On-Sky Tests of an A/R Coated Silicon Grism on board NICS@TNG

TL;DR

This paper reports on the design, fabrication, and on-sky testing of a silicon grism with anti-reflection coating for near-infrared astronomy, demonstrating its performance on the TNG telescope.

Contribution

It presents the first successful on-sky test of an A/R coated silicon grism fabricated with complex litho-masking and bonding techniques.

Findings

Successful fabrication of silicon grism with 363 grooves/mm

Effective application of A/R coating without performance loss

On-sky testing confirms operational performance

Abstract

We present the results of our project for the design and construction and on-sky test of silicon grisms. The fabrication of such devices is a complex and critical process involving litho-masking, anisotropic etching and direct bonding techniques. After the successful fabrication of the silicon grating, we have optimized the bonding of the grating onto the hypotenuse of a silicon prism to get the final prototype. After some critical phases during the experimentation a silicon grism with 363 grooves/mm and a blaze angle of 14 degrees has been eventually fabricated. The application of an A/R coating on both the surfaces has been the last step: this procedure is critical because of the groove geometry of the diffraction grating, whose performace might be compromised by the coating. Then, the grism was inserted in the filter wheel of the Near Infrared camera NICS, at the focal plane of the National Galileo Telescope (TNG), the 3.5 m Italian facility in the Canary Islands (E). The result of the on-sky tests are given in detail.