Laser frequency combs (LFCs) are well on their way to becoming the next-generation calibration sources for precision astronomical spectroscopy1,2,3,4,5,6. This development is considered key in the hunt for low-mass rocky exoplanets around solar-type stars whose discovery with the radial-velocity method requires cm s–1 Doppler precision7. In order to prove such precise calibration with an LFC, it must be compared to another calibrator of at least the same precision. Being the best available spectrograph calibrator, this means comparing it to a second—fully independent—LFC. Here, we report on a test in which two separate LFCs were used to simultaneously calibrate an astronomical spectrograph. Our installation of two LFCs at the ultra-stable two-channel spectrograph HARPS allowed characterization of their relative stability and consistency in calibration at the highest available level. Although the test was limited in time, the results confirm the 1 cm s–1 stability that has long been anticipated by the astronomical community.
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The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request. The HARPS data are publicly available from http://archive.eso.org/wdb/wdb/eso/repro/form and https://www.eso.org/sci/activities/instcomm/harps_lfc.html. LFC2/LFC2 series: 2015-04-17, 14:40:26–16:14:46 utc, LFC2/LFC1 series: 2015-04-17, 18:26:12–20:06:36 utc.
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We cordially thank the technical staff of the La Silla Observatory for their assistance during the installation and the test run. We are equally grateful to P. St. J. Russell and his group from the Max Planck Institute for the Science of Light for their support on the development of tapered photonic crystal fibres to broaden the LFC spectrum. B.L.C.M., I.d.C.L. and J.R.d.M. acknowledge CNPq, CAPES and FAPERN Brazilian agencies.
The authors declare no competing interests.
Peer review information Nature Astronomy thanks Richard McCracken, David F. Phillips and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Probst, R.A., Milaković, D., Toledo-Padrón, B. et al. A crucial test for astronomical spectrograph calibration with frequency combs. Nat Astron 4, 603–608 (2020). https://doi.org/10.1038/s41550-020-1010-x
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