Thursday, July 24, 2014

Geared to the Stars

The telescope at Ladd Observatory uses a clock drive to compensate for the Earth's rotation and track the stars. Modern telescopes use electric motors but this one was built in 1891 before electric power distribution was common. The Observatory originally had gas lamps for lighting and the telegraph system was powered by "gravity cell" batteries. The telescope's mechanical clock drive is weight driven with the speed regulated by a centrifugal friction governor.

The clock drive showing the governor
A closeup of the clock drive showing the governor in motion.
An optical sensor and precision timer are used to measure the rotation rate.

The governor can be seen at the bottom of this figure just below the polar axis of the mount. A shaft connects the clock to the telescope drive train. Not shown is the stack of 400 pounds of driving weights below the clock. A hand crank is used to wind the clock by raising the weights. It will run for about 3 hours before it needs to be rewound.

Equatorial Mounting for Telescopes from Saegmuller patent.
Equatorial Mounting for Telescopes.
by George N. Saegmuller, 1888
Patent No. 395002

A thorough restoration of the drive was done in 2012 by David Gow. There's a photo album that shows the drive on a test stand before it was re-installed and the video below shows the governor in action. The photos and video are by Jim Hendrickson of the Skyscrapers Amateur Astronomical Society of Rhode Island.

The governor makes one rotation in about 392 milliseconds, a rate of about 153.2 rpm. There are thumb screws on the weights which can be used to adjust the speed. The gear ratio of the drive train is 220000:1 meaning that the governor will make 220,000 revolutions per one rotation of the telescope about the polar axis, which is one sidereal day.

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