Virtual Museum of Measuring Engines

"When I came to McCormick Observatory-I remember very well there was only one office with four people in it and one measuring engine machine with people standing in line, so to speak. And there was one mechanical desk computer. And these people produced more parallaxes per year than anybody has done ever since; not because they were poor and had limited facilities, but because they relied on what is most important of all, hard work."
-Peter van de Kamp, 1969

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.

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History of photographic measurements in astronomy

History of astrometric observations in astronomy

History of photometric observations in astronomy

Gaertner Single Screw Measuring Engine (1916)

One of the first measuring machines ever designed for photographic work, this engine allowed precise measurements of both stellar and spectral line positions.

Gaertner Single Screw Measuring Engine (1926)

With similar design as the original Gaertner engine, this machine was bought to supplement the use of the other as observers at McCormick Observatory produced more photographic plates to measure.

Blink Comparator (1920's)

This machine allowed astronomers to examine two plates at one time in order to more easily detect stellar motions and changes in brightness.

Brashear Blink Comparator (1950)

Variable blink settings, better lighting, and easier movement of plates allowed for more precise measurements and easier detections of stellar movement and variability.

Gaertner Single Screw Measuring Engine (1960's)

Specially designed to measure astronomical spectra with a precision down to a small fraction of a micron.

Mann 422F13 Measuring Engine (1967)

This was the first engine owned by McCormick Observatory which could measure in two directions using two separate precision screws, allowing much better accuracy in all measurements.

Mann Comparator (late 1960's)

With a huge projection screen and two-coordinate measuring system, this engine would have been useful for making accurate measurements in two directions.

Grant 2-Coordinate Measuring Engine (1970)

The Grant engine required much less skill to operate, was much faster, and was 10-30 percent more accurate than previous engines.

Boller and Chivens Microphotometer (1970's)

The microphotometer was built specially for accurately measuring the positions of spectral lines.

Type 621 Mann Comparator (1981)

A large measuring stage allowed operators to make very precise measurements on very large photographic plates and then send them straight to a computer for analysis.

Perkin-Elmer PDS Microdensitometer Model 1010GM (1989)

Fully automated and microcomputer-controlled, this machine made faster, extremely accurate, and reproduceable measurements over a larger range in photographic plate density.

Glossary of Terms

References and Bibliography

Last updated on Tuesday, June 22, 1999 by afs5z@virginia.edu