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Comment: Re:Duration - Why Not Longer? (Score 5, Informative) 26 26

It looks like prof_bart was about to answer these questions but got cut off.

> What kind device or systems keep the orientation correct? Gyroscopes?

We measure the pointing with gyroscopes for short time scales and star cameras to correct them on longer time scales, along with several other coarse sensors. We use 2 redundant sets of 3 orthogonal 1-d optical gyroscopes and 2 redundant 1-Mpix cameras behind 200 f/2 Nikon lenses. We expect to measure our pointing to less than 5 arcsec.

> If there are gyroscopes, are they a major percent of payload weight?

The two sets together weigh about 5 lbs, so less than 0.1% of the total weight.

We control the azimuth pointing with a 5-ft diameter reaction wheel, made of light-weight aluminum honeycomb with ~100 lbs of brass weights at the perimeter. The moment of inertia of the wheel is approximately 1% of that of the entire gondola. We keep the reaction wheel spinning at reasonable rates (~ 10 rpm) with a torque motor in the pivot. Elevation pointing is accomplished with a torque motor and a fluid balancing system to offset the cryogen boil-off.

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