New stigmatic, coma-free, concave-grating spectrograph*

Tandem use of two classical concave gratings makes it possible to design a double-dispersion spectrograph that is essentially free of astigmatism and coma over a large wavelength range near normal incidence. The first grating is used as a Wadsworth collimator. Light of different wavelengths is dispersed by the Wadsworth collimator so that it illuminates different portions of the second grating. The second grating is placed so that it acts as a Wadsworth camera, in which the light bundle of a certain wavelength illuminating a particular section of the second grating is diffracted along the local normal of that section. In this way, the Wadsworth condition for stigmatic and coma-free imaging is almost fulfilled for all wavelengths. Only two reflecting surfaces are needed. The instrument is a double-dispersion spectrograph with additive dispersion. It does not use an intermediate slit, but has the stray-light-suppression characteristics of such a mount. A comparison of its imaging capabilities with other stigmatic concave-grating spectrographs is presented.