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A spectrograph for exoplanet observations calibrated at the centimetre-per-second level


The best spectrographs are limited in stability by their calibration light source1. Laser frequency combs are the ideal calibrators for astronomical spectrographs2. They emit a spectrum of lines that are equally spaced in frequency3 and that are as accurate and stable as the atomic clock relative to which the comb is stabilized. Absolute calibration4 provides the radial velocity of an astronomical object relative to the observer (on Earth). For the detection of Earth-mass exoplanets5,6 in Earth-like orbits around solar-type stars, or of cosmic acceleration7,8,9, the observable is a tiny velocity change of less than 10 cm s−1, where the repeatability of the calibration—the variation in stability across observations—is important. Hitherto, only laboratory systems10,11,12 or spectrograph calibrations of limited performance4,13,14 have been demonstrated. Here we report the calibration of an astronomical spectrograph with a short-term Doppler shift repeatability of 2.5 cm s−1, and use it to monitor the star HD 75289 and recompute the orbit of its planet. This repeatability should make it possible to detect Earth-like planets in the habitable zone of star or even to measure the cosmic acceleration directly.

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Figure 1: Experimental set-up and visualization of the raw data.
Figure 2: Overview over the data taken during the two measurement campaigns in November 2010 and January 2011.
Figure 3: Two-sample deviation of the two long series
Figure 4: Observation of the radial velocity of the star HD 75289 using the LFC for calibration.


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We would like to thank the staff at La Silla Observatory for their support during our campaigns; S. Stark, H. Hundertmark and P. St J. Russell for supplying us with the tapered PCF; C. Lovis, B. Chazelas and F. Pepe for discussions and help with the data reduction; and C. Buggle and A. Thaller for engineering support. T.W.H. acknowledges support from the Max Planck Foundation.

Author information




T.W., R.A.P. and T.S. designed and set up the LFC; T.W., G.L.C., R.A.P., T.S., A.M., L.P., J.I.G.H., T.U. and R.H. participated in data acquisition; T.W., G.L.C., L.P., T.U. and R.H. evaluated and analysed the data; L.P., T.W.H., T.U. and R.H. initiated and supervised the experiment; and T.W. wrote the manuscript. All authors discussed the results and commented on the manuscript.

Corresponding authors

Correspondence to Tobias Wilken or Ronald Holzwarth.

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The authors declare no competing financial interests.

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Wilken, T., Curto, G., Probst, R. et al. A spectrograph for exoplanet observations calibrated at the centimetre-per-second level. Nature 485, 611–614 (2012).

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