How do we know when we get there?
Guide Stars. Hubble uses stars to navigate, just like sea captains did when they sailed the oceans hundreds of years ago.
Guide Stars are stars that HST uses to find its place. Every star in the
sky has a certain position and Hubble knows where many of them are. There
are almost 20 million stars stored in the Guide Star Catalog in Hubble's
computer.
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Precision instruments, called Fine Guidance Sensors, look through the telescope and search for Guide Stars located around Hubble's new target. When they find them, the lock Hubble into position. Fine Guidance Sensors can hold the telescope on target as l
ong as astronomers need in order to take a picture.
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