Establishing wavelength standards in the near Infra-red: Th-Ar

Abstract

We describe the joint efforts by the European Southern Observatory (ESO), the Space Telescope European Co-ordinating Facility (ST-ECF), and the United States National Institute of Standards and Technology (NIST) to establish the Th-Ar hollow cathode lamp as a standard for the calibration of VLT (Very Large Telescope) spectrographs in the near infrared (IR). In the near IR only a limited number of wavelength standards are available. Th-Ar hollow cathode lamps provide a rich spectrum in the ultraviolet (UV)-visible region and have been used in astronomy for a long time. 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. Based on these data and additional measurements investigating other properties relevant for operations we conclude that Th-Ar lamps hold the promise of becoming a standard source for wavelength calibration in near IR astronomy. © 2008 Springer-Verlag Berlin Heidelberg.