Starting in the autumn of 1914, astronomers at McCormick Observatory undertook an extensive program in photographic astrometry, measuring the distances to stars. The director of the observatory at that time, Samuel Alfred Mitchell, realized that photography was a most efficient way to do this sort of work, for reasons including higher accuracy, permanence of record, and an ease in the labor involved, as compared to visual methods. At that time, only the distances to about 100 stars were known and only a handful of observatories were participating in this work, so Mitchell decided to dedicate the observatory to the determinations of parallaxes.
During the 1920's, astronomers at McCormick Observatory also became active in the observation of proper motions of stars, i.e., the relative angular motions of stars. And then in the 1930's several of the astronomers turned toward photometry, which is the science of measuring the brightnesses of stars. Each of these disciplines -- parallax, proper motion, and photometry -- required the astronomer to take an image of an object on a photographic plate, to develop the plate, and then to measure the location or brightness of the object on that plate. From the time that photography began to be used in astronomy, engineers developed ways to measure these plates with increasing precision. Today, images are taken with electronic cameras and measurements are made on computers. But since 1914, and continuing to today, a variety of machines have been invented to measure the vast collections of astronomical photographs in observatories around the world. The observatory at University of Virginia has been taking photographs for almost one hundred years and so its collection of both photographic plates and measuring engines is quite extensive and encompasses more than 160,000 plates. Some of the plate measuring machines that have been used at McCormick Observatory since its early days are listed below, along with the approximate dates of their arrivals at the observatory.
History of astrometric observations in astronomy
History of photometric observations in astronomy
Gaertner Single Screw Measuring Engine (1926)
Blink Comparator (1920's)
Brashear Blink Comparator (1950)
Gaertner Single Screw Measuring Engine (1960's)
Mann 422F13 Measuring Engine (1967)
Mann Comparator (late 1960's)
Grant 2-Coordinate Measuring Engine (1970)
Boller and Chivens Microphotometer (1970's)
Type 621 Mann Comparator (1981)
Perkin-Elmer PDS Microdensitometer Model 1010GM (1989)
Glossary of Terms
References and Bibliography