Volume 576, April 2015
|Number of page(s)||17|
|Section||Numerical methods and codes|
|Published online||01 April 2015|
Molecfit: A general tool for telluric absorption correction⋆
II. Quantitative evaluation on ESO-VLT/X-Shooterspectra
1 Institute for Astro- and Particle Physics, University of Innsbruck, Technikerstr. 25/8, 6020 Innsbruck, Austria
2 University of Vienna, Department of Astrophysics, Türkenschanzstr. 17 (Sternwarte), 1170 Vienna, Austria
3 European Southern Observatory, Alonso de Córdova 3107, 19001 Casilla, Santiago 19, Chile
4 Instituto de Astronomía, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile
5 International Graduate School of Science and Engineering, Technische Universität München, Boltzmannstr. 17, 85748 Garching bei München, Germany
6 Space Telescope Science Institute, 3700 San Martin Dr, Baltimore, MD 21218, USA
7 Josef-Führer-Straße 33, 80997 München, Germany
8 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
Received: 30 March 2014
Accepted: 9 December 2014
Context. Absorption by molecules in the Earth’s atmosphere strongly affects ground-based astronomical observations. The resulting absorption line strength and shape depend on the highly variable physical state of the atmosphere, i.e. pressure, temperature, and mixing ratio of the different molecules involved. Usually, supplementary observations of so-called telluric standard stars (TSS) are needed to correct for this effect, which is expensive in terms of telescope time. We have developed the software package molecfit to provide synthetic transmission spectra based on parameters obtained by fitting narrow ranges of the observed spectra of scientific objects. These spectra are calculated by means of the radiative transfer code LBLRTM and an atmospheric model. In this way, the telluric absorption correction for suitable objects can be performed without any additional calibration observations of TSS.
Aims. We evaluate the quality of the telluric absorption correction using molecfit with a set of archival ESO-VLT/X-Shooter visible and near-infrared spectra.
Methods. Thanks to the wavelength coverage from the U to the K band, X-Shooter is well suited to investigate the quality of the telluric absorption correction with respect to the observing conditions, the instrumental set-up, input parameters of the code, the signal-to-noise of the input spectrum, and the atmospheric profiles. These investigations are based on two figures of merit, Ioff and Ires, that describe the systematic offsets and the remaining small-scale residuals of the corrections. We also compare the quality of the telluric absorption correction achieved with molecfit to the classical method based on a telluric standard star.
Results. The evaluation of the telluric correction with molecfit shows a convincing removal of atmospheric absorption features. The comparison with the classical method reveals that molecfit performs better because it is not prone to the bad continuum reconstruction, noise, and intrinsic spectral features introduced by the telluric standard star.
Conclusions. Fitted synthetic transmission spectra are an excellent alternative to the correction based on telluric standard stars. Moreover, molecfit offers wide flexibility for adaption to various instruments and observing sites.
Key words: radiative transfer / atmospheric effects / instrumentation: spectrographs / methods: data analysis / methods: numerical / techniques: spectroscopic
© ESO, 2015
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