Astronomy Principles and Practice Fourth Edition.pdf

Spectrometry 323
Figure 19.11. A single-beam polarimeter.
19.8.4 Polarimetry
In order to make measurements of polarization in the optical region, the necessary related photometric
determinations (see equation (15.31)) 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 a more 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 19.11. 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.
19.9 Spectrometry
It is sufficient for some spectrophotometric purposes to use colour filters to isolate spectral regions.
These may be simple dye filters, by which a passband is achieved by spectrally selective absorption,
or interference filters, whereby a passband is achieved by interference in thin layers of materials,
with chosen refractive indices and thicknesses, vacuum-deposited on a glass substrate. Dye filters
have transmission efficiencies which are typically 10%, while the efficiency of an interference filter is
much higher, being typically 50%. However, for other spectrophotometric studies, where, for example,
detailed measurements are made on the profiles of individual spectral lines, filters cannot be used to
isolate the wavelengths of interest. Other equipment must be employed, which is capable of looking
at a continuous range of wavelengths, and with spectral passbands much narrower than those provided
by colour filters.
The conventional way of doing this is by using a spectrometer. With this type of device, the
light contained in an image in the focal plane of a telescope is dispersed and a spectrum obtained. By
allowing the spectrum to fall on a photographic plate or a CCD chip, a broad range of wavelengths may