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 HARMONI integral field spectrograph of the Extreme 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 μm 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.