A new Generation of Spectrometer Calibration Techniques Based on Optical Frequency Combs
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].
KeywordsGlobal Position System Frequency Shift Transmission Maximum Cavity Resonance Frequency Comb
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- 2.H.U. Käufl, P. Ballester, P. Biereichel, et al.: Ground-based Instrumentation for Astronomy. In: Proceedings of the SPIE, vol 5492, ed. by A.F.M. Moorwood and M. Iye (SPIE, Glasgow, Scotland, United Kingdom 2004) pp. 1218–1227Google Scholar
- 3.J. Stenger, H. Schnatz, C. Tamm, H.R. Telle: Phys. Rev. Lett. 88, 073601-1-4 (2002)Google Scholar
- 5.C. Araujo-Hauck, L. Pasquini, A. Manescau, et al.: The 2007 ESO Instrument Calibration Workshop (2008)Google Scholar
- 7.H.U. Käufl, et al.: The 2007 ESO Instrument Calibration Workshop (2008)Google Scholar
- 9.C. Lovis, F. Pepe: The 2007 ESO Instrument Calibration Workshop (2008)Google Scholar
- 10.R. Bacon, Y. Georgelin, G. Monnet: Bull. CFHT, 23 (1990)Google Scholar
- 11.C.B. Foltz, F.H. Chaffee, D.B. Quellette, et al.: MMT Tech. Mem. 85–4, 1 (1985)Google Scholar