Stigmatic performance of an EUV spectrograph with a single toroidal grating

The stigmatic performance of conventionally ruled toroidal gratings used near normal incidence is explored in detail. A single-grating spectrograph, particularly suitable for observing the dynamics of the solar corona in the 520-630-A wavelength range, is described. The arrangement uses a 3600-line/mm grating with a horizontal radius of curvature of 2 m that generates images with a blur of less than 20 microm over an area 2.6 mm high x 80 mm wide, while over an area 6 mm x 80 mm the blur does not exceed 40 microm. In terms of wavelength intervals these blurs correspond to 28 mA and 56 mA, and the width of the area covers a spectral range of 110 A. If the spectrograph is equipped with a 20-microm wide entrance slit, and if this slit is placed in the focal plane of a telescope with 4-m focal length, spatial resolution elements of 1 x 1 (sec of arc)(2) and 1 x 2 (sec of arc)(2) over a slit height of 2 min of arc and 5 min of arc, respectively, result.