Temperature-dependent refractive index measurements of S-FPL51, S-FTM16, and S-TIM28 to cryogenic temperatures

Using NASA Goddard Space Flight Center’s Cryogenic High Accuracy Refraction Measuring System (CHARMS), we measured absolute refractive indices for three infrared glasses from Ohara for lens designs for instruments at two of the world’s largest, ground-based, astronomical observatories – the present W.M. Keck Observatory and the future Giant Magellan Telescope (GMT). MOSFIRE (Keck), a near-infrared multi-object spectrograph and wide-field camera, has demonstrated diffraction limited performance at 120 K in part owing to our absolute refractive index measurements of Ohara S-FPL51 and S-FTM16 covering wavelengths and temperatures from 0.5 to 2.6 μm and 30 to 300 K, respectively. Measured index uncertainties range from 0.7–3.5E-5 and 0.7–2.9E-5 for S-FPL51 and S-FTM16, respectively, depending on wavelength and temperature, and for the latter on which test prism. NIRMOS (GMT), a near infrared multiple object imager/spectrograph, uses S-TIM28 in its imaging lens design. We measured S-TIM28’s indices for wavelengths and temperatures from 0.40 to 2.8 μm and 25 to 300 K, respectively with uncertainties ranging from 1.6– 3.0E-5. Absolute indices and their wavelength and temperature derivatives for these infrared glasses are reported along with coefficients for temperature-dependent Sellmeier fits of the measured index data to enable accurate computation of index to other wavelengths and temperatures. We compare our measurements to those in the literature.