  A new astronomical photo-polarimeter that can measure linear polarization of
point sources simultaneously in three spectral bands was designed and built in
Indian Institute of Astrophysics. The polarimeter has a Calcite
beam-displacement prism as the analyzer. The ordinary and extra-ordinary
emerging beams in each spectral band are quasi-simultaneously detected by the
same photomultiplier by using a high speed rotating chopper. The effective
chopping frequency can be set to as high as 200 Hz. 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. A detailed analysis shows 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 position angle of polarization
produced by the Glan-Taylor prism in the light path is found to be slightly
wavelength dependent, indicating that the fixed super-achromatic halfwave plate
in the beam does not fully compensate for the variation in the position angle
of the effective optical axis of the rotating plate. However, the total
amplitude of variation in the U-I spectral region is only 0.92 degree. The
polarization efficiency is also found to be wavelength-dependent with a total
amplitude of 0.271% in the U-I region; its mean value is 99.211%. The
instrumental polarization is found to be very low. It is nearly constant in the
V-I spectral region (~0.04%), and apparently, it increases slightly towards the
ultraviolet. The observations of polarized stars show that the agreement
between the measured polarization values and those available in the literature
to be excellent.
