Cryogenic refractive indices for S-FPL55, S-LAL8, and S-BAH28 glasses for the HARMONI visible camera

This paper reports on absolute refractive index measurements for the S-FPL55, S-LAL8, and S-BAH28 glass materials that are used in the visible channel of the High Angular Resolution Monolithic Optical and Near-infrared Integral (HARMONI) integral field spectrograph of the Extremely Large Telescope (ELT). These measurements were carried out by using the world-class Cryogenic High Accuracy Refraction Measurement System (CHARMS) refractometer (located at the Goddard Space Flight Center) in the 0.420-3.5 pm wavelength range and in the cryogenic temperature range of 105-310K. We also report on the validity of a cold-lens system design that is based on extrapolated 300 K refractive index data in comparison with directly measured cryogenic data obtained with the CHARMS refractometer.