  We present the transmission spectra of the hot-Jupiter HD209458b taken with
the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. Our
analysis combines data at two resolutions and applies a complete pixel-by-pixel
limb-darkening correction to fully reveal the spectral line shapes of
atmospheric absorption features. Terrestrial-based Na I and H I contamination
are identified which mask the strong exoplanetary absorption signature in the
Na core, which we find reaches total absorption levels of ~0.11% in a 4.4 Ang
band. The Na spectral line profile is characterized by a wide absorption
profile at the lowest absorption depths, and a sharp transition to a narrow
absorption profile at higher absorption values. The transmission spectra also
shows the presence of an additional absorber at ~6,250 Ang, observed at both
medium and low resolutions. We performed various limb-darkening tests,
including using high precision limb-darkening measurements of the sun to
characterize a general trend of Atlas models to slightly overestimate the
amount of limb-darkening at all wavelengths, likely due to the limitations of
the model's one-dimensional nature. We conclude that, despite these
limitations, Atlas models can still successfully model limb-darkening in high
signal-to-noise transits of solar-type stars, like HD209458, to a high level of
precision over the entire optical regime (3,000-10,000 Ang) at transit phases
between 2nd and 3rd contact.
