The Hubble Space Telescope (HST) is the first big optical space observatory telescope. Being above the atmosphere means it can see the sky more clearly than a telescope on the ground. The atmosphere blurs starlight before it reaches Earth. Named after the astronomer Edwin Hubble, the Hubble Space Telescope can observe 24 hours a day. The main mirror is 94.5 inches (2.4 meters) across. The telescope can take pictures of things so far away it would be nearly impossible to see them from anywhere else.