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Electronographic polarimetry - The Durham polarimeter
The present investigation is concerned with an instrument for mappingthe linear polarization of extended astronomical objects at opticalwavelengths. The first important studies of this type were based uponthe use of photographic techniques and photoelectric observations.Later, improvements in detector technology have made it possible toobserve fainter objects at higher spatial resolution. A description ispresented of the principles of operation of the Durham Universitypolarimeter which has been extensively used in the observations. Thepolarimeter uses a 40-mm McMullan electronographic camera to providesimultaneous detection at all the image points. The selected approachprovides advantages of increased speed, linearity, resolution, dynamicrange, and storage capacity in comparison to photographic detection.
| New map of the optical polarisation of galaxy M82
Observations of the linear optical polarization of the galaxy M82 wereobtained during the period Feb. 27-Mar. 15, 1975 using a polarimetrictechique derived from that of Ohman (1939). A rotating achromatichalf-wave plate and a Wollaston prism were used to separate orthogonallypolarized beams. The optical polarization map shows a centrallysymmetric pattern which permits the derivation of the position of thecentral source of radiation. A singular nucleus could not be identified;there is no detailed correspondence between the various central featuresin M82. The most intense radio component has no special position in theinfrared source, which is not coincident with the center of symmetry.The optically brightest spot is close to the center of the infraredsource but not to the other features. It is suggested that galaxies ofthe type of M82 might represent a connecting link between Seyferts andnormal galaxies.
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