Precision Measuring Machine

The precision measuring machine (PMM) in its clean room at the USNO Flagstaff Station. Built by Anorad, Inc. of New York, the base of this machine is a single block of granite weighing roughly 10 tons. It rests on a concrete pier which is isolated from the floor of the room to prevent vibration of the measuring machine. Twin CCD (charge coupled device) cameras are set up to ``fly'' a constant distance above the photographic plates, stopping every few seconds to take digital ``snapshots'' of a small area of the photographs. The plates rest on a horizonal ``plate stage'' supported by four air bearings which float on a thin cushion of air above the granite base. These air bearings further isolate the photographic plates from vibration and insure that they move very smoothly while being digitized.

The images taken by the CCD cameras are measured and analyzed while the plates are still being digitized, so positions and magnitudes of all the stars have been computed by the time a plate has been scanned, usually in less than an hour for each plate. The measuring/analysis time varies depending on the number of stars on the plate. Photographs taken in the Milky Way have so many stars on them, they appear almost black, and these plates can take several hours to measure.

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