Calibration of the ZnSe pre-disperser on ESO's cryogenic IR echelle spectrograph (CRIRES): comparison of the first results from CRIRES and the laboratory data from CHARMS

The European Southern Observatory (ESO), the Space Telescope European Co-ordinating Facility (ST-ECF) and Goddard Space Flight Center (NASA) are collaborating to study the refractive index of ZnSe at cryogenic temperatures. The pre-disperser prism of ESO's Cryogenic high-resolution IR Echelle Spectrograph (CRIRES) for the Very Large Telescope (VLT) is made of ZnSe. CRIRES covers the wavelength range from 950 - 5000 nm at a resolution of 100,000 and is operated at about 65 K. Recent measurements at NASA GSFC's cryogenic high accuracy refraction measuring system (CHARMS) have established the index of refraction for ZnSe both as a function of wavelength and temperature. These data are being used as input for a physical model that provides the wavelength calibration for CRIRES. Here we present the latest results from CHARMS and a comparison with measurements obtained during CRIRES' laboratory testing. Our results highlight the value of high accuracy laboratory measurements of the optical properties of materials for the design and operation of astronomical instrumentation. This also illustrates the use of such data in instrument physical models for high fidelity calibration of spectrographs.