Comb-calibrated solar spectroscopy through a multiplexed single-mode fiber channel

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. − 1 m s 1 over large time spans. LFCs hold promise to enable RV measurements at an accuracy of − 1 cm s 1 over arbitrary time horizons, and a − 2.5 cm s 1 calibration repeatability has already been demonstrated on a time scale of hours [9]. Unlike other calibration sources, LFCs are not only repeatable, but to the same degree also accurate, i.e. they allow assigning wavelengths, or optical frequencies, that come close to the true values. Most applications would profit from LFCs mainly for OPEN ACCESS RECEIVED their high repeatability, but the high accuracy of LFCs can also be beneficial, e.g. to compare spectra from different instruments. This property has also been exploited by [10] to create an improved atlas of the solar lines in the visible.Current high-precision spectrographs are usually connected to their telescopes via optical fibers, to detach them from telescope guiding, increasing the stability of the measurements. The spectrographs are usually equipped with a second fiber channel that is used to simultaneously calibrate the spectrograph during each measurement, to track spectrograph drifts. Differential drifts between the two channels are minimized by a rigid mechanical connection between the fiber outputs. However, since it is challenging to couple light from astronomical objects efficiently into single-mode fibers (SMFs), current high-precision spectrographs are fed with multimode fibers. While an SMF guides only a single, approximately Gaussian shaped beam profile, a multimode fiber supports a variety of beam profiles. The profile at the output of a multimode fiber depends on the beam alignment at its input, and therefore on telescope guiding. This translates into uncertainties in RV measurements.In this work, the use of SMFs for LFC-based calibration of astronomical spectrographs is explored. The tests presented here have been made at the Vacuum Tower Telescope (VTT), a solar telescope in Tenerife, Canary Islands, that is operated by the KIS. The very high optical powers available from the Sun allow the use of SMFs to feed the spectrograph b...

The photospheric solar oxygen project

“…The set-up provides a spectral purity and fidelity that is, to the best of our knowledge, not attainable with any other instrument currently available for solar observations. An early version of the instrument is described by Doerr et al (2012a,b) and by Probst et al (2015). The spectra used in this work were obtained during several campaigns in the testing and commissioning phase of the instrument in August and December 2013 and in March and July 2014.…”

Efficient photonic reformatting of celestial light for diffraction-limited spectroscopy

“…A further and potentially powerful benefit of the PIMMS concept is its potential to enable fibre-fed spectrographs which are precisely calibrated (Probst et al 2015) and free of modal noise. Modal noise is a phenomenon present in multimode optical-fibre-fed spectrographs, where changes in the modal pattern at the output of the fibre effectively result in variations in the spectrograph linefunction (Lemke et al 2011;McCoy et al 2012).…”

LARS: An Absolute Reference Spectrograph for solar observations

“…The general design of an astro-comb has already been described by Steinmetz et al (2008) and Wilken et al (2012). and Probst et al (2015) discussed the prototype version of the present instrument in great detail.…”

“…This paper describes the LARS instrument in its current science-ready configuration that was reached after substantial upgrades and modifications in 2016 and 2017, compared to the prototype version described in earlier works (Doerr et al 2012a,b;Probst et al 2015). In Sect.…”