Molecfit: A general tool for telluric absorption correction

I. Method and application to ESO instruments ^⋆,⋆⋆

¹ European Southern Observatory, Casilla 19001, Alonso de Cordova 3107 Vitacura, Santiago, Chile
e-mail: asmette@eso.org
² now at ESA/Space Telescope Science Institute, 3700 San Martin Dr, Baltimore, MD 21218, USA
³ Institute for Astro and Particle Physics, Universität Innsbruck, Technikerstrasse 25, 6020 Innsbruck, Austria
⁴ Josef-Führer-Straße 33, 80997 München, Germany
⁵ University of Vienna, Department of Astrophysics, Türkenschanzstr. 17 (Sternwarte), 1180 Vienna, Austria
⁶ Instituto de Astronomía, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile
⁷ International Graduate School of Science and Engineering, Technische Universität München, Boltzmannstr. 17, 85748 Garching bei München, Germany
⁸ Universidad de Concepción, Casilla 160-C, Concepción, Chile
⁹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany

Received: 2 April 2014
Accepted: 23 December 2014

Abstract

Context. The interaction of the light from astronomical objects with the constituents of the Earth’s atmosphere leads to the formation of telluric absorption lines in ground-based collected spectra. Correcting for these lines, mostly affecting the red and infrared region of the spectrum, usually relies on observations of specific stars obtained close in time and airmass to the science targets, therefore using precious observing time.

Aims. We present molecfit, a tool to correct for telluric absorption lines based on synthetic modelling of the Earth’s atmospheric transmission. Molecfit is versatile and can be used with data obtained with various ground-based telescopes and instruments.

Methods. Molecfit combines a publicly available radiative transfer code, a molecular line database, atmospheric profiles, and various kernels to model the instrument line spread function. The atmospheric profiles are created by merging a standard atmospheric profile representative of a given observatory’s climate, of local meteorological data, and of dynamically retrieved altitude profiles for temperature, pressure, and humidity. We discuss the various ingredients of the method, its applicability, and its limitations. We also show examples of telluric line correction on spectra obtained with a suite of ESO Very Large Telescope (VLT) instruments.

Results. Compared to previous similar tools, molecfit takes the best results for temperature, pressure, and humidity in the atmosphere above the observatory into account. As a result, the standard deviation of the residuals after correction of unsaturated telluric lines is frequently better than 2% of the continuum.

Conclusions. Molecfit is able to accurately model and correct for telluric lines over a broad range of wavelengths and spectral resolutions. The accuracy reached is comparable to or better than the typical accuracy achieved using a telluric standard star observation. The availability of such a general tool for telluric absorption correction may improve future observational and analysing strategies, as well as empower users of archival data.