HiRES: a high-resolution stigmatic EUV spectroheliometer for studies of the fine scale structure of the solar chromosphere, transition region and corona

We describe the design of a high-resolution stigmatic extreme-ultraviolet (EUV) spectroheliometer, configured for flight on a Black Brant sounding rocket, which consists of a 45-cm Gregory telescope coupled to a spectrometer employing a single toroidal diffraction grating in a normal-incidence Rowland circle mounting and an imaging pulse-counting Multi-Anode Microchannel Array (MAMA) detector system. The toroidal diffraction grating is fabricated by a new technique employing an elastically-deformable sub-master grating which is replicated in a spherical form and then mechanically distorted to produce the desired aspect ratio of the toroidal surface for stigmatic imaging over the selected wavelength range. The spectroheliometer will produce spatially-resolved spectra of the chromosphere, transition-region and corona with an angular resolution of 0.4 arc sec or better a spectral resolution λ/Δλ of about 10⁴ in first order, and a temporal resolution of the order of seconds. Because of the geometric fidelity of the MAMA detector system, the spectroheliometer will be able to determine. Doppler shifts to a resolution of at least 2 angstrom at wavelengths near 600 angstrom (approx.1.0 km s^-1), depending on the level of the accumulated signal. The unique characteristics of the spectroheliometer will be used in combination with plasma-diagnostic techniques to study the temperature, density and velocity structures of specific features in the solar outer atmosphere.