In astrophotography, plate-solving is the process of correlating stars in the field of view of a telescope with stars in a reference catalog. The name refers to the photographic glass plates that were in use for survey catalogs during 19th and 20th centuries.
By matching the relative position of light sources in an input capture with known star positions from its catalog, the plate-solver returns the sky coordinates of the field of view. Automated observatories use those results to align their target iteratively, but also to refine their mount model and optimize their slewing time. Information such as optical reversal, size of the field of view and pixel scale are very important to reduce the complexity of the task.
Human observers generally target a well-known star then hop from star to star until they are close enough to the target object. Conversely, plate-solvers match triangle proportions against a huge number of reference combinations, taking the astronomer’s old joke “can you see those three stars shaping a triangle?” to the letter.
(pic: widefield Markarian Chain in Leo, 2016, solved by astrometry.net)