures from 0.1 to 1 arc second across. A square aperture 4.3 arc seconds on a side was also used to acquire targets. Fine-Guidance Sensors (FGS) is one of the key elements in the system that holds the spacecraft steady; these sensors are locked onto two guide stars to keep Hubble in the same relative position to these stars. FGS may ... guide stars. Once found, they lock onto the guide stars and send information to the flight computer to keep the guide stars within their field of view. The HST has a skeleton to hold the optics, instruments, and spacecraft sys...

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