Passage: 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.

Question: What are binary stars often mistakened by?
optical doubles