A coarse grating or series of parallel rods placed over the front lens (the objective) of a telescope to create fringes alongside star images. These fringes are short spectra, but if the spacing of the grating is very coarse (about 10 mm) they will effectively be so short that they are subsidiary star images at a known distance from a star. The grating can be rotated, enabling the position angle and separation of double stars to be measured. A fine grating may be used to create spectra, in the same manner as an objective prism.
Subjects: Astronomy and Astrophysics.