A full polarization measurement takes four exposures. If the source is variable (in polarization) within the time it takes to complete one full polarization measurement, the results will come out wrong. If, however, the instrumental gain ratio is known and stable in time (see Section 3.2), two exposures (with the halfwave plate at 0 and 22.5 degrees) are sufficient to derive the polarization vector. The gain ratio can be readily derived from a full polarization measurement (i.e. four settings of the halfwave plate) of a constant source. Tests have shown that this gain ratio (averaged over the full Dekker aperture) was stable to within 1 % during a night, except in the cutoff region of the dichroic. Do not expect the gain ratio to be the same after moving any of the optics, since the throughput of many of the optical components depends strongly on polarization and the beams within the spectrograph are highly polarized.