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-Å wavelength range, is described. The arrangement uses a 3600-line/mm grating with a horizontal radius of curvature of 2 μm that generates images with a blur of less than 20 μm over an area 2.6 mm high × 80 mm wide, while over an area 6 mm × 80 mm the blur does not exceed 40 μm. In terms of wavelength intervals these blurs correspond to 28 mÅ and 56 mÅ, and the width of the area covers a spectral range of 110 Å. If the spectrograph is equipped with a 20-μm 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 × 1 (sec of arc)2 and 1 × 2 (sec of arc)2 over a slit height of 2 min of arc and 5 min of arc, respectively, result. © 1979 Optical Society of America.