The GALAXY measuring engine was designed for the automatic measurement of glass photographic plates taken with Schmidt telescopes. These plates contain the negative images of tens of thousands of stars, with image sizes from 15 μm upwards. To carry out this task GALAXY works in two modes: search phase and measurement phase.

In search phase, GALAXY scans preset areas of the plate with a spot of light from a CRT in a linear raster scan. The transmission through the plate is monitored and each time the light spot crosses a star image, the reduction in transmission is recorded. The coordinates of the star are derived to a nominal accuracy of 16 μm and these coordinates output to paper tape.