Design and Fabrication of a UV Fused Silica GRISM Prototype for the Hobby-Eberly Telescope

Abstract

Modern telescope imaging spectrographs utilize high-quality gratings to achieve high resolution and throughput. However, when it comes to UV wavelengths, traditional volume phase holographic gratings (VPHGs) mostly used in telescopes may not provide ideal efficiency due to the absorption caused by the dichromate layer. In this study, we introduce a prototype of a fused silica transmission GRISM (prism-grating-prism) that offers several notable advantages over emerging VPHGs. Firstly, it exhibits a peak diffraction efficiency up to 95% and the efficiency is higher than 80% in the wavelength range 370-470nm. Secondly, it demonstrates high mechanical stability in humid or other harsh environments. Thirdly, it provides significant angular dispersion while maintaining a nearly constant angular dispersion profile. Here, we report a universal and convenient optical design method for the GRISM to meet the requirements of the Hobby-Eberly Telescope (HET). Additionally, we provide the fabrication procedure involving holographic lithography, ion-beam etching, and wafer direct bonding technology.