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Silicon immersion gratings exploit the large index of refraction of silicon (nNIR ∼ 3.4) to compactly disperse light at high resolution for near-infrared spectrographs. These instruments are crucial for many astronomical studies, including measurements of stellar and exoplanet atmospheres. In this work, we characterize silicon immersion gratings fabricated by patterning the silicon with contact photolithography, followed by wet-etching in potassium hydroxide. In this process, we take advantage of the different etch rates between the (100) and (111) planes of silicon to form the blazed grooves. However, the designed blaze tends to differ slightly from the blaze of the grating once etched. While this difference can be corrected when mounting the gratings, it can come at the cost of reducing the instrument’s total throughput. Accurate measures of the fabricated blaze prior to machining the immersion grating entrance face can help reach the optimal instrument efficiency. We present here a procedure for determining the fabricated blaze of silicon immersion gratings through an optical method that measures the intensity of light reflected off each facet of our etched grating. Using a rotation stage, we align diffraction orders near the peak of the blaze function in the Littrow configuration and fit a theoretical blaze function to the resulting intensity profile. We perform this procedure on 17 of our in-house gratings including immersion gratings manufactured for the Giant Magellan Telescope Near-Infrared Spectrograph (GMTNIRS) and report the resulting blaze measurements with a precision of ∼ 0.06 degrees.
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