A new Generation of Spectrometer Calibration Techniques Based on Optical Frequency Combs

  • P. O. Schmidt
  • S. Kimeswenger
  • H. U. Käufl
Part of the ESO Astrophysics Symposia European Southern Observatory book series (ESO)


Typical astronomical spectrographs have a resolution λ/°λ ranging between a few hundred to 200,000. Deconvolution and correlation techniques are being employed with a significance down to 1/1000th of a pixel. HeAr and ThAr lamps are usually used for calibration in low and high resolution spectroscopy, respectively. Unfortunately, the emitted lines typically cover only a small fraction of the spectrometer’s spectral range. Furthermore, their exact position depends strongly on environmental conditions. A problem is the strong intensity variation between different lines1 (intensity ratios >300). In addition, the brightness of the lamps is insufficient to illuminate a spectrograph via an integrating sphere, which in turn is important to calibrate a long-slit spectrograph, as this is the only way to assure a uniform illumination of the spectrograph pupil.

Laboratory precision laser spectroscopy has experienced a major advance with the development of optical frequency combs generated by pulsed femto-second lasers. These lasers emit a broad spectrum (several hundred nanometers in the visible and near infra-red) of equally-spaced “comb” lines with almost uniform intensity (intensity ratios typically <10). Self-referencing of the laser establishes a precise ruler in frequency space that can be stabilized to the 10-18 uncertainty level, reaching absolute frequency inaccuracies at the 10-12 level per day when using the Global Positioning System’s (GPS) time signal as the reference. The exploration of the merits of this new technology holds the promise for broad-band, highly accurate and reproducible calibration required for reliable operation of current and next generation astronomic spectrometers. Similar techniques are also proposed in[5, 6].


Global Position System Frequency Shift Transmission Maximum Cavity Resonance Frequency Comb 
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Copyright information

© Springer-Verlag Berlin Heidelberg 2008

Authors and Affiliations

  • P. O. Schmidt
    • 1
  • S. Kimeswenger
    • 2
  • H. U. Käufl
    • 3
  1. 1.Institute of Experimental PhysicsAustria
  2. 2.Institute of Astro- and Particle PhysicsAustria
  3. 3.ESO85748 GarchingGermany

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