 To measure these various properties, there have been many designs of polarimeters, some archaic and some in current use.The most sensitive are based on interferometers, while more conventional polarimeters are based on arrangements of polarising filters, wave plates or other devices.Polarimetry is used in many areas of astronomy to study physical characteristics of sources including active galactic nuclei and blazars, exoplanets, gas and dust in the interstellar medium, supernovae, gamma-ray bursts, stellar rotation, stellar magnetic fields, debris disks, reflection in binary stars and the cosmic microwave background radiation.