The Hubble Space Telescope uses mirrors to collect science data. Hubble's main mirror is about eight feet in diameter. Light enters the telescope and strikes the main mirror, which reflects the light forward to a smaller (12-inch) secondary mirror. This small mirror reflects the light again, sending it through a two-foot hole in the center of the large mirror to the science instruments. These powerful instruments analyze the incoming light stream and translate it into information and images for scientists back on Earth.

After Hubble's launch in 1990, NASA discovered a flaw in the large, main mirror. The flaw was tiny — about 1/50th the thickness of a piece of paper — but significant enough to distort Hubble's vision. During the First Servicing Mission, astronauts added corrective optics to compensate for the flaw. The optics acted like eyeglasses to correct Hubble's vision.
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