Hartmann Screen

an attachment for investigating the quality of a telescope objective. An opaque circular screen with apertures, it is mounted in front of the objective of a telescope directed toward a star. During the investigations photographs are taken on plates set up closer to and farther from the objective than its focal plane; the rays that pass through the apertures in the Hartmann screen leave traces on these photographs. Knowing the distance between the photographic plates when taking the pictures and measuring the location of the traces, one may reconstruct the ray paths and determine the best focusing plane, the radius of the least circle of aberration, and the zonal and local errors of the objective. The screen is named after the German optician J. Hartmann (1904).