Image photometry: Polarimetry
 
In order to make measurements of polarization in the optical region, the necessary related photometric determinations need to be made with high accuracy. Most precision polarimeters, therefore, make use of photoelectric detectors and, as a consequence, they are able to make measurements of only one star at a time. There are several designs of photographic polarimeter which allow star field surveys to be undertaken but the results from individual stars are of inferior quality in comparison with the photoelectric measurements.
In the optical region, the amount of polarization in the light from an astronomical object is usually very small and great care is needed to avoid the introduction of systematic errors. In principle, the polarization can be measured by rotating polarization-sensitive optical elements in the beam, prior to the detector. The simplest design would use a piece of Polaroid in the beam and would require the signal output to be recorded as the rotational setting of the Polaroid is adjusted. A more elegant version would use amore efficient polarizer in the form of a birefringent prism. The improvement might require the use of an extra lens to provide a collimated beam for the prism. The basic elements of this type of polarimeter are illustrated in figure 1. Non-polarizing colour filters may be used to limit the spectral passband. Calibration filters may also be available so that known amounts of polarization may be added and the instrument’s response checked. From the experiences of various observers, catalogues of stars exhibiting zero polarization and others with accurately determined values have been established for reference measurement and these are constantly under review.

Figure 1. A single-beam polarimeter.