  Well over 700 exoplanets have been detected to date. Only a handful of these
have been observed directly. Direct observation is extremely challenging due to
the small separation and very large contrast involved. Imaging polarimetry
offers a way to decrease the contrast between the unpolarized starlight and the
light that has become linearly polarized after scattering by circumstellar
material. This material can be the dust and debris found in circumstellar
disks, but also the atmosphere or surface of an exoplanet. We present the
design, calibration approach, polarimetric performance and sample observation
results of the Extreme Polarimeter, an imaging polarimeter for the study of
circumstellar environments in scattered light at visible wavelengths. The
polarimeter uses the beam-exchange technique, in which the two orthogonal
polarization states are imaged simultaneously and a polarization modulator
swaps the polarization states of the two beams before the next image is taken.
The instrument currently operates without the aid of Adaptive Optics. To reduce
the effects of atmospheric seeing on the polarimetry, the images are taken at a
frame rate of 35 fps, and large numbers of frames are combined to obtain the
polarization images. Four successful observing runs have been performed using
this instrument at the 4.2 m William Herschel Telescope on La Palma, targeting
young stars with protoplanetary disks as well as evolved stars surrounded by
dusty envelopes. In terms of fractional polarization, the instrument
sensitivity is better than 10^-4. The contrast achieved between the central
star and the circumstellar source is of the order 10^-6. We show that our
calibration approach yields absolute polarization errors below 1%.
