Ultraviolet conical diffraction: a near-stigmatic tandem grating mounting spectrometer.

Conical diffraction grating mounts (gratings used off-plane) offer many new possibilities in the ultraviolet and extreme ultraviolet wavelength domain. Up to now, single conical diffraction gratings, whether coupled or not to grazing incidence gratings, have had limited spectral resolution and disadvantages owing to the simultaneous monochromatic slit image rotation with the grating rotation during the wavelength scan. Here we propose a tandem conical diffraction mount that improves the spectral resolution and nearly eliminates the monochromatic slit image rotation. This results in the improvement of spectral image quality. Through the example of a complete solar telescope-spectrometer instrumentation, the qualities (spectral and angular resolution-2 pm and 1 sec of arc) and limits (lower efficiency) of the mounting are compared to other recent solar instrumentations proposed in the 30-135-nm wavelength range.