The best spectrographs are limited in stability by their calibration light source. Laser frequency combs are the ideal calibrators for astronomical spectrographs. They emit a spectrum of lines that are equally spaced in frequency and that are as accurate and stable as the atomic clock relative to which the comb is stabilized. Absolute calibration provides the radial velocity of an astronomical object relative to the observer (on Earth). For the detection of Earth-mass exoplanets in Earth-like orbits around solar-type stars, or of cosmic acceleration, 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 systems or spectrograph calibrations of limited performance 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 HD75289 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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When the Sun is observed in X-ray or extreme ultraviolet wavelengths, hundreds of bright and compact structures with a rounded shape and sizes similar to that of our planet Earth can be easily distinguished in the solar corona. These structures are known as Coronal Bright Points or CBPs and they consist of sets of magnetic loops that connect areas of opposite magnetic polarity on the solar surface. These loops confine the solar plasma and in them, by mechanisms that have been debated for many years among solar physicists, the gas remains with temperatures of several million degrees, emittingAdvertised on