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Quantification of broadband chromatic drifts in Fabry–Pérot resonators for exoplanet science

Nature Astronomy volume 9, pages 589–597 (2025)Cite this article

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Abstract

Finding an Earth–Sun analogue is one of the longest-standing goals in astronomy. The detection of such a system using the radial velocity (RV) technique is highly challenging, and would require coordinated advances in astronomical telescopes, fibre optics, precision spectrographs, large-format detector arrays and data processing. Measurements at the necessary 10−10 level over multiyear periods would also require a highly precise calibrator. Here we explore simple and robust white-light-illuminated Fabry–Pérot (FP) etalons as spectral calibrators for precise RV measurements. We track the frequencies of up to 13,000 FP modes against laser frequency combs at two state-of-the-art spectrographs and trace unexpected chromatic variations of the modes to subpicometre changes in the dielectric layers of the broad-bandwidth FP mirrors, corresponding to a RV precision at the centimetres per second level. These results represent critical progress in precision RV measurements in two ways—they validate FP etalons as a more powerful stand-alone calibration tool and demonstrate the capability of laser frequency combs to extend RV measurement precision at the centimetres per second level over periods approaching a year. These advances highlight a path to achieving spectroscopic calibration at levels that will be critical for finding Earths like our own.

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Fig. 1: Introduction to etalon drift measurement.
Fig. 2: Etalon mirror coatings.
Fig. 3: Fits to the drift data.

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Data availability

All data needed to reproduce the results of this Article are available at https://scholar.colorado.edu/concern/datasets/m326m3334 (ref. 51).

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Acknowledgements

We acknowledge helpful discussions with J. Stürmer on mirror coatings, who has also pointed out the importance of relaxation of the stress in the outer dielectric coating layers and its impact on the variation of the mirror properties. We also acknowledge useful technical conversations with M. Rebello de Sousa Dias about optical modelling and material properties. We are grateful for financial support from NIST award number 70NANB18H006 from the US Department of Commerce (C.F.) and from NSF grant numbers AAG 2108512 (S.M.), ATI 2009889 (S.M.), ATI 2009982 (S.A.D.), ATI 2009955 (R.C.T.), AST 1310875 (S.A.D.) and AST 1310885 (S.M.). NEID DRP development is supported by JPL Subcontract 1644767 (C.F.B.). M.K.K. acknowledges support from the NSF Graduate Research Fellowship Program. The Hobby–Eberly Telescope (HET) is a joint project of the University of Texas at Austin, the Pennsylvania State University, Ludwig-Maximilians-Universität München and Georg-August-Universität Göttingen. The HET is named in honour of its principal benefactors, William P. Hobby and Robert E. Eberly. The Center for Exoplanets and Habitable Worlds and the Penn State Extraterrestrial Intelligence Center are supported by Penn State and its Eberly College of Science.

Author information

Authors and Affiliations

  1. Department of Physics, University of Colorado Boulder, Boulder, CO, USA

    Molly Kate Kreider, Connor Fredrick & Scott A. Diddams

  2. Electrical Computer and Energy Engineering, University of Colorado Boulder, Boulder, CO, USA

    Molly Kate Kreider, Connor Fredrick & Scott A. Diddams

  3. Time and Frequency Division, National Institute of Standards and Technology, Boulder, CO, USA

    Molly Kate Kreider, Connor Fredrick & Scott A. Diddams

  4. Department of Physics and Astronomy, Carleton College, Northfield, MN, USA

    Ryan C. Terrien

  5. Department of Astronomy & Astrophysics, The Pennsylvania State University, University Park, PA, USA

    Suvrath Mahadevan, Fred Hearty & Jason T. Wright

  6. Astrobiology Research Center, The Pennsylvania State University, University Park, PA, USA

    Suvrath Mahadevan

  7. Center for Exoplanets and Habitable Worlds, The Pennsylvania State University, University Park, PA, USA

    Suvrath Mahadevan, Fred Hearty & Jason T. Wright

  8. Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research, Mumbai, India

    Joe P. Ninan

  9. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

    Samuel Halverson

  10. Steward Observatory, University of Arizona, Tucson, AZ, USA

    Chad F. Bender

  11. LightMachinery Inc., Nepean, Ontario, Canada

    Daniel Mitchell

  12. NSF NOIRLab, Tucson, AZ, USA

    Jayadev Rajagopal

  13. Schmidt Sciences, New York, NY, USA

    Arpita Roy

  14. School of Mathematical and Physical Sciences, Macquarie University, North Ryde, New South Wales, Australia

    Christian Schwab

  15. Penn State Extraterrestrial Intelligence Center, University Park, PA, USA

    Jason T. Wright

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Contributions

R.C.T., S.M., J.P.N., S.H., C.F.B., F.H., J.R., A.R., C.S. and J.T.W. built the NEID and HPF instruments and acquired the etalon drift data. M.K.K., C.F. and S.A.D. developed the model to explain the etalon drift. M.K.K., C.F., S.A.D., R.C.T. and S.M. analysed the data and wrote the initial draft of the paper. D.M. provided information on the mirror coating structure and advised on drift sources. All authors discussed the results and contributed to revising and editing the final paper.

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Correspondence to Molly Kate Kreider or Scott A. Diddams.

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Nature Astronomy thanks Alexandre Cabral and Fei Zhao for their contribution to the peer review of this work.

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Supplementary Figs. 1–5 and Notes 1–4.

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Kreider, M.K., Fredrick, C., Diddams, S.A. et al. Quantification of broadband chromatic drifts in Fabry–Pérot resonators for exoplanet science. Nat Astron 9, 589–597 (2025). https://doi.org/10.1038/s41550-025-02486-x

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