  We have measured a precise optical transmission spectrum for WASP-52b, a
highly inflated hot Jupiter with an equilibrium temperature of 1300 K. Two
transits of the planet were observed spectroscopically at low resolution with
the auxiliary-port camera (ACAM) on the William Herschel Telescope (WHT),
covering a wide range of 4000-8750 \AA. We use a Gaussian process approach to
model the correlated noise in the multi-wavelength light curves, resulting in a
high precision relative transmission spectrum with errors on the order of a
pressure scale height. We attempted to fit a variety of different
representative model atmospheres to the transmission spectrum, but did not find
a satisfactory match to the entire spectral range. For the majority of the
covered wavelength range (4000-7750 \AA) the spectrum is flat, and can be
explained by an optically thick and grey cloud layer at 0.1 mbar, but this is
inconsistent with a slightly deeper transit at wavelengths $> 7750$ \AA. We
were not able to find an obvious systematic source for this feature, so this
opacity may be the result of an additional unknown absorber.
