Blink comparator
Encyclopedia
A blink comparator was a viewing apparatus used by astronomer
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...

s to find differences between two photographs
Photography
Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...

 of the night sky shot using optical telescopes such as astrograph
Astrograph
An astrograph is a telescope designed for the sole purpose of astrophotography. Astrographs are usually used in wide field surveys of the night sky as well as detection of objects such as asteroids, meteors, and comets.-Design:...

s. 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.

Applications

In photographs taken a few days apart, rapidly moving objects such as asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s and comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

s would stand out, because they would appear to be jumping back and forth between two positions, while all the other fixed stars stood still. Photographs taken at longer intervals could be used to detect stars with large proper motion
Proper motion
The proper motion of a star is its angular change in position over time as seen from the center of mass of the solar system. It is measured in seconds of arc per year, arcsec/yr, where 3600 arcseconds equal one degree. This contrasts with radial velocity, which is the time rate of change in...

, or variable star
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...

s, or to distinguish binary star
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...

s from optical doubles.

The most notable body to be found using this technique was Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

, discovered by Clyde Tombaugh
Clyde Tombaugh
Clyde William Tombaugh was an American astronomer. Although he is best known for discovering the dwarf planet Pluto in 1930, the first object to be discovered in what would later be identified as the Kuiper Belt, Tombaugh also discovered many asteroids; he also called for serious scientific...

 in 1930.

The Projection Blink Comparator (PROBLICOM), invented by amateur astronomer Ben Mayer
Ben Mayer
Ben Mayer was an amateur astronomer perhaps best known for the invention of the Projection Blink Comparator . This low-cost version of a tool used by professional astronomers allowed amateurs to contribute to some phases of serious research.Mayer was the first ever to photograph a nova in its...

, was a low-cost version of the professional tool. It consisted of two slide projectors with a rotating occulting disk that alternately blocked the images from the projectors. This tool allowed amateur astronomers
Amateur astronomy
Amateur astronomy, also called backyard astronomy and stargazing, is a hobby whose participants enjoy watching the night sky , and the plethora of objects found in it, mainly with portable telescopes and binoculars...

 to contribute to some phases of serious research.

In modern times, CCDs
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...

 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.

Modern replacements

However, the blinking technique is less used than before because image differencing
Image differencing
Image differencing is an image processing technique used to determine changes between images. The difference between two images is calculated by finding the difference between each pixel in each image, and generating an image based on the result...

algorithms are often used to detect moving objects more effectively than human eyes are capable of. Or, to measure the precise position of a known object whose direction and rate of motion are known, a "track and stack" technique is used in which multiple images are superimposed by software in such a way 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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