  Context. Transit events of extrasolar planets offer the opportunity to study
the composition of their atmospheres. Previous work on transmission
spectroscopy of the close-in gas giant TrES-3 b revealed an increase in
absorption towards blue wavelengths of very large amplitude in terms of
atmospheric pressure scale heights, too large to be explained by
Rayleigh-scattering in the planetary atmosphere. Aims. We present a follow-up
study of the optical transmission spectrum of the hot Jupiter TrES-3 b to
investigate the strong increase in opacity towards short wavelengths found by a
previous study. Furthermore, we aim to estimate the effect of stellar spots on
the transmission spectrum. Methods. This work uses previously published long
slit spectroscopy transit data of the Gran Telescopio Canarias (GTC) and
published broad band observations as well as new observations in different
bands from the near-UV to the near-IR, for a homogeneous transit light curve
analysis. Additionally, a long-term photometric monitoring of the TrES-3 host
star was performed. Results. Our newly analysed GTC spectroscopic transit
observations show a slope of much lower amplitude than previous studies. We
conclude from our results the previously reported increasing signal towards
short wavelengths is not intrinsic to the TrES-3 system. Furthermore, the broad
band spectrum favours a flat spectrum. Long-term photometric monitoring rules
out a significant modification of the transmission spectrum by unocculted star
spots.
