Searches for extrasolar planets using the periodic Doppler shift of stellar spectral lines have recently achieved a precision of 60 cm s −1 (ref 1), which is sufficient to find a 5-Earth-mass planet in a Mercury-like orbit around a Sun-like star. To find a 1-Earth-mass planet in an Earthlike orbit, a precision of ∼ 5 cm s −1 is necessary. The combination of a laser frequency comb with a Fabry-Pérot filtering cavity has been suggested as a promising approach to achieve such Doppler shift resolution via improved spectrograph wavelength calibration 2−4 , with recent encouraging results 5 . Here we report the fabrication of such a filtered laser comb with up to 40-GHz (∼ 1-Å) line spacing, generated from a 1-GHz repetition-rate source, without compromising long-term stability, reproducibility or spectral resolution. This wide-line-spacing comb, or 'astro-comb', is well matched to the resolving power of high-resolution astrophysical spectrographs. The astro-comb should allow a precision as high as 1 cm s −1 in astronomical radial velocity measurements.The accuracy and long-term stability of state-of-theart astrophysical spectrographs are currently limited by the wavelength-calibration source 6,7 , typically either thorium-argon lamps or iodine absorption cells 8 . In addition, existing calibration sources are limited in the red-tonear-IR spectral bands most useful for exoplanet searches around M stars 9 and dark matter studies in globular clusters 10 . Iodine cells have very few spectral lines in the red and near-IR spectral bands, while thorium-argon lamps have limited lines and unstable bright features that saturate spectrograph detectors. Recently, laser frequency combs 11 have been suggested as potentially superior wavelength calibrators 2,3 because of their good longterm stability and reproducibility, and because they have useful lines in the red-to-near-IR range. The absolute optical frequencies of the comb lines are determined by f = f ceo + m × f rep , where f rep is the repetition rate, f ceo is the carrier-envelope offset frequency and m is an integer. Both f rep and f ceo can be synchronized with radio-frequency oscillators referenced to atomic clocks. For example, using the generally available Global Positioning System (GPS), the frequencies of comb lines have long-term fractional stability and accuracy of better than 10 −12 . For the calibration of an astrophysical spectrograph, fractional stability and accuracy of 3 × 10 −11 are sufficient to measure a velocity variation of 1 cm s −1 in astronomical objects. In addition, using GPS as the absolute reference allows the comparison of measurements at different observatories.For existing laser combs, f rep is usually < 1 GHz (ref. 12), which would require a spectrograph with a resolving power of R = λ/δλ 10 5 to resolve individual comb lines (here δλ is the smallest difference in wavelengths that can be resolved at wavelength λ). In practice, astrophysical spectrographs tend to have a resolving power of R ∼ 10 4 − 10 5 owing to physical limitations on the in...

Radial velocity perturbations induced by stellar surface inhomogeneities including spots, plages and granules currently limit the detection of Earth-twins using Doppler spectroscopy. Such stellar noise is poorly understood for stars other than the Sun because their surface is unresolved. In particular, the effects of stellar surface inhomogeneities on observed stellar radial velocities are extremely difficult to characterize, and thus developing optimal correction techniques to extract true stellar radial velocities is extremely challenging. In this paper, we present preliminary results of a solar telescope built to feed full-disk sunlight into the HARPS-N spectrograph, which is in turn calibrated with an astro-comb. This setup enables long-term observation of the Sun as a star with state-of-the-art sensitivity to radial velocity changes. Over seven days of observing in 2014, we show an average 50 cm s −1 radial velocity rms over a few hours of observation. After correcting observed radial velocities for spot and plage perturbations using full-disk photometry of the Sun, we lower by a factor of two the weekly radial velocity rms to 60 cm s −1 . The solar telescope is now entering routine operation, and will observe the Sun every clear day for several hours. We will use these radial velocities combined with data from solar satellites to improve our understanding of stellar noise and develop optimal correction methods. If successful, these new methods should enable the detection of Venus over the next two to three years, thus demonstrating the possibility of detecting Earth-twins around other solar-like stars using the radial velocity technique.

Searches for Earth-like exoplanets using the periodic Doppler shift of stellar absorption lines require 10 cm∕s precision in the measurement of stellar radial velocity (RV) over timescales of years. Current techniques have led to the discovery of short-period exoplanets that induce RV wobbles as small as ≈1 m∕s on their parent stars. It has been suggested that order-of-magnitude improved RV precision may be achievable using an astro-comb, a laser frequency comb optimized for astrophysical spectrograph wavelength calibration. Here we report the development of a broadband visible-wavelength astro-comb and its operation with the HARPS-N spectrograph at the Telescopio Nazionale Galileo in the Canary Islands. This green astrocomb has >7000 narrow (<1 MHz) spectral lines spaced by 16 GHz with relatively uniform line power from 500 to 620 nm. The line frequencies are locked to GPS, enabling us to realize HARPS-N wavelength calibration with RV measurement precision and stability <10 cm∕s.

Abstract:We deployed two wavelength calibrators based on laser frequency combs ("astro-combs") at an astronomical telescope. One astrocomb operated over a 100 nm band in the deep red (∼ 800 nm) and a second operated over a 20 nm band in the blue (∼ 400 nm). We used these red and blue astro-combs to calibrate a high-resolution astrophysical spectrograph integrated with a 1.5 m telescope, and demonstrated calibration precision and stability sufficient to enable detection of changes in stellar radial velocity < 1 m/s.

Abstract:We demonstrate a tunable laser frequency comb operating near 420 nm with mode spacing of 20-50 GHz, usable bandwidth of 15 nm and output power per line of ~20 nW. Using the TRES spectrograph at the Fred Lawrence Whipple Observatory, we characterize this system to an accuracy below 1m/s, suitable for calibrating high-resolution astrophysical spectrographs used, e.g., in exoplanet studies.