|
An instrument for comparing two photographs of a region of sky, usually a pair taken at different times. The purpose of the comparison is to show up any images that are different in position or brightness on the two photographs. This is achieved by means of an optical system that brings images of the two photographs into exact coincidence while they are illuminated alternately. An object whose brightness differs appears to blink on and off as the illumination is changed; an object that is at a different position appears to jump between the two locations.
|