  Using STIS - HST observations, we have detected an extended upper atmosphere
in the extrasolar planet HD209458b . The spectra show an absorption of
(15+/-4)% of the stellar Ly-alpha emission line during the planetary transit in
front of the star. This is interpreted as the signature of HI evaporation
taking place around the planet within an extended upper atmosphere
(Vidal-Madjar et al. 2003). We present the data analysis of these observations.
A particular attention is paid to the background correction and the airglow
subtraction.
