Abstract
An account is given of a fundamentally different approach to the use of variable line-space gratings for grazing incidence spectrometers. In the new approach, the gratings are flat and they are placed to intercept the converging beam from a collecting mirror rather than the diverging beam from a slit. This results in (1) small aberrations over a wide instantaneous range in wavelength, (2) a modest required variation in line spacing across the ruled width, (3) a simultaneous minimization of both the spectral and image height aberrations, and (4) a completely stigmatic zero order image. The slitless arrangement common to these designs is very compact, having no additional length behind the focal plane of the collecting mirror. In-plane and conical fan designs are considered. One combination design, in the form of an echelle spectrometer, is discussed. The designs are considered to represent ideal candidates for moderate to high resolution spectrometers on such missions as the Far Ultraviolet Spectroscopic Explorer (FUSE, now Columbus) and the Advanced X-ray Astrophysics Facility (AXAF).
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