objective gratingSee objective prism.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
objective grating[äb′jek·tiv ′grād·iŋ]
A series of equally spaced parallel wires placed over the objective lens of a telescope; photographic magnitudes of stars are calculated from the relative brightnesses of images in the resulting diffraction pattern.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.