  We designed and built a new astronomical photo-polarimeter that can measure
linear polarization simultaneously in three spectral bands. It has a Calcite
beamdisplacement prism as the analyzer. The ordinary and extra-ordinary
emerging beams in each spectral bands are quasi-simultaneously detected by the
same photomultiplier by using a high speed rotating chopper. A rotating
superachromatic Pancharatnam halfwave plate is used to modulate the light
incident on the analyzer. The spectral bands are isolated using appropriate
dichroic and glass filters. We show that the reduction of 50% in the efficiency
of the polarimeter because of the fact that the intensities of the two beams
are measured alternately is partly compensated by the reduced time to be spent
on the observation of the sky background. The use of a beam-displacement prism
as the analyzer completely removes the polarization of background skylight,
which is a major source of error during moonlit nights, especially, in the case
of faint stars. The field trials that were carried out by observing several
polarized and unpolarized stars show the performance of the polarimeter to be
satisfactory.
