In-Situ Determination of Astro-Comb Calibrator Lines to Better

High precision wavelength calibrators for high-resolution astrophysical spectrographs will be key components of new precision radial velocity (RV) observations including the search for Earth-like exoplanets and direct observation of cosmological deceleration. The astro-comb, a novel high-precision calibrator, is a combination of an octave-spanning femtosecond laser frequency comb and a Fabry-Pérot cavity used to achieve calibrator line spacings that can be resolved by an astrophysical spectrograph. Systematic spectral shifts associated with the cavity can be 0.1-1 MHz, corresponding to RV errors of 10-100 cm/s, due to the dispersive properties of the cavity mirrors over broad spectral widths. Although these systematic shifts are very stable, their correction is crucial to high accuracy astrophysical spectroscopy. Here, we demonstrate an in-situ technique to determine the systematic shifts of astro-comb lines due to finite Fabry-Pérot cavity dispersion. The technique employed practically at a telescope-based spectrograph to enable wavelength calibration accuracy better than 10 cm/s. © 2010 Optical Society of America References and links 1. C. Lovis, M. Mayor, F. Pepe, Y. Alibert, W. Benz, F. Bouchy, A. C. M. Correia, J. Laskar, C. Mordasini, D. Queloz, N. C. Santos, S. Udry, J.-L. Bertaux, and J.-P. Sivan, “An extrasolar planetary system with three neptune-mass planets,” Nature 441, 305–309 (2006). 2. A. Sandage, “The change of redshift and apparent luminosity of galaxies due to the deceleration of selected expanding universes,” Astrophys. J. 136, 319–333 (1962). 3. A. Loeb, “Direct measurement of cosmological parameters from the cosmic deceleration of selected expanding universes,” Astrophys. J. 499, L111–L114 (1998). 4. T. Udem, R. Holzwarth, and T. W. Hansch, “Optical frequency metrology,” Nature 416, 233–237 (2002). 5. T. Steinmetz, T. Wilken, C. Araujo-Hauck, R. Holzwarth, T. W. Hansch, L. Pasquini, A. Manescau, S. D’Odorico, M. T. Murphy, T. Kentischer, W. Schmidt, and T. Udem, “Laser frequency combs for astronomical observations,” Science 321, 1335–1337 (2008). 6. C.-H. Li, A. Benedick, P. Fendel, A. Glenday, F. Kärtner, D. Phillips, D. Sasselov, A. Szentgyorgyi, and R. Walsworth, “A laser frequency comb that enables radial velocity measurements with a precision of 1 cm s−1,” Nature 208, 610–612 (2008). 7. S. Udry, X. Bonfils, X. Delfosse, T. Forveille, M. Mayor, C. Perrier, F. Bouchy, C. Lovis, F. Pepe, D. Queloz, and J.-L. Bertaux, “The harps search for southern extra-solar planets. xi. super-earths (5 and 8 M⊕) in a 3-planet system,” Astron. Astrophys. 469, L43–L47 (2007). 8. C. Lovis, F. Pepe, F. Bouchy, G. L. Curto, M. Mayor, L. Pasquini, D. Queloz, G. Rupprecht, S. Udry, and S. Zucker, “The exoplanet hunter harps: unequalled accuracy and perspectives toward 1 cm s−1 precision,” Proc. SPIE 6269, 62690P1 – 62690P23 (2006). 9. M. J. Thorpe, R. J. Jones, K. D. Moll, J. Ye, and R. Lalezari, “Precise measurements of optical cavity dispersion and mirror coating properties via femtosecond combs,” Opt. Express 13, 882–888 (2005). 10. A. Schliesser, C. Gohle, T. Udem, and T. W. Hänsch, “Complete characterization of a broadband high-nesse cavity using an optical frequency comb,” Opt. Express 14, 5975–5983 (2006). 11. A. Yariv and P. Yeh, Photonics (Oxford University Press, Oxford, 2006). 12. G. Furez, “Design and application of high resolution and multiobject spectrographs: Dynamical studies of open clusters,” Ph.D. thesis, University of Szeged, Hungary (2008). 13. P. E. Ciddor, “Refractive index of air: new equations for the visible and near infrared,” Appl. Optics 35, 1566– 1573 (1996).