  Spectra of astronomical targets acquired from ground-based instruments are
affected by the atmospheric transmission. The authors and their institutes are
developing a web-based service, TAPAS (Transmissions of the AtmosPhere for
AStromomical data). This service, freely available, is developed and maintained
within the atmospheric ETHER data center. 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 informations), and the
atmospheric transmission is computed from LBLRTM software and HITRAN data base
for a number of gases: O2, H2O, O3, CO2, and Rayleigh extinction. The first
purpose of TAPAS output is to allow identification of observed spectral
features as being from atmospheric or astrophysical origin. The returned
transmission may also serve for characterizing the spectrometer in 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. In this paper, we describe
briefly 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. The address is http://ether.ipsl.jussieu.fr/tapas/
