Precision and consistency of astrocombs. (arXiv:2002.05182v1 [astro-ph.IM])

<a href="http://arxiv.org/find/astro-ph/1/au:+Milakovic_D/0/1/0/all/0/1">Dinko Milaković</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pasquini_L/0/1/0/all/0/1">Luca Pasquini</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Webb_J/0/1/0/all/0/1">John K Webb</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Curto_G/0/1/0/all/0/1">Gaspare Lo Curto</a>

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 ($approx

60{rm cms}^{-1}$ in our raw data) which were removed. Thirdly, astrocombs

achieve a precision of $lesssim 4{rm cms}^{-1}$ in a single exposure

($approx 10% $ above the measured photon-limited precision) and $1 {rm

cms}^{-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.

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 ($approx

60{rm cms}^{-1}$ in our raw data) which were removed. Thirdly, astrocombs

achieve a precision of $lesssim 4{rm cms}^{-1}$ in a single exposure

($approx 10% $ above the measured photon-limited precision) and $1 {rm

cms}^{-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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