A blink comparator was a viewing apparatus used by astronomers to find differences between two photographs of the night sky. It permitted rapidly switching from viewing one photograph to viewing the other, "blinking" back and forth between the two taken of the same area of the sky at different times. This allowed the user to more easily spot objects in the night sky that changed position. It was also sometimes known as a blink microscope. It was invented by physicist Carl Pulfrich at Carl Zeiss AG, then constituted as Carl-Zeiss-Stiftung, in 1904.
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 other fixed 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 Projection Blink Comparator (PROBLICOM), invented by amateur astronomer Ben Mayer, was a low-cost version of the professional tool. It consisted of two slide projectors with a rotating occluding disk that alternatingly blocked the images from the projectors. This tool allowed amateur astronomers to contribute to some phases of serious research.
In modern times, CCDs have largely replaced photographic plates and astronomical images are stored digitally on computer. 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.
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