  We present an optical to near-infrared transmission spectrum of the hot
Jupiter HAT-P-1b, based on HST observations, covering the spectral regime from
0.29 to 1.027{\mu}m with STIS, which is coupled with a recent WFC3 transit
(1.087 to 1.687{\mu}m). We derive refined physical parameters of the HAT-P-1
system, including an improved orbital ephemeris. The transmission spectrum
shows a strong absorption signature shortward of 0.55{\mu}m, with a strong
blueward slope into the near-ultraviolet. We detect atmospheric sodium
absorption at a 3.3{\sigma} significance level, but find no evidence for the
potassium feature. The red data implies a marginally flat spectrum with a
tentative absorption enhancement at wavelength longer than ~0.85{\mu}m. The
STIS and WFC3 spectra differ significantly in absolute radius level (4.3 +/-
1.6 pressure scale heights), implying strong optical absorption in the
atmosphere of HAT-P-1b. The optical to near-infrared difference cannot be
explained by stellar activity, as simulta- neous stellar activity monitoring of
the G0V HAT-P-1b host star and its identical companion show no significant
activity that could explain the result. We compare the complete STIS and WFC3
transmission spectrum with theoretical atmospheric mod- els which include haze,
sodium and an extra optical absorber. We find that both an optical absorber and
a super-solar sodium to water abundance ratio might be a sce- nario explaining
the HAT-P-1b observations. Our results suggest that strong optical absorbers
may be a dominant atmospheric feature in some hot Jupiter exoplanets.
