The spectrum of Th-Ar hollow cathode lamps in the 900-4500 nm region: establishing wavelength standards for the calibration of VLT spectrographs

Abstract

The European Southern Observatory (ESO), the Space Telescope European Co-ordinating Facility (ST-ECF), and the US National Institute of Standards and Technology (NIST) are collaborating to study Th-Ar hollow cathode lamps as used for the calibration of VLT (Very Large Telescope) spectrographs. In the near IR only a limited number of wavelength standards are available. The density and distribution of lines in Ne or Kr lamps, for example, are inadequate for high-resolution spectroscopy. Th-Ar hollow cathode lamps provide a rich spectrum in the UV-visible region and have been used in astronomy for a long time; current examples at ESO include the spectrographs UVES and FLAMES. The Th spectrum from 278 nm to about 1000 nm was studied at high resolution about 20 years ago (Palmer and Engleman, 1983). Two studies of the Th-Ar spectrum in the near IR have recently been published, but neither work is directly applicable to the calibration of IR astronomical spectrographs at ESO. We report new measurements using the 2-m UV/visible/IR Fourier transform spectrometer (FTS) at NIST that establish more than 2000 lines as wavelength standards in the range 900 nm to 4500 nm. This line list is used as input for a physical model that provides the wavelength calibration for the Cryogenic High-Resolution IR Echelle Spectrometer (CRIRES), ESO's new high resolution (R~100,000) IR spectrograph at the VLT. We also present first calibration results from laboratory testing of CRIRES. The newly established wavelength standards will also be available for use by X-shooter and other spectrographs in the future. Measurements of the variation of the spectrum of Th-Ar lamps as a function of operating current allow us to optimise the spectral output in terms of relative intensity and line density for operation on the telescope. Since Th and Ar line intensities show a different response with respect to operating current, such measurements can be used as a diagnostic tool for distinguishing the gas and metal lines. Our findings show that Th-Ar lamps hold the promise of becoming a standard source for wavelength calibration in near IR astronomy.