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Wavelength Standards for IR Spectrographs at ESO: Supporting High-Resolution Spectroscopy

This document discusses the establishment of wavelength standards for high-resolution infrared spectrographs at the European Southern Observatory (ESO). It focuses on Th-Ar hollow cathode lamps as a calibration source, highlighting the accuracy and stability of measurements. Key aspects include the number of spectral lines available, variability, and the need for reliable light sources for wavelength calibration, particularly in near-IR photometry. The ongoing work involves optimizing the use of gas cells for calibration and ensuring precise measurements of molecular transitions.

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Wavelength Standards for IR Spectrographs at ESO: Supporting High-Resolution Spectroscopy

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  1. Wavelength Standards for IR Spectrographs at ESO Supporting high resolution spectroscopy Florian Kerber, ESO

  2. Wavelength Calibration - Near IR • Night Sky OH Meinel Bands (Abrams et al., 1994; Rousselot et al. 2000) • Present on science exposure • Number of lines at given wavelength and resolution • Variability • Stability at high resolution ? (< 25 m s-1) • Lack of good sources • Lamps - Ne, Kr • Gas cells ESAC, March 22nd, 2007

  3. Th-Ar hollow cathode lamp Anode Kerber et al. 2007, ASP Conf. Series. 364, p.461 Cathode ESAC, March 22nd, 2007

  4. Hollow cathode lamp • Introduced by Paschen (1916) • Study of hyperfine structure • Atomic Absorption (AA) Spectroscopy (Walsh 1950s) • Lifetime - Getter • Quantitative chemical analysis • Great variety of materials • Mass production for commercial market ESAC, March 22nd, 2007

  5. Th-Ar lamp:Visible and Near IR • Why Thorium ? • Nature: 232Th - no isotope splitting • zero nuclear spin - no hyperfine structure • Established standard source in Visual • Palmer & Engleman (1983) 278 - 1000 nm • FEROS, FLAMES, HARPS, UVES, X-shooter ESAC, March 22nd, 2007

  6. CRIRES • Cryogenic High Resolution InfraRed Echelle Spectrometer (CRIRES) at VLT • 950 - 5000 nm, • Resolution / 100,000 • Project to establish wavelength standards (NIST) ESAC, March 22nd, 2007

  7. Th-Ar Lamp Project • Physical Modelling Group (ESO INS) • F. Kerber, P. Bristow • NIST Atomic Spectroscopy Group • J. Reader, G. Nave, C.J. Sansonetti • Space Telescope European Coordinating Facility (ST-ECF) • M.R. Rosa • ESO La Silla-Paranal Observatory • G. Lo Curto ESAC, March 22nd, 2007

  8. Th-Ar lamp:Visible and Near IR • Th-Ar is work in progress for near-IR • Hinkle et al. (2001), 1000 - 2500 nm, 500 lines • Gaps in wavelength coverage • FTS and grating spectrometer data • Engleman et al. (2003), 1000 - 5000 nm, > 5000 lines • High current source, rich Th spectrum • Differences in spectrum but … ESAC, March 22nd, 2007

  9. Visible/IR FTS at NIST ESAC, March 22nd, 2007

  10. NIST Experimental Work • 2-m UV/Visible/IR FTS • CaF2 beamsplitter, silver coated mirrors, InSb detectors • Two different settings optimized for wavelength • 800 - 2000 nm, resolution 0.01 cm-1 • > 2000 nm, resolution 0.005 cm-1 • Data acquisition times: up to 20 hours • Radiometric standard lamp ESAC, March 22nd, 2007

  11. Th-Ar ESAC, March 22nd, 2007

  12. CRIRES FTS ESAC, March 22nd, 2007

  13. Th-Ar at NIST: Results • Total ~2500 lines between 750 nm and 4800 nm • accuracy ~ 0.001 cm-1 for strong lines • Absolute accuracy in wavelength: ~1.4 x 10-8 • calibration using laser measurements of Th lines (DeGraffenreid & Sansonetti 2002) • 5 x 10-8 1/100 of a pixel in CRIRES • Engleman et al. (2003) and our work agree: • Th: (6 ± 2) x 10-9 (weighted mean) • Ar: (-2 ± 0.2) x 10-8 ESAC, March 22nd, 2007

  14. Results • The absolute accuracy in wavelength is about 1.4 10-8 • calibration using laser measurements of Th lines (DeGraffenreid & Sansonetti 2002, JOSA, B19, 1715) • Engleman et al (2003) and our work agree: • Th: 6 x 10-9 (weighted mean) • Ar: 2 x 10-8 • CRIRES wavelength calibration: 5 x 10-8 corresponds to ~1/100 of a pixel • Th-Ar lamp is able to support CRIRES science ESAC, March 22nd, 2007

  15. At ESO Additional Measurements FTS Parabolic Mirror Integrating Sphere Elliptical Mirror Sample Compartment ESAC, March 22nd, 2007

  16. Spectrum - Operating Current ESAC, March 22nd, 2007

  17. Gas Cells - Status and next Steps • CRIRES commissioning: efficient use of the N2O gas cells (l=18 cm) as calibration source: ~2200 - 4100 nm • primary calibration molecule from Heterodyne frequency measurements at NIST (Maki & Wells) • Fundamental mode: 523 – 2845 cm -1 (1912 – 3515 nm) • Characterize spectrum of the CRIRES gas cell (P,T) Q2 2007 • optimal choice of fill gas pressure - measurements using ESO’s FTS • Wavelength Standards for first overtone transitions of N2O • Possible use of OCS (carbonyl sulfide) as calibrator (Q2 07) • separate gas cell or in a mixture with N2O • Long-term Stability of spectrum (Q4 2007) ESAC, March 22nd, 2007

  18. Gas Cells - N2O ESAC, March 22nd, 2007

  19. Summary: CRIRES Calibration • Calibration Reference Data • Established Wavelength Standards Th-Ar: 950 - 2200 nm, ~5000 lines; N2O: 2200 - 4100 nm • Laboratory “ground truth” Accuracy few parts in 10-8; ≤ 1/100 pixel • Properties of Sources • Optimize use: Th-Ar - current; N2O - gas pressure • Paradigma for wavelength calibration in near IR • very similar to the UV-visible region ESAC, March 22nd, 2007

  20. Outlook • CRIRES Science Verification: • Nissen et al. astro-ph/0702689: Sulphur abundances • CRIRES operational: April 1st, 2007 • ESO archive • Spectral Data Model - Simple Line Access Protocol • Publish IR data set for Th: VOTable - metadata • Q2/Q3 2007 • linking astronomical and atomic physics communities ESAC, March 22nd, 2007

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