The Observational Procedure

At least three exposures with different analyser or half-wave plate positions are required to estimate the degree and orientation of the polarization. However, if you can, you should take some extra analyser positions. Not only does this reduce the noise, but it also allows consistency checks to be performed, since the recorded intensity should be a sinusoidal function of analyser or half-wave plate position. POLPACK provides facilities to reject data values which are more than a given distance from the best fitting sine curve (more details here).