The Telescope

ubble's three Fine Guidance Sensors — its targeting cameras — provide feedback used to maneuver the telescope and perform celestial measurements. Two of the sensors point the telescope at an astronomical target and then hold that target in a scientific instrument's field of view. The third sensor is available to perform scientific observations.

The sensors aim the telescope by locking onto "guide stars" and measure the position of the telescope relative to the object being viewed. Adjustments based on these constant, minute measurements keep Hubble pointed precisely in the right direction.

Hubble is, in principle, free to roll about its optical axis (not end-over-end, but more of a "log roll"). This freedom is limited, however, by the need to keep sunlight shining on the solar arrays, and by a thermal design that assumes that the Sun always heats the same side of the telescope.

Hubble's Steering Committee

Hubble's pointing control system uses the Fine Guidance Sensors to point the telescope at a target with an accuracy of 0.01 arcsec. The sensors detect when the telescope drifts even a miniscule amount and return it to its target. This gives Hubble the ability to remain pointed at that target with no more than 0.007 arcsec of deviation over long periods of time. This level of stability and precision is like being able to hold a laser beam focused on a dime 400 miles away (about the distance from Washington, D.C. to Detroit, MI) for 24 hours.

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Measuring the Universe

Astrometry is the science that determines the precise positions and motions of stars and other celestial objects. These measurements are helping to advance knowledge of stars' distances, masses, and motions. The Fine Guidance Sensors can provide star positions that are about 10 times more precise than those observed from a ground-based telescope. When used as science instruments, the sensors allow Hubble to:

• Search for a "wobble" in the motion of nearby stars, which may indicate that they have planets orbiting around them
• Determine if certain stars are actually double stars
• Measure the angular diameter of stars and other celestial objects
• Refine the positions and the absolute magnitude (brightness) scale for stars
• Help determine the true distance scale for the universe.

The Fine Guidance Sensors, which were built by the Perkin-Elmer Corporation, are refurbished and monitored by Goodrich Corporation Optical and Space Systems in Danbury, CT.

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