Cryogenic refractive index of S-FTM16 and S-NPH2 glasses for the Magellan infrared multi-object spectrograph (MIRMOS)

Abstract

The Magellan Infrared Multi-Object Spectrograph (MIRMOS) is an astronomical instrument to be delivered to the Magellan Telescopes at the Las Campanas Observatory in Chile (expected delivery in 2027). The instrument has been designed to address frontier scientific questions in extragalactic, cosmological, and exoplanetary science. This spectrograph is optimized for faint object spectroscopy observing the full near infrared (NIR) and short-wave infrared (SWIR) spectrum from 0.89 to 2.4 microns on the 6.5 m Magellan Telescope. This workhorse instrument is designed as a wide-field, ground-based complement to the space-based James Webb Space Telescope (JWST) and will have unprecedented, simultaneous wavelength coverage that will make it roughly five times faster than existing instruments in performing galactic surveys. The development of MIRMOS is being carried out by the Carnegie Observatories. Successful implementation of Carnegie’s MIRMOS optical design requires accurate knowledge of absolute refractive indices of two glasses used in its design, S-FTM16 and S-NPH2, at the instrument’s operating temperature of 120 K. This paper is devoted to the characterization of cryogenic refractive index of these two glasses using the world-class, Cryogenic High Accuracy Refraction Measuring System (CHARMS) facility at NASA's Goddard Space Flight Center (GSFC) over the entire spectral range of the instrument. We report on those characterizations in the wavelength range from 0.54 to 2.44 μm and temperature range from 50 to 310 K.