Detecting polarization of the light reflected from an exoplanet requires
extremely high-precision polarimeters and highaccuracy calibration
techniques. The polarimetric precision of a few parts per million (ppm),
approaching the photon noise, was demonstrated for the Sun and bright
distant stars by several groups and instruments. However, the accuracy
of absolute polarimetric calibration strongly depends on the polarimeter
design and observing conditions, which results in largely unknown
systematic errors hindering the exoplanet polarization detection. Here
we discuss some of the crucial aspects of exoplanet polarimetric data
acquisition, e.g., effects of seeing, sky polarization, telescope
polarization, etc. We simulate examples of polarimetric measurements
with various levels of random and systematic errors. They demonstrate
that sparse measurements (ten or less) and unknown systematic errors can
hinder exoplanet signal detection even when the signal is significantly
larger than the polarimetric precision. We discuss various approaches
which help improve random errors (precision) and mitigate systematic
errors (accuracy) caused by various effects. We also discuss the
performance of polarimeters with different designs and indicate their
strengths and weaknesses in terms of precision and accuracy.