Usually, a further two exposures are taken at half-wave plate positions of 45 degrees and 67.5 degrees (referred to here as exposures and ). These provide some redundancy in the data and enable internal consistency checks to be made during the data reduction stage.
These exposures are denoted by the letter to indicate that they are target exposures. In addition to these target exposures, some flat-field exposures are also required. These are used to correct for any spatial variation in the sensitivity of the system, and consist of exposures of a photometrically flat surface. Ideally, the flat-field source should be unpolarized. However, if the additional target exposures and are taken, then a spatially constant polarization across the flat-field source can be corrected for during data reduction. Since the polarization of the flat-field surface is rarely known to be zero, these additional target exposures should always be taken.
It is important that the flat-field has a good signal-to-noise ratio.
For this reason, it is common practice to take one or more flat-field
exposures at each half-wave plate position, and stack them together into a
single master flat-field (although it is not strictly necessary to use
different half-wave plate positions). Further discussion of the flat-fielding
procedure is given here.