We introduce an optical system design of the calibration system for Giant Magellan Telescope Near-Infrared Spectrograph (GMTNIRS), capable of operating across a wavelength range of 1.08 - 5.4 μm. The calibration system fulfills several critical functions, including flat-fielding, wavelength calibration, dark current measurements, and focusing of the spectrograph. The system consists of flat lamp collimator, illuminator, relay optics, and three targets – the USAF 1951 resolution target, a pinhole, and a dark mirror. The focal ratio of the output beam in image space is designed to be 8, replicating the Giant Magellan Telescope. The flatness of the light from the calibration system is evaluated using a non-sequential ray tracing method, confirming over 99% flatness across the slit area.
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