  In this chapter we present a brief summary of methods, instruments and
calibration techniques used in modern astronomical polarimetry in the optical
wavelengths. We describe the properties of various polarization devices and
detectors used for optical broadband, imaging and spectropolarimetry, and
discuss their advantages and disadvantages. The necessity of a proper
calibration of the raw polarization data is emphasized and methods of the
determination and subtraction of instrumental polarization are considered. We
also present a few examples of high-precision measurements of optical
polarization of black hole X-ray binaries and massive binary stars made with
our DiPol-2 polarimeter, which allowed us to constrain the sources of optical
emission in black hole X-ray binaries and measure orbital parameters of massive
stellar binaries.

Instrumentation and Methods for Astrophysics;High Energy Astrophysical Phenomena;Solar and Stellar Astrophysics