Cryogenic Refractive Indices of S-LAH55, S-LAH55V, S-LAH59, S-LAM3, S-NBM51, S-NPH2, S-PHM52, and S-TIH14 Glasses

The Transiting Exoplanet Survey Satellite (TESS) is an explorer-class planet finder whose principal goal is to detect small planets with bright host stars in the solar neighborhood. The TESS payload consists of four identical cameras with seven lens elements, each made from various Ohara glass. The successful implementation of both the panchromatic and the thermal aspect of these lens assemblies requires accurate knowledge of the thermal and spectral dependence of the refractive index in the wavelength and temperature ranges of operation. Hence, this paper reports measurements of the refractive index for the following Ohara glasses over the wavelength range 0.42 – 1.10 μm and temperature range ~120 – 300 K: S-LAH55, S-LAH55V, S-LAH59, S-LAM3, S-NBM51, S-NPH2, S-PHM52, and S-TIH14. The measurements were performed using the Cryogenic High Accuracy Refraction Measuring System (CHARMS) facility at NASA Goddard Space Flight Center. Dense coverage of absolute refractive indices of these glasses over the aforementioned wavelength and temperature ranges allowed accurate determination of spectral thermo-optic coefficients (dn/dT) and spectral dispersions (dn/dλ) as a function of temperature. A comparison of measured, temperature-dependent indices to literature values is presented for S-PHM52 and S-TIH14. A comparison to Ohara’s catalog indices at room temperature is presented for all of the materials.