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How does Hubble stay pointed?

Date: Thu Nov 26 04:26:15 1998
Posted by Gerald Steede
Grade level:
School: No school entered.
City: Russellville State/Province: Arkansas
Country: United States


While browsing the other night I seen a photo from Hubble deepfield exposed for ten days, it was dated 1-15-96, and was of an dark area in Ursa Major. It looked very sharp, considering the legenth of the exposure. I would like to understand how the telescope is held on target that long, are there internal movements to correct and if so how do they overcome the law that states every force has an equal force in the opposite direction?

Pointing telescopes on Earth is pretty hard. The telescope must compensate for the Earth's rotation, and drive motors can be glitchy, making the 'scope jerk instead of track smoothly. Out in space, the problems are different. Since we know that an object will stay the way it's set up unless acted upon by an outside force, the Hubble Space Telescope (HST) tends to stay pointed pretty much in the same place once it's on target.

However, there are a myriad of forces acting on it. For one, tides from the Earth (an effect of gravity) tend to make the telescope try to line up with its long axis pointed toward the Earth. Even sunlight applies a pressure to the telescope which can (over time) make it move. To compensate for all these and other forces, HST has a series of gyroscopes on board which help it stay aimed. There are instruments on board called Fine Guidance Sensors (FGS), which are pointed at relatively bright stars near the target of interest. They lock on to the stars, and if HST starts to drift or goes off-target in some way, the FGSs sense it and tell the gyroscopes to compensate. This is amazingly accurate; here is a quote I found on the Space Telescope Science Institute's website (searching on 'pointing accuracy'):

Telescope stability: Hubble is the most precisely pointed
machine ever devised for astronomy. Its requirements for
pointing stability and pointing accuracy are expressed in
terms of multiple-zero decimals. The telescope  must be able
to maintain lock on a target for 24 hours without deviating
more than 7/1,000ths (0.007) of an arc second (2 millionths
of a degree) which is about the width of a human hair seen at a
distance of a mile. A laser with the stability and precision
of the Hubble, mounted on top of the United States
Capitol could hold a steady beam on a dime suspended over
the World Trade Center in New York, over 200 miles distant.
This level of stability and precision is comparable to sinking
a hole-in-one on a Los Angeles gold course from a tee in
Washington, DC, over 2,000 miles away, in 19 out of 20 attempts.

©2008 Phil Plait. All Rights Reserved.

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