A novel quadruple beam imaging polarimeter and its application to Comet Tanaka-Machholz 1992 X

For observations of solar system objects an imaging polarimeter has been constructed as an auxiliary instrument for a f/15 focal reducer. With this instrument simultaneous measurement of the Q and U Stokes parameters is possible. It contains no moving parts such as a moving lambda/2 retarder. The polarizer consists of two Wollaston prisms which are combined to form a single optical element. Their polarization angles differ by 45 degrees. This compound polarizer is placed in the exit pupil of the afocal telescope-collimator system of the focal reducer and splits the exit pupil into halves. In this way four polarized beams with E-vector orientations 0 degrees, 90 degrees, 45 degrees, and 135 degrees emerge from the exit pupil. These are intercepted by the camera lens of the focal reducer system and imaged simultaneously on the CCD detector. With a properly designed field mask at the Cassegrain focus, the four beams are imaged without overlap. As a demonstration of the capabilities of this quadruple-beam Wollaston-prism polarimeter, we present observations of comet Tanaka-Machholz 1992 X in May 1992. The advantages, shortcomings, and possible improvements of the instrument are discussed.