Transmission of the AtmosPhere for AStronomical data: TAPAS upgrade

¹ LIRA, Observatoire de Paris, PSL University, CNRS, 5 Place Jules Janssen, 92190 Meudon, France
² LATMOS, Sorbonne-Université, Paris, France
³ ACRI-ST, 260 Route du Pin Montard, BP234, 06904 Sophia-Antipolis, France
⁴ Institut Pierre Simon Laplace (IPSL), AERIS Data Centre, 75252 Paris, France
⁵ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile

^★ Corresponding author: rosine.lallement@obspm.fr

Received: 3 April 2025
Accepted: 1 August 2025

Abstract

Context. Current molecular databases and realistic global atmospheric models allow us to predict accurate atmospheric transmittance spectra. Observers with ground-based spectrographs may use this information to identify the telluric absorption lines, to correct their astronomical spectra for these lines fully or partially, or take them into account in forward models.

Aims. The TAPAS online service provides atmospheric transmittance spectra of the most important species, as well as Rayleigh extinction, adapted to any observing location, date, and direction. We describe recent updates, improvements, and additional tools.

Methods. TAPAS interpolates the location in the atmospheric profiles of temperature, pressure, H₂O, O₂ and O₃ that are extracted from the meteorological field of the European Centre for Medium Term Weather Forecast (ECMWF) for the date and time of the observation. The composite profiles are produced by a Data Terra/AERIS/ESPRI product called Arletty, and they are supplemented by auxiliary climatological models for additional species. The transmittance spectra are computed with the code LBLRTM. The default width of the spectral pixels is chosen to ensure that the shapes of all the absorption lines are reproduced for each species. Major improvements with respect to the previous TAPAS are the extension of the wavelength range in the near-UV down to 300 nm and the extension in the near-IR up to 3500 nm; the use of the recent version of the HITRAN database (HITRAN2020); the addition of NO₂ transmittance to complement H₂O, O₂, O₃, N₂O, CO₂, and CH₄; an increased accessibility and a reduced time to obtain the results; and the possibility to force the total H₂O column to match the column measured at the observatory at the time of record.

Results. We show O₃ absorption in the near-UV and near-IR and NO₂ absorption in the visible. We illustrate the quality of TAPAS by means of comparisons between models and ESO/VLT/CRIRES recorded spectra of a hot star with a spectral resolution of ~130 000 in two intervals in the near-IR with strong H₂O, N₂O, CO₂, and CH₄ absorption. We describe the measurement of an instrumental line spread function based on TAPAS O₂ lines and a method using the singular value decomposition technique that can be made entirely automated.

Conclusions. The new TAPAS tool provides realistic simulations of the telluric lines. It gives access to the weakest H₂O or O₂ lines, and to the very weak highly irregular NO₂ lines. It can be used to improve the wavelength assignment when calibration lamps provide only a few emission lines, and to accurately measure the line spread function in most regions in which telluric features are present. The extended wavelength range will be particularly useful for future or recent spectrographs in the near-UV and in the near-IR.