Frequency Comb Calibration of Astronomical Spectrographs

Summary

We have constructed portable frequency combs that have provided in situ calibration of astronomical spectrographs, to aid in the search for exo-planets.

Description

Credit: CU/NIST/Penn State

NIST researchers and collaborators measured the frequencies, or colors, of infrared starlight (three solid bands with faint tick marks indicating where light is absorbed by the atmosphere) by comparing the missing light to a laser frequency comb reference "ruler" (sets of bright vertical bars indicating precise wavelengths, which increase from left to right). The three sets of starlight and comb light are shown in false color, from deeper orange (the most light) to orange-white (slightly less light) to black (very little light).

A powerful technique for finding exo-planets is based on measuring the tiny Doppler shifts in the light from a star that is induced by the presence of orbiting planets. While many exo-planets have been discovered using this Doppler radial velocity (RV) technique, it remains a significant technical challenge to reduce systematic uncertainties to the point of detecting an Earth-analog planet.  Towards this goal, it is now recognized that laser frequency combs (sometimes called "astrocombs" for this application) offer new opportunities for the highest precision astronomical spectroscopy measurements (e.g. 1 cm/s RV, 3×10⁻¹¹ fractionally, or ~6 kHz at 1550 nm) by providing a broad bandwidth, precisely tunable calibration spectrum that has an absolute accuracy traceable to the SI second. The uniformly spaced, bright, and narrow features of the comb spectrum are ideal for wavelength calibrations, while the absolute traceability of the comb enables comparison of observations made on timescales from days to years and even from different observatories.

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Optical physics and Time and frequency