Amazing Space logoServicing Mission 4 > SM4 Glossary

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A light-sensitive picture element on a charge-coupled device (CCD), or some other kind of digital imaging device. A pixel is a tiny cell that, when placed together with other pixels, produces a complete image.

Primary Mirror

A large mirror in a reflecting telescope that captures light from celestial objects and focuses it toward a smaller secondary mirror. The primary mirror in the Hubble Space Telescope measures 2.4 meters (94.5 inches) in diameter.

Rate Sensor Units (RSUs)

Boxes that house Hubble’s gyroscopes. Each rate sensor unit contains two gyroscopes. Astronauts remove the rate sensor units when they replace gyroscopes, so gyroscopes are always replaced two at a time.