A crucial test for astronomical spectrograph calibration with frequency combs


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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Fig. 1: Instrumental set-up.
Fig. 2: Two LFCs on the HARPS spectrograph.
Fig. 3: Relative stability measurement of two LFCs.
Fig. 4: Calibration reproducibility with two different LFCs characterized by the mean shift in line positions.
Fig. 5: Observation of Ceres.

Data availability

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.

Author information

R.H., F.G., L.P., G.L.C., J.R.d.M., R.R. and M.E. initiated the project. G.L.C., T.S., H.K., G.A., Y.W., A.S.M., F.K., R.A.P., A.M., B.L.C.M., I.d.C.L., L.P., O.M., E.P., J.R.d.M., R.H. and J.U. were at the La Silla Observatory for on-site activities during the campaign. T.S., Y.W., H.K., A.B. and F.G. provided and operated the LFCs. O.M., E.P. and J.U. programmed the software to integrate the LFCs into the infrastructure of the observatory. G.L.C., B.L.C.M. and F.K. operated the spectrograph. A.M. and G.A. designed, built and optimized the light injection and scrambling in multimode fibres. R.A.P., D.M., G.L.C., A.S.M. and I.d.C.L. analysed the LFC data. B.T.-P. and J.I.G.H. analysed the Ceres observation. R.H., Th.U., T.W.H., L.P., R.R. and J.R.d.M. supervised the work. R.A.P. wrote the manuscript. All authors discussed and commented on the manuscript.

Correspondence to Rafael A. Probst.

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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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Supplementary discussion, Figs. 1–8 and references.

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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 (2020). https://doi.org/10.1038/s41550-020-1010-x

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