High accuracy, absolute, cryogenic refractive index measurements of infrared lens materials.pdf

High accuracy, absolute, cryogenic refractive index measurements of infrared lens materials for JWST NIRCam using CHARMS Douglas B. Levitona*, Bradley J. Freya, Todd Kvammeb aNASA Goddard Space Flight Center, Greenbelt, MD 20771 bLockheed Martin Corporation, Palo Alto, CA 94304 ABSTRACT The refractive optical design of the James Webb Space Telescope (JWST) Near Infrared Camera (NIRCam) uses three infrared materials in its lenses: LiF, BaF2, and ZnSe. In order to provide the instrument’s optical designers with accurate, heretofore unavailable data for absolute refractive index based on actual cryogenic measurements, two prismatic samples of each material were measured using the cryogenic, high accuracy, refraction measuring system (CHARMS) at NASA’s Goddard Space Flight Center (GSFC), densely covering the temperature range from 15 to 320 K and wavelength range from 0.4 to 5.6 microns. Data reduction methods are discussed and graphical and tabulated data for absolute refractive index, dispersion, and thermo-optic coefficient for these three materials are presented for selected wavelengths and temperatures along with estimates of index uncertainty. Coefficients for temperature-dependent Sellmeier fits of measured index are also presented with an example of their usage to predict absolute index at any wavelength or temperature within the applicable range of those parameters. Keywords: Cryogenic, refractive index, infrared, refractometer, NIRCam, CHARMS, Sellmeier, lithium fluoride, barium fluoride, zinc selenide 1. INTRODUCTION The Near Infrared Camera (NIRCam) is not only the primary science camera for the James Webb Space Telescope (JWST), it is also the observatory’s wavefront sensor. The camera is designed to function at a nominal operating temperature of 37 K. It contains two essentially identical imaging channels which are fed by a dichroic beamsplitter, which separates incoming light from the JWST optical telescope into