blink comparator (blink microscope) Instrument that enables two photographs of the same area of sky, taken at different times, to be rapidly alternated to compare them. Any object that has changed position or brightness during the photograph intervals will show up.
The comparator has two optical paths so that the two photographs can be seen together in one viewing eyepiece. By careful adjustment, the separate images are brought into exact coincidence and then alternately illuminated, changing from one to the other about once a second. All features that are identical appear unchanged, but any object that is on only one of the photographs is seen to blink on and off. An object that has changed its position between the times the photographs were taken appears to jump to and fro and an object that has changed in brightness is seen to pulsate.
The eye is very efficient at detecting the few varying objects among what can be tens of thousands of star images. This simple technique makes possible the discovery of stars of large proper motion, minor planets, comets or variable stars, without the need individually to compare every star image on two photographs.
These days blink comparators are particularly used by hunters of novae and asteroids but past examples of their work include: the discovery of PLUTO by Clyde TOMBAUGH; the catalogue of over 100,000 stars brighter than magnitude 14.5 with detectable proper motion, produced by W.J. LUYTEN; and the majority of the nearly 30,000 known variable stars discovered at various observatories around the world.