Abstract
A visible astrocomb spanning 555-890 nm has been implemented on the 10-m Southern African Large Telescope, delivering complete calibration of one channel of its high-resolution spectrograph and an accurate determination of its resolving power. A novel co-coupling method allowed simultaneous observation of on-sky, Th-Ar lamp and astrocomb channels, reducing the wavelength calibration uncertainty by a factor of two compared to that obtained using only Th-Ar lines. The excellent passive stability of the master frequency comb laser enabled broadband astrocomb generation without the need for carrier-envelope offset frequency locking, and an atomically referenced narrow linewidth diode laser provided an absolute fiducial marker for wavelength calibration. The simple astrocomb architecture enabled routine operation by non-specialists in an actual telescope environment. On-sky spectroscopy results are presented with direct calibration achieved entirely using the astrocomb.
Highlights
Astronomical spectrographs combining high resolution (R >> 10,000) with high stability provide unique scientific capabilities, e.g. for observing the small Doppler shifts which are signatures of Earth-like exoplanets or sensitively measuring isotopic ratios in ancient stars to reveal details of the early universe
A visible astrocomb spanning 555–890 nm has been implemented on the 10-m Southern African Large Telescope, delivering complete calibration of one channel of its highresolution spectrograph and an accurate determination of its resolving power
On-sky spectroscopy results are presented with direct calibration achieved entirely using the astrocomb
Summary
Astronomical spectrographs combining high resolution (R >> 10,000) with high stability provide unique scientific capabilities, e.g. for observing the small Doppler shifts which are signatures of Earth-like exoplanets or sensitively measuring isotopic ratios in ancient stars to reveal details of the early universe. Using Yb:fiber this approach has been combined with an additional frequency doubling stage to generate an astrocomb spanning 450–600 nm [4,5] These astrocombs harness the excellent comb stability of fiber lasers, the lower native repetition frequency requires additional filtering to achieve the necessary mode spacing for spectrograph calibration. The reduced operational demands of this astrocomb, compared with those designed exclusively for precision radial velocity measurements, allow for simpler Fabry-Pérot etalon coatings and a partially stabilized master frequency comb. This has reduced the overall complexity of the astrocomb and enabled the system to be operated fully by astronomers with no prior laser experience
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