Editing Blink comparator

{{Short description|Instrument for comparing small differences between two photographs of the night sky}}
{{More citations needed|date=May 2007}}
[[File:Lowell blink comparator.jpg|thumb|This blink comparator at [[Lowell Observatory]] was used in the discovery of Pluto in 1930.]]
A '''blink comparator''' is a viewing apparatus formerly used by [[astronomer]]s to find differences between two [[Astrophotography|photographs of the night sky]]. It permits rapid switching from viewing one photograph to viewing the other, "blinking" back and forth between the two images taken of the same area of the sky at different times. This allows the user to more easily spot objects in the night sky that have changed position or brightness. It was also sometimes known as a '''blink microscope'''. It was invented in 1904 by physicist [[Carl Pulfrich]] at [[Carl Zeiss AG]], then constituted as Carl-Zeiss-Stiftung.<ref>Zeiss inventions http://www.zeiss.com/corporate/en_de/events/international-year-of-light/optical-technologies.html</ref>
[[File:Animation.gif|thumb|The movement of an asteroid in a blink comparator]]
In photographs taken a few days apart, rapidly moving objects such as [[asteroid]]s and [[comet]]s 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 star]]s, or to distinguish [[binary star]]s from [[Double star|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 projector]]s with a rotating occluding disk that alternately blocks the images from the projectors. This tool allowed [[amateur astronomy|amateur astronomers]] to contribute to some phases of serious research.<ref>{{cite journal | url=https://ui.adsabs.harvard.edu/abs/1986ESASP.250c.229M/abstract | bibcode=1986ESASP.250c.229M | title=Steblicom/Problicom/Viblicom, AN International Search for Comets | last1=Mayer | first1=Ben | journal=Eslab Symposium on the Exploration of Halley's Comet | date=1986 | volume=250 | page=229 }}</ref>

== Modern replacements ==
In modern times, [[charge-coupled device]]s (CCDs) have largely replaced [[photographic plate]]s, 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.<ref>Blink Comparator on PC http://www.instructables.com/id/Blink_Comparator_on_Personal_Computer {{Webarchive|url=https://web.archive.org/web/20200325193143/http://www.instructables.com/id/Blink_Comparator_on_Personal_Computer |date=2020-03-25 }}</ref><ref>{{cite web | url=https://skyserver.sdss.org/dr5/en/proj/user/asteroids/ | title=Finding Asteroids }}</ref>

The blinking technique is less used today, because [[image differencing]] algorithms detect moving objects more effectively than [[Visual system|human eyes]] can. To measure the precise position of a known object whose direction and rate of motion are known, a "track [[Shift-and-add|and stack]]" software technique is used.<ref>http://www.astrometrica.at/</ref> 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 trail]]s. 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.

== See also ==
{{Commons category|Blink comparators}}
* [[Hinman collator]]
* [[Visual comparison]]

== References ==
{{Reflist}}<!-- added under references heading by script-assisted edit -->

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[[Category:Astronomical imaging]]
[[Category:Optical devices]]
[[Category:Observational astronomy]]
[[Category:Products introduced in 1904]]
[[Category:1904 in science]]
[[Category:Carl Zeiss AG]]