The advances in two dimensional array detector technology in the
optical and near infrared wavelength bands have made new kinds
of imaging astronomical observations feasible. Astronomical polarimetry
is one field which has
gained tremendously from these developments. The limitations of using
aperture photometry for polarimetry were so severe that any serious study
was rendered time consuming and difficult. On the other hand, imaging
polarimetry with its capabilities for multiplexing,
simultaneous sky measurement, seeing-limited resolution etc. offer great
advantages over aperture polarimetry. Astronomers have recognized this
potential and have developed several new observational techniques in the optical
(e.g. Scarrott 1991;
Jannuzi et al. 1993;
Jarrett et al. 1994;
Wolstencroft
et al. 1995;
Simmons et al. 1995) and near infrared
(e.g. Kastner & Weintraub
1994; Moore & Yamashita 1995;
Weintraub et al. 1995) wavelengths to study
phenomena in a variety of Galactic and extragalactic astrophysical objects.
In this paper, we report the design and construction of an imaging polarimeter
(IMPOL) which uses a cooled CCD array as detector. It was developed at the
Inter-University Centre for Astronomy and Astrophysics 
, India.
The principle of the instrument
(Sen & Tandon 1994) is based on a combination
of ideas suggested
by Ohman (1939) and
Appenzeller (1967). An instrument of this type has
been constructed at the University of Durham and
has been in use for some time now
(Scarrott et al. 1983). Section 2 is
a description of the instrument - the different subsections dealing
with various aspects ranging from design guidelines to instrument control
and user-interface. The dominant sources of errors in the measurement are
investigated in Sect. 3, while an estimate of the performance of the
instrument under two typical observing conditions is given in Sect. 4.
In Sect. 5 we discuss the observational procedure and
the data-analysis method. Section 6 contains results obtained during
commissioning of the instrument. The last section (Sect. 7) contains a summary.