A Laser Frequency Comb System for Absolute Calibration of the VTT Echelle Spectrograph

Doerr, Hans-Peter; Steinmetz, Tilo; Holzwarth, Ronald; Kentischer, T.; Schmidt, Wolfgang

doi:10.1007/s11207-012-9960-5

Cited by 15 publications

(12 citation statements)

References 18 publications

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“…Similar short-term precision using astrocombs has also been demonstrated on several other high-resolution spectrographs (e.g. Ycas et al 2012;Phillips et al 2012;Doerr et al 2012;Glenday et al 2015;Brucalassi et al 2016;McCracken et al 2017). All these previous studies referenced the astrocomb to itself, so any possible systematic effects arising in the astrocomb system itself may go undetected.…”

Section: Introductionsupporting

confidence: 69%

Precision and consistency of astrocombs

Milaković

Pasquini

Webb

et al. 2020

Monthly Notices of the Royal Astronomical Society

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Astrocombs are ideal spectrograph calibrators whose limiting precision can be derived using a second, independent, astrocomb system. We therefore analyse data from two astrocombs (one 18 GHz and one 25 GHz) used simultaneously on the HARPS spectrograph at the European Southern Observatory. The first aim of this paper is to quantify the wavelength repeatability achieved by a particular astrocomb. The second aim is to measure wavelength calibration consistency between independent astrocombs, that is to place limits or measure any possible zero-point offsets. We present three main findings, each with important implications for exoplanet detection, varying fundamental constant and redshift drift measurements. Firstly, wavelength calibration procedures are important: using multiple segmented polynomials within one echelle order results in significantly better wavelength calibration compared to using a single higher-order polynomial. Segmented polynomials should be used in all applications aimed at precise spectral line position measurements. Secondly, we found that changing astrocombs causes significant zero-point offsets (≈ 60 cm s −1 in our raw data) which were removed. Thirdly, astrocombs achieve a precision of 4 cm s −1 in a single exposure (≈ 10% above the measured photon-limited precision) and 1 cm s −1 when time-averaged over a few hours, confirming previous results. Astrocombs therefore provide the technological requirements necessary for detecting Earth-Sun analogues, measuring variations of fundamental constants and the redshift drift.

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Section: Introductionsupporting

confidence: 69%

Precision and consistency of astrocombs

Milaković

Pasquini

Webb

et al. 2020

Monthly Notices of the Royal Astronomical Society

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“…The systematic effect of the varying line-of-sight component of Earth rotation is only on the order of 75 ms −1 and was neglected. The thermal drift of the spectrograph of the VTT is on the order of 50 ms −1 per hour (Doerr et al 2012) and was also neglected.…”

Section: Doppler Shiftsmentioning

confidence: 99%

Dynamics of the Fe II 396.94 nm emission line observed at solar disk center

Schmidt

Fisher

2013

A&A

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Aims. We have studied the structure and dynamics of the upper photosphere and lower chromosphere of the quiet Sun. Methods. We analyzed a sequence of scans of slit spectra of the Ca II H line at 396.8 nm, with a spectral resolution of 850 000, and a spatial resolution of about one arcsec, taken in the quiet Sun at disk center, with a duration of about 55 min and an area coverage of 185 × 3.5 . We used statistical methods to characterize size, lifetime, and dynamics of emission features of the Fe II line at 396.94 nm. Results. We have identified about 780 distinct features where the Fe II line shows emission stronger than 3% above the local continuum. Their lifetime is on the order of one minute, and the spatial extent is about or less than 2 . On average, Fe II emission features show a significant redshift of 1.3 km s −1 , with respect to the position of the absorption line. The redshift of the Fe II emission is absent close to the solar limb. The Fe II emission is coupled with a strongly enhanced intensity level of the Ca II wings. A time line analysis shows that other photospheric lines show significant redshift one minute before, and a blueshift after, the occurrence of an emission event. A redshift of the Ca II H line core precedes the redshift of the Fe II line by about 30 s. The occurrence of the H2v emissions is strongly reduced before and significantly increased after the Fe II emission events. Conclusions. The temporal behavior of the Fe II emission line and both the Ca II H line core and H2v emission suggest a connection between Fe II emission and chromospheric activity. The presence of a significant redshift during the Fe II emissions and the absence of this redshift near the limb suggest that the observed redshift is indeed caused by a downflow in the line-forming region. From our result concerning the intensity fluctuations in the core of the Fe II line, and from the magnitude of the downflow velocities we conclude that the Fe II emissions at 396.94 nm occur in the photosphere.

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“…Laser frequency combs (LFCs) have revolutionized precision spectroscopy in atomic and molecular physics [1], enabling measurements of transition frequencies with unprecedented accuracy. Recently, they have grown into high-precision frequency calibrators for astronomical spectrographs [2][3][4][5][6][7][8][9][10]. An LFC represents an absolute optical frequency reference, directly linked to an accurate radio-frequency (RF) standard such as an atomic clock.…”

Section: Introductionmentioning

confidence: 99%

“…The LFC as installed at the VTT in October 2011 is described in [7]. In May 2012, just before making the tests reported here, the system was upgraded to enable higher calibration accuracy and more reliable operation [8].…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2015

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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...

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A Laser Frequency Comb System for Absolute Calibration of the VTT Echelle Spectrograph

Abstract: A wavelength calibration system based on a laser frequency comb (LFC) was developed in a co-operation between the Kiepenheuer

Cited by 15 publications

References 18 publications

Precision and consistency of astrocombs

Precision and consistency of astrocombs

Dynamics of the Fe II 396.94 nm emission line observed at solar disk center

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

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