The Spectrum of Th-Ar Hollow Cathode Lamps in the900-4500 nm Region: Establishing Wavelength Standardsfor the Calibration of VLT Spectrographs

摘要

We summarize the history, design and operation of hollow cathodelamps, including technical information that is usually not provided in literaturereadily available to the astronomical community. We show that modern hollowcathode lamps come close to being an "ideal" calibration lamp as described bya user's wish list. We provide examples of the use of such lamps for wavelengthcalibration of astronomical spectrographs both on the ground and in space. Wethen describe the joint efforts by the European Southern Observatory (ESO),the Space Telescope European Co-ordinating Facility (ST-ECF), and the UnitedStates National Institute of Standards and Technology (NIST) to establish theTh-Ar hollow cathode lamp as a standard for the calibration of VLT (VeryLarge Telescope) spectrographs in the near infrared (IR). In the near-IR onlya limited number of wavelength standards are available. Th-Ar hollow cathodelamps provide a rich spectrum in the ultraviolet (UV)-visible region and havebeen used in astronomy for a long time. We report new measurements usingthe 2-m UV/visible/IR Fourier transform spectrometer (FTS) at NIST thatestablish more than 2000 lines as wavelength standards in the range 900 nmto 4500 nm. This line list is used as input for a physical model that providesthe wavelength calibration for the Cryogenic High-Resolution IR Echelle Spec-trometer (CRIRES), ESO's new high-resolution (R n100 000) IR spectrographat the VLT. Based on these data and additional measurements investigatingother properties relevant for operations we conclude that Th-Ar lamps hold thepromise of becoming a standard source for wavelength calibration in near-IRastronomy.