``Classical'' polarimetry consists mainly of 2 classes: photographic imaging polarimetry of low precision (1 %) and modulator/photomultiplier polarimetry, capable of very high precision (better than 0.01 %). Both types of polarimetry have astrophysically relevant applications and there is a continuum of potential applications between these extremes, notably the fairly unexplored technique of spectropolarimetry with good resolution and nominally 0.1 % precision, to which the ISIS system is eminently suited. Because of the number of photons required, relatively little precision polarimetry exists, and what there is, is generally broadband. Array detectors can make an enormous impact on polarimetric practice, if they can be made to work at sufficiently high precision. The WHT, with several instruments in fully computerised systems, can make a very significant polarimetric contribution in many astrophysical areas. ISIS spectropolarimetry is now in operation, FOS has been tried and should serve well, while TAURUS and UES will follow in due course. For polarimetry, all 3 instruments will use CCD detectors, since IPCS has a maximum count rate which is crippling for polarimetry of even moderate precision.
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Figure 1: A polarization modulator.
The photographic imaging polarimeters have developed into electronographic and CCD equivalents which are very successful for relatively faint, highly polarized objects. The modulator/PMT polarimeters are found in many varieties, but are limited to only a few simultaneous channels. A very important achievement would be to devise a modulator/array-detector equivalent; rapid-readout CCDs may be the answer in the near future, but that time has not come yet. Of the LPO instruments, Peoples Photometer and Multi Purpose Fotometer have polarimetric facilities using modulators and are fully described in their User Manuals. See also Fig. 1.
The essential feature of modulator polarimeters is that they, in one way or another, allow measurement of the 2 orthogonally-polarized intensities with one and the same detector within a very short time interval, shorter than any of the time constants of system sensitivity variations; ``system'' here includes atmosphere, telescope, instrument optics and detector. Since detector gain is in general a function of polarization, one must stabilize the polarization of the light striking the detector; this is why the last element of a polarization modulator is always a polarizer (generally a linear polarizer; it is referred to as the ``analyser''). To make the detector see, in light of constant polarization, alternately one and the other orthogonal vibration of the light incident on the polarimeter, waveplates of various kinds are used. Rotation or periodic modification of the retardation of such a waveplate converts incident polarization in a time-varying manner; the analyser converts this into an intensity modulation. Since only linear polarizers exist as single components, it is convenient if the polarization emerging from the modulating element is linear, rotating. Modulators for circular polarization therefore tend to employ quarterwave plates, those for linear polarization halfwave plates. The required modulation rate is dictated by the time constants of the gain variations, those of the atmosphere as seen by the telescope generally being the most important. For a 4-metre telescope, about 10 Hz is expected to be sufficient for all but the very highest precision. When a 2-beam analyser is used, atmospheric noise can be eliminated by a ratio measurement, the modulation speed becomes irrelevant and only discrete states of the modulator are needed; the modulator then becomes a polarization-switching device. For reliable measurements at the very highest accuracy, modulation and a 2-beam analyser are usually incorporated into a single instrument, both techniques contributing to the robustness of the instrument.
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Figure 2: ISIS polarimeter schematic, with representative beam
cross-sections and polarization ellipses.
Since the modulator and analyser convert the incident polarization
into an intensity modulation of light of constant polarization, the
particular state of polarization at the detector is no longer of
interest. See Fig. 2. The polarization effects of gratings, a
well-known scourge of precision spectrophotometry, are
irrelevant in a modulation spectropolarimeter; they cause a
wavelength dependence of system gain (different for the 2 orthogonally
polarized spectra), which is of no interest, unless one also wishes to
use the same data for spectrophotometry, in which case such phenomena
as Wood's anomalies might become important.
This completes the thumbnail sketch of astronomical polarimetry. For more detail consult the references in Appendix I.