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8 Polarimetric Principles and Instruments
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81 Introduction
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Polarization phenomena are related to orientational qualities of the electromagnetic waves making up a beam of radiation In order to explore these and to make quantitative measurements of the polarization, a modulator and detector system requires to be tted in an instrument for the attachment to a telescope In most cases, some of the included optical elements need to be rotatable in order to tease out the polarimetric information Not all polarimeters are designed to measure simultaneously all the Stokes parameters of the collected light By far the majority of polarimetry concerns the measurement of the linear parameters: fQ, Ug, or fq, ug, or fp, g Most instruments are speci cally designed to do so using a modulator that alters the ux passed to the detector according to the strength of Q and U in the collected radiation The values of the parameters describing the linear polarization are calculated from measurements of the modulations in the recorded signal produced by the detector The circular parameter, V or v, may be measured using an instrument designed for linear polarization studies, but with the appropriate insertion of a quarter-wave plate prior to the modulator Other designs are immediately better suited for measuring V or v, alone in isolation Again, these latter instruments can be adapted to measure the linear parameters by the addition of a quarter-wave plate prior to the modulator Occasionally, instruments are designed to measure both the linear and circular components simultaneously In any instrument, particularly as higher and higher accuracy is demanded, care must be taken to check out any deleterious effects of cross-talk between the various Stokes parameters This is particularly important when measuring circular polarization, as it is usually very small, and is subject to contamination by linear-to-circular conversions within the instrument Also, the very requirement of having rotatable elements may in itself be a source of spurious signals, or an instrumental polarization, as a result of displacements of the light beams The effects of such disturbances must be calibrated out and compensation applied The last element of any modulator system is a polarizer and, as a consequence, the subsequent optical train receives radiation which is completely polarized Any polarizing properties of the later optics are irrelevant in terms of biasing or distortStellar Polarimetry David Clarke Copyright 2010 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim ISBN: 978-3-527-40895-5
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Stellar P larimetry
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Fig 81 A sketch of a basic double-beam polarimeter of the late 1950s era (Communication to the author by W Hiltner in 1959)
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ing the measurement of the original collected radiation The relevant orientations of the various optical parts in the overall train are important, however, in determining the ux level arriving at the detector and in controlling the strength of the output signal As discussed earlier (Section 73), in reference to instrumental polarization associated with the telescope, any optics can introduce distortion to the measurement and for this reason the modulator is placed early in the system, usually immediately after the eld stop The basic parts of the simplest kind of double-beam photoelectric polarimeter are sketched in Figure 81 In general, a modulator comprises either rotatable elements or devices whose retardation is adjustable by some controlled external stimulus, or a combination of both For those instruments employing rotatable components, the modulation may be achieved by continuous rotation, or by adjustment to click-stop preferred orientations Just as in the discipline of spectrometry, with its plethora of optical systems, various kinds of polarimetric elements have been incorporated within instruments, and a range of techniques has developed for determining the Stokes parameters of any collected radiation A simple descriptive overview, although not complete, has been provided by Oetken (1970) A survey of techniques, polarimetric devices and instruments has also been made by Serkowski (1974a, 1974b) While comparing the performance and different designs of astronomical polarimeters, Cox (1983) has used the concept of information capacity
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