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Abstract
In this paper we present the design and fabrication of the reflection varied-line-space concave grating (VLSCG) for the project of CAFE (the Census of warm-hot intergalactic medium, Accretion, and Feedback Explorer), which aims to detect and map the warm-hot circumgalactic medium via OVI emission at 103.2 nm and 103.8 nm, using two off-Rowland-circle spectrograph channels. High diffraction efficiency at LUV is supposed for the VLSCG, and an aperture ratio as small as $F/3.6$ is desired for a compact design. The gratings are fabricated by holographic lithography and ion beam etching techniques. We introduce an additional lens into the normal holographic exposing system to generate the varied-line-space grating patterns. Grooves with triangle profiles are obtained to increase the diffraction efficiency by oblique ion beam bombardment during the etching process. Finally, several VLSCGs with a central line density of 3300 lines/mm have been fabricated successfully. The measured results show that the groove efficiency reaches 51% at 106.4 nm and 31% at 127.4 nm. We imitated the working optical path of the spectrometer and used the ${-}{1}$ order of the VLSCG to measure the image near the exit slit. The results showed that the image of the point source has a vertical extent of 0.68 mm, and the aberrations have been corrected.
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