Blink comparator
A device for blinking between sky photos to spot changes.
A blink comparator is a device once used by astronomers to compare two photographs of the same patch of night sky. By rapidly switching between the two images—"blinking" back and forth—the viewer could spot objects that had shifted in position or changed in brightness. Also called a blink microscope, it was invented in 1904 by physicist Carl Pulfrich at Carl Zeiss AG (then the Carl-Zeiss-Stiftung).
When photographs taken a few days apart were compared, fast-moving objects like asteroids and comets appeared to jump between two spots, while distant stars stayed fixed. Longer intervals between photos helped reveal stars with large proper motion, variable stars, or binary stars that were not merely optical doubles. The most famous discovery using this technique was Pluto, found by Clyde Tombaugh in 1930.
A low-cost version, the Projection Blink Comparator (PROBLICOM), was invented by amateur astronomer Ben Mayer. It used two slide projectors and a rotating disk that alternately blocked each image, letting amateurs contribute to serious research.
In modern times, charge-coupled devices (CCDs) have replaced photographic plates, storing images digitally. The blinking technique is now done on a computer screen instead of with a physical apparatus. It is less common today because image-differencing algorithms detect moving objects more effectively than the human eye. For known objects with predictable motion, a "track and stack" method superimposes multiple images so the moving object stays fixed, appearing as a dot among star trails—especially useful for very faint objects.
- Inventor
- Carl Pulfrich
- Invention year
- 1904
- Inventor affiliation
- Carl Zeiss AG, then constituted as Carl-Zeiss-Stiftung
- Notable discovery
- Pluto (discovered by Clyde Tombaugh in 1930)
- Low cost version
- Projection Blink Comparator (PROBLICOM), invented by amateur astronomer Ben Mayer
Lore & Background
The blink comparator was invented in 1904 by physicist Carl Pulfrich at Carl Zeiss AG, then constituted as Carl-Zeiss-Stiftung. It was also sometimes known as a blink microscope. In photographs taken a few days apart, rapidly moving objects such as asteroids and comets would stand out, because they would appear to be jumping back and forth between two positions, while all the distant stars remained stationary. Photographs taken at longer intervals could be used to detect stars with large proper motion, or variable stars, or to distinguish binary stars from optical doubles.
The most notable object in the Solar System to be found using this technique is Pluto, discovered by Clyde Tombaugh in 1930. The Projection Blink Comparator (PROBLICOM), invented by amateur astronomer Ben Mayer, is a low-cost version of the professional tool. It consists of two slide projectors with a rotating occluding disk that alternately blocks the images from the projectors. This tool allowed amateur astronomers to contribute to some phases of serious research.
Reader's Guide
In modern times, charge-coupled devices (CCDs) have largely replaced photographic plates, as astronomical images are stored digitally on computers. The blinking technique can easily be performed on a computer screen rather than with a physical blink comparator apparatus as before. The blinking technique is less used today, because image differencing algorithms detect moving objects more effectively than human eyes can. To measure the precise position of a known object whose direction and rate of motion are known, a 'track and stack' software technique is used. Multiple images are superimposed such that the moving object is fixed in place; the moving object then stands out as a dot among the star trails. This is particularly effective in cases where the moving object is very faint and superimposing multiple images of it permits it to be seen better.
Did You Know?
- Pluto was discovered in 1930 by Clyde Tombaugh using the blink comparator technique.
- The Projection Blink Comparator (PROBLICOM) was a low-cost version invented by amateur astronomer Ben Mayer.
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