Wavelength Calibration of Near‐Infrared Spectra

Abstract

We report new reference wavelengths emitted by a Th/Ar hollow cathode lamp in the range 900 nm to 4500 nm. OCIS codes: (300.6340) Spectroscopy, infrared; (300.6210) Spectroscopy, atomic. Astronomical spectroscopy in the near infra-red (IR) has relied on atmospheric features of the night sky for wavelength calibration. These standards are convenient, since they are imprinted on the spectrum of any astronomical target. However, they usually have low signal-to-noise ratio when observed at high resolution, with intensities that are site dependent and vary from night to night. To achieve the full accuracy of which they are capable, high-resolution astronomical spectrographs, such as the Cryogenic High-Resolution IR Echelle Spectrograph (CRIRES) on the European Southern Observatory's Very Large Telescope (VLT), require external calibration sources such as lamps or gas cells. Use of these external sources has been limited in the near-IR because of the lack of a sufficiently dense and accurate grid of IR wavelength standards. Thorium lamps provide a rich spectrum in the ultraviolet (UV) and visible regions and have been used for calibration of astronomical spectrographs for a long time. The Th-Ne spectrum from 278 nm to about 1 µm was studied using high-resolution Fourier transform spectroscopy (FTS) about 20 years ago [1]. 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 spectrographs such as CRIRES. Hinkle et al. [2], produced an atlas of the Th-Ar spectrum covering selected regions in the range 1 µm to 2.5 µm, but this atlas has many gaps in its wavelength coverage. Engleman, Hinkle & Wallace [3], measured more than 5000 lines emitted by a high-current hollow cathode lamp. This lamp produced a rich Th spectrum, but the spectrum is very different from the low current lamps used on astronomical telescopes. We thus decided to measure the spectrum of commercial Th-Ar lamps in order to determine a set of IR wavelength standards suitable for use on CRIRES. Spectra of three types of Th-Ar lamps were recorded on the NIST 2-m FTS. The first lamp had a Th cathode with a closed end. The second lamp contained a cathode with both ends open, and was used to determine if an open-ended cathode would reduce the continuum that becomes increasingly prominent for wavelengths longer than 2.5 µm. Both of these lamps had quartz windows that limited the longest wavelength to about 3.5 µm. A third lamp with a closed-end cathode and a sapphire window was also observed. This lamp produced a good spectrum up to 5.5 µm. The lamps were run at a current of 20 mA. The spectrometer was fitted with a CaF2 beamsplitter, silver coated mirrors, and InSb detectors. The resolution was 0.005 cm-1 to 0.01 cm-1 and up to 250 interferograms were co-added for total integration times of up to 20 hours. Tungsten standard lamps provided radiometric calibration. Analysis of the spectra is still ongoing, but we expect to measure a total of about 2500 lines in the range 900 nm to 4800 nm. Our preliminary data are being used for the construction of a physical model of CRIRES. Such models have been successfully used for calibration of other astronomical spectrographs on the VLT [4] and the Hubble Space Telescope. The CRIRES model will be used to derive a more accurate wavelength calibration than is possible with an empirical approach based on fitting a polynomial to wavelength standards observed in the spectrum. References [1] B. A. Palmer and R. Engleman Jr., "Atlas of the thorium spectrum," Los Alamos National Laboratory Report, LA-9615 (1983) [2] K.