A crucial test for astronomical spectrograph calibration with frequency combs

Relative stability measurement of two laser frequency combs (LFCs). LFC1 (18 GHz mode spacing) is in channel A and LFC2 (25 GHz mode spacing) in channel B. a, Series of 100 spectrograph calibrations with one exposure every 61 s, 102 min in total. b, Results obtained with binned exposures of increasing size. The filled circles represent the standard deviation in A-B. The error bars quantify the uncertainty of the standard deviation estimated from the size of the statistical sample.

Relative stability measurement of two laser frequency combs (LFCs) of 18 and 25 GHz mode spacing, respectively, in channel A and B. a: Series of 100 spectrograph calibrations. b: Results obtained with binned exposures of increasing size.

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06/24/2020 - 17:45

Authors

R. A. Probst et al.

Max Planck Institute of Quantum Optics

References

2020 Nature Astronomy 4 603

Laser frequency combs (LFCs) are well on their way to becoming the next-generation calibration sources for precision astronomical spectroscopy. 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 Doppler precision. 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 stability that has long been anticipated by the astronomical community.

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