  Imaging polarimetry is a powerful tool for detecting and characterizing
exoplanets and circumstellar environments. Polarimetry allows a separation of
the light coming from an unpolarized source such as a star and the polarized
source such as a planet or a protoplanetary disk. Future facilities like SPHERE
at the VLT or EPICS at the E-ELT will incorporate imaging polarimetry to detect
exoplanets. The Extreme Polarimeter (ExPo) is a dual-beam imaging polarimeter
that currently can reach contrast ratios of 10^5, enough to characterize
circumstellar environments. We present the data reduction steps for a dual-beam
imaging polarimeter that can reach contrast ratios of 10^5. The data obtained
with ExPo at the William Herschel Telescope (WHT) are analyzed. Instrumental
artifacts and noise sources are discussed for an unpolarized star and for a
protoplanetary disk (AB Aurigae). The combination of fast modulation and
dual-beam techniques allow us to minimize instrumental artifacts. A proper data
processing and alignment of the images is fundamental when dealing with large
contrasts. Imaging polarimetry proves to be a powerful method to resolve
circumstellar environments even without a coronagraph mask or an Adaptive
Optics system.
