An Imaging Polarimeter (IMPOL) has been constructed which has a field of view of about 6.5 for a 1.2 m, f/13 telescope. An off-axis acquisition and guidance unit capable of using stars as faint as is also built in the instrument so that long exposures of faint extended objects like reflection nebulae etc. can be taken while keeping the image fixed on the CCD face to within one half of a pixel - this stability of the image is a necessary condition to achieve a high accuracy in relative photometry between the different frames used to estimate the polarization. Observations of nearby standard polarized and unpolarized stars show that for wideband observations, there is no discernible depolarization and the instrumental polarization is less than 0.05%. Preliminary results of wideband polarimetry of stellar fields, using a PSF fitting technique for determining the centroids of the stellar images and aperture photometry to derive the intensities of the individual images, give close to photon-noise limited accuracy of 0.15% for stars with about an hour of total exposure time. For extended objects with brightness about 20th mag per square arcseconds and a background of about the same brightness, it is estimated that an accuracy of about 1% is possible with 60 minutes of total exposure time and an aperture of 10 sq arcsec.
The project has been funded by the Department of Science and Technology of the Government of India. The authors also wish to thank the Physical Research Laboratory, Ahmedabad for providing telescope time, P.A. Chordia and R. Bedade for their involvement in developing the electronic circuitry for the instrument and M.S. Deshpande for her assistance during its commissioning. Thanks are also due to Dr. A. Saha and an anonymous referee for their valuable comments.