The Cryogenic Refractive Indices of S‐FTM16, a Unique Optical Glass for Near‐Infrared Instruments

The Ohara glass S-FTM16 is of considerable interest for near-infrared opticaldesigns because it transmits well through the K band and because negativeS-FTM16 elements can be used to accurately achromatize positive calciumfluoride elements in refractive collimators and cameras. Glass manufacturershave sophisticated equipment to measure the refractive index at roomtemperature, but cannot typically measure the refractive index at cryogenictemperatures. Near-infrared optics, however, are operated at cryogenictemperatures to reduce thermal background. Thus we need to know the temperaturedependence of S-FTM16's refractive index. We report here our measurements ofthe thermal dependence of S-FTM16's refractive index between room temperatureand ~77 K. Within our measurement errors we find no evidence for a wavelengthdependence or a nonlinear temperature term so our series of measurements can bereduced to a single number. We find that Delta n_{abs} / Delta T = -2.4x10^{-6}K^{-1} between 298 K and ~77 K and in the wavelength range 0.6 micron to 2.6micron. We estimate that the systematic error (which dominates the measurementerror) in our measurement is 10%, sufficiently low for most purposes. We alsofind the integrated linear thermal expansion of S-FTM16 between 298 K and 77 Kis -0.00167 m m^{-1}.Comment: 8 pages, including 9 figures. Uses emulateapj.cls. Accepted for publication in PAS