Comparison of grating designs for the Lyman Far-Ultraviolet Spectroscopic Explorer spectrograph.

Various grating designs have been proposed by several investigators for possible use with the Lyman/Far-Ultraviolet Spectroscopic Explorer mission. The image quality, the feasibility, and the efficiency of five designs are compared, each using a distinct type of grating: (1) a grating ruled on a deformed ellipsoidal or toroidal blank, (2) an ellipsoidal grating recorded holographically with two auxiliary spherical mirrors, (3) a spherical holographic grating recorded with two auxiliary spherical holographic gratings, (4) a spherical ruled grating with variable spacing and straight grooves, and (5) a spherical ruled grating with a groove pattern that is determined theoretically (hybrid grating). From a purely theoretical viewpoint, grating (5) provides the finest images, followed by gratings, (3), (1), (4), and (2). In view of the current technological limitations, the order of practical importance is gratings (4), (1), (2), (3), and (5).