TAPAS, a web-based service of atmospheric transmission computation for astronomy

Spectra of astronomical targets acquired from ground-based instruments are affected by the atmospheric transmission. Aims. The authors and their institutes are developing a web-based service⋆, TAPAS (Transmissions Atmosphe riques Personnalise es pour l'AStronomie, or Transmissions of the AtmosPhere for AStromomical data). This service, freely available, is developed and maintained within the atmospheric ETHER data center. Methods. TAPAS computes the atmospheric transmission in the line-of-sight to the target indicated by the user. The user files a request indicating the time, ground location, and either the equatorial coordinates of the target or the zenith angle of the line-of sight (LOS). The actual atmospheric profile (temperature, pressure, humidity, ozone content) at that time and place is retrieved from the ETHER atmospheric data base (from a combination of ECMWF meteorological field and other information), and the atmospheric transmission is computed from LBLRTM software and HITRAN data base for a number of gases: O2, H2O, O3, CO2, CH4, N2O, and Rayleigh extinction. The first purpose of TAPAS output is to allow identifying of observed spectral features having an atmospheric or astrophysical origin. The returned transmission may also serve for characterizing the spectrometer on the wavelength scale and instrument line spectral function (ILSF) by comparing one observed spectrum of an atmospheric feature to the transmission. Finally, the TOA "Top Of Atmosphere" spectrum may be obtained either by division of the observed spectrum by the computed transmission or other techniques developed on purpose. The obtention of transmissions for individual species allows more potentialities and better adjustments to the data. Results. In this paper, we briefly describe the mechanism of computation of the atmospheric transmissions, and we show some results for O2 and H2O atmospheric absorption. The wavelength range is presently 500-2500 nm, but may be extended in the future. Conclusions. It is hoped that this service will help many astronomers in their research. The user may also contribute to the general knowledge of the atmospheric transmission, if he/she finds systematic discrepancies between synthetic transmissions and the observed spectra. This has already happened in the recent past.