Abstract
We investigate a new scheme for astronomical spectrograph calibration using the laser frequency comb at the Solar Vacuum Tower Telescope on Tenerife. Our concept is based upon a single-mode fiber channel, that simultaneously feeds the spectrograph with comb light and sunlight. This yields nearly perfect spatial mode matching between the two sources. In combination with the absolute calibration provided by the frequency comb, this method enables extremely robust and accurate spectroscopic measurements. The performance of this scheme is compared to a sequence of alternating comb and sunlight, and to absorption lines from Earth’s atmosphere. We also show how the method can be used for radial-velocity detection by measuring the well-explored 5 min oscillations averaged over the full solar disk. Our method is currently restricted to solar spectroscopy, but with further evolving fiber-injection techniques it could become an option even for faint astronomical targets.
Highlights
Laser frequency combs (LFCs) have revolutionized precision spectroscopy in atomic and molecular physics [1], enabling measurements of transition frequencies with unprecedented accuracy
This occurs due to modal interference. This point is critical for the LFC, owing to its high degree of coherence. Another concept that we introduce for spectrograph calibration with an LFC is multiplexing of the fiber channel, which involves coupling both comb and sunlight into one single fiber
The observed solar spectrum contained several O2 lines of telluric origin, i.e. stemming from Earth’s atmosphere. Their line centers are independent of telescope guiding, and to compare the LFC with these lines, all recorded solar spectra are of use
Summary
100012, Peoples Republic of China 3 Universitäts-Sternwarte München, Scheinerstr. 1, D-81679 München, Germany 4 Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. Our concept is based upon a single-mode fiber channel, that simultaneously feeds the spectrograph with comb light and sunlight. This yields nearly perfect spatial mode matching between the two sources. In combination with the absolute calibration provided by the frequency comb, this method enables extremely robust and accurate spectroscopic measurements. The performance of this scheme is compared to a sequence of alternating comb and sunlight, and to absorption lines from Earth’s atmosphere. Our method is currently restricted to solar spectroscopy, but with further evolving fiber-injection techniques it could become an option even for faint astronomical targets
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