Linear polarimetry of extended objects

For extended objects it is possible to do imaging linear polarimetry through a number of polaroids mounted at different rotation angles in the ALFOSC filter or grism wheel. A minimum of 3 angles are needed: 0°, 45° and 90°, but the optimum is 4. This mode is sensitive to variations in the sky transparency, and observations should be made only in photometric conditions. Also, a sufficient number of repeated cycles are needed to verify the stability of the results. The polaroids produce negligible focus offsets and no field vignetting. But the efficiency is low because of total absorption of one of the linearly polarized components.