Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

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.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

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 and 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.


  1. 1.

    Glenday, A. G. et al. Operation of a broadband visible-wavelength astro-comb with a high-resolution astrophysical spectrograph. Optica 2, 250–254 (2015).

    ADS  Article  Google Scholar 

  2. 2.

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

    ADS  Article  Google Scholar 

  3. 3.

    Molaro, P. et al. A frequency comb calibrated solar atlas. Astron. Astrophys. 560, A61 (2013).

    Article  Google Scholar 

  4. 4.

    Yi, X. et al. Demonstration of a near-IR line-referenced electro-optical laser frequency comb for precision radial velocity measurements in astronomy. Nat. Commun. 7, 10436 (2016).

    ADS  Article  Google Scholar 

  5. 5.

    Löhner-Böttcher, J. et al. LARS: an Absolute Reference Spectrograph for solar observations. Astron. Astrophys. 607, A12 (2017).

    Article  Google Scholar 

  6. 6.

    Obrzud, E. et al. A microphotonic astrocomb. Nat. Photon. 13, 31–36 (2019).

    ADS  Article  Google Scholar 

  7. 7.

    Fischer, D. A. et al. State of the field: extreme precision radial velocities. Publ. Astron. Soc. Pac. 128, 66001 (2016).

    Article  Google Scholar 

  8. 8.

    Udem, T., Holzwarth, R. & Hänsch, T. W. Optical frequency metrology. Nature 416, 233–237 (2002).

    ADS  Article  Google Scholar 

  9. 9.

    Murphy, M. T. et al. High-precision wavelength calibration of astronomical spectrographs with laser frequency combs. Mon. Not. R. Astron. Soc. 380, 839–847 (2007).

    ADS  Article  Google Scholar 

  10. 10.

    McCracken, R. A. et al. Wavelength calibration of a high resolution spectrograph with a partially stabilized 15-GHz astrocomb from 550 to 890 nm. Opt. Express 25, 6450–6460 (2017).

    ADS  Article  Google Scholar 

  11. 11.

    Ycas, G. G. et al. Demonstration of on-sky calibration of astronomical spectra using a 25 GHz near-IR laser frequency comb. Opt. Express 20, 6631–6643 (2012).

    ADS  Article  Google Scholar 

  12. 12.

    Steinmetz, T. et al. Laser frequency combs for astronomical observations. Science 321, 1335–1337 (2008).

    ADS  Article  Google Scholar 

  13. 13.

    Probst, R. A. et al. A laser frequency comb featuring sub-cm/s precision for routine operation on HARPS. Proc. SPIE 9147, 91471C (2014).

    Google Scholar 

  14. 14.

    McCracken, R. A., Charsley, J. M. & Reid, D. T. A decade of astrocombs: recent advances in frequency combs for astronomy. Opt. Express 25, 15058–15078 (2017).

    ADS  Article  Google Scholar 

  15. 15.

    Suh, M. et al. Searching for exoplanets using a microresonator astrocomb. Nat. Photon. 13, 25–30 (2019).

    ADS  Article  Google Scholar 

  16. 16.

    Kerber, F., Nave, G. & Sansonetti, C. J. The spectrum of Th-Ar hollow cathode lamps in the 691–5804 nm region: establishing wavelength standards for the calibration of infrared spectrographs. Astrophys. J. Suppl. Ser. 178, 374–381 (2008).

    ADS  Article  Google Scholar 

  17. 17.

    Lovis, C. & Pepe, F. A new list of thorium and argon spectral lines in the visible. Astron. Astrophys. 468, 1115–1121 (2007).

    ADS  Article  Google Scholar 

  18. 18.

    Nave, G. et al. The dirt in astronomy’s genie lamp: ThO contamination of Th-Ar calibration lamps. Proc. SPIE 10704, 1070407 (2018).

    Google Scholar 

  19. 19.

    Mayor, M. & Queloz, D. A Jupiter-mass companion to a solar-type star. Nature 378, 355–359 (1995).

    ADS  Article  Google Scholar 

  20. 20.

    Anglada-Escudé, G. et al. A terrestrial planet candidate in a temperate orbit around Proxima Centauri. Nature 536, 437–440 (2016).

    ADS  Article  Google Scholar 

  21. 21.

    Astudillo-Defru, N. et al. The HARPS search for southern extra-solar planets XLII. A system of Earth-mass planets around the nearby M dwarf YZ Ceti. Astron. Astrophys. 605, L11 (2017).

    ADS  Article  Google Scholar 

  22. 22.

    Pepe, F. et al. The HARPS search for Earth-like planets in the habitable zone I. Very low-mass planets around HD 20794, HD 85512, and HD 192310. Astron. Astrophys. 534, A58 (2011).

    Article  Google Scholar 

  23. 23.

    Pepe, F., Ehrenreich, D. & Meyer, M. R. Instrumentation for the detection and characterization of exoplanets. Nature 513, 358–366 (2014).

    ADS  Article  Google Scholar 

  24. 24.

    Liske, J. et al. Cosmic dynamics in the era of Extremely Large Telescopes. Mon. Not. R. Astron. Soc. 386, 1192–1218 (2008).

    ADS  Article  Google Scholar 

  25. 25.

    Ravi, A. et al. Probing dark matter using precision measurements of stellar accelerations. Phys. Rev. Lett. 123, 091101 (2019).

    ADS  Article  Google Scholar 

  26. 26.

    Webb, J. K. et al. Indications of a spatial variation of the fine structure constant. Phys. Rev. Lett. 107, 191101 (2011).

    ADS  Article  Google Scholar 

  27. 27.

    Mayor, M. et al. Setting new standards with HARPS. Messenger 114, 20–24 (2003).

    ADS  Google Scholar 

  28. 28.

    Brucalassi, A. et al. Stability of the FOCES spectrograph using an astro-frequency comb as calibrator. Proc. SPIE 9908, 99085W (2016).

    Article  Google Scholar 

  29. 29.

    Hao, Z. et al. Calibration tests of a 25-GHz mode-spacing broadband astro-comb on the fiber-fed High Resolution Spectrograph (HRS) of the Chinese 2.16-m telescope. Publ. Astron. Soc. Pac. 130, 125001 (2018).

    ADS  Article  Google Scholar 

  30. 30.

    Probst, R. A. et al. Relative stability of two laser frequency combs for routine operation on HARPS and FOCES. Proc. SPIE 9908, 990864 (2016).

    Article  Google Scholar 

  31. 31.

    Wilken, T. et al. High-precision calibration of spectrographs. Mon. Not. R. Astron. Soc. 405, L16–L20 (2010).

    ADS  Article  Google Scholar 

  32. 32.

    Pfeiffer, M. J., Frank, C., Baumüller, D., Fuhrmann, K. & Gehren, T. FOCES—a fibre optics Cassegrain échelle spectrograph. Astron. Astrophys. Suppl. Ser. 130, 381–393 (1998).

    ADS  Article  Google Scholar 

  33. 33.

    Probst, R. A. et al. Spectrally flattened, broadband astronomical frequency combs. In CLEO 2015 SW4G.7 (Optical Society of America, 2015).

  34. 34.

    Probst, R. A. et al. Spectral flattening of supercontinua with a spatial light modulator. Proc. SPIE 8864, 88641Z (2013).

    Article  Google Scholar 

  35. 35.

    Mahadevan, S., Halverson, S., Ramsey, L. & Venditti, N. Suppression of fiber modal noise induced radial velocity errors for bright emission-line calibration sources. Astrophys. J. 786, 18 (2014).

    ADS  Article  Google Scholar 

  36. 36.

    Hunter, T. R. & Ramsey, L. W. Scrambling properties of optical fibers and the performance of a double scrambler. Publ. Astron. Soc. Pac. 104, 1244–1251 (1992).

    ADS  Article  Google Scholar 

  37. 37.

    HORIZONS Web-Interface (Jet Propulsion Laboratory, California Institute of Technology, NASA).

  38. 38.

    Lanza, A. F., Molaro, P., Monaco, L. & Haywood, R. D. Long-term radial-velocity variations of the Sun as a star: the HARPS view. Astron. Astrophys. 587, A103 (2016).

    ADS  Article  Google Scholar 

  39. 39.

    Dumusque, X. et al. HARPS-N observes the Sun as a star. Astrophys. J. Lett. 814, L21 (2015).

    ADS  Article  Google Scholar 

  40. 40.

    Probst, R. A. et al. Nonlinear amplification of side-modes in frequency combs. Opt. Express 21, 11670–11687 (2013).

    ADS  Article  Google Scholar 

  41. 41.

    Horne, K. An optimal extraction algorithm for CCD spectroscopy. Publ. Astron. Soc. Pac. 98, 609–617 (1986).

    ADS  Article  Google Scholar 

Download references


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.

Corresponding author

Correspondence to Rafael A. Probst.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

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.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Supplementary discussion, Figs. 1–8 and references.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

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).

Download citation

Further reading


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing