Polarimetry is a powerful technique to study the ongoing astrophysical processes in celestial objects. When polarimetry is conducted for a single point object, we normally place the object on the axis of the telescope (i.e. the field angle of the object is zero). As each unpolarized ray of light falls on the metal coated mirror surface, it gets polarized due to oblique incidence. The ray after being reflected from the primary mirror also gets reflected at the secondary mirror and thereby the polarization state of the ray gets complicated with a mixture of linear and circular polarization. But if the object is on the axis of the telescope, we have all the rays incident on primary parallel to the telescope axis. Thereby, in case of prime focus (or Cassegrain focus), we will have a total circular symmetry for these rays and the net polarization effect for all the rays considered together will be zero. Thus the instrumental polarization will always be zero for an on-axis object point due to the above mechanism.
However, for an off-axis object point, the field angle will produce some finite value of instrumental polarization. When the field angle is different from zero, the Stokes parameters Q, U and V for the rays for an unpolarized star, add up to give a non-zero instrumental polarization effect. Actually compared to the other sources of instrumental polarization, this effect produces polarization values too small to be detected by any polarimeter. Also objects are normally not observed off-axis and thereby such effects are normally ignored.
As discussed in detail by Serkowski (1974), in polarimetry there are mainly
two kinds of errors: (i) the uncertainty (or noise) (
) in the
estimated values of polarization due to photon count statistics, which is in
general 
(Sen et al. 1990); where 
is the
uncertainty in measured intensity (I) and (ii) "instrumental polarization''
arising due to the polarimeter optics, which is mostly a systematic error.
However, such an instrumental polarization can also arise due to the
telescope optics itself and will be discussed in detail in this paper.
The "instrumental polarization'' due to the polarimeter can be caused by the
chromaticity and incidence angle dependent performance of the optical components
(like polarizers, analyzers etc.) and also unnecessary reflection from such
components (Serkowski 1974). In addition, errors in polarization measurements
can also be due to the varying sky background. Unless very bright objects are
observed, during polarimetry one normally gets photon noise limited polarization
values. Typically a present day polarimeter (Sen & Tandon 1994;
Rampraksh
et al. 1996) can give 
due to the polarimeter optics
and 
due to photon noise, when a 17.5 mag (arcsec)-2
source is observed for 1000 s.
These days, imaging polarimetry (or area scan polarimetry) is emerging as a new area of observational astronomy. Quite a good amount of work has been done in this area over the last two decades. The Durham University group with their imaging polarimeter sometimes cover a field angle up to 1 arcmin (Scarrott et al. 1983, 1991). Similarly, astronomers from MPIK, Germany have used their polarimeter to cover field angles as large as 1 arcmin (Röser 1981; Röser & Meisenheimer 1986; Schlötelburg et al. 1988). Other imaging polarimetry works can also be mentioned in these connections. Renard et al. (1992) have covered a field angle up to 30 arcsec in their measurement of comet Levy. Sen et al. (1990) in their imaging polarimetry work on comet P/Halley have covered a field angle as large as 10 arcmin. Also astronomers are increasingly using larger formats for their CCDs. So if we take a CCD of 20 mm dimension and assume a plate scale 30 arcsec/mm for prime focus and 10 arcsec/mm for Cassegrain focus; the maximum field angle that an off-axis star will cover will be 300 and 100 arcsec, respectively.
Thus, it is important to estimate such instrumental polarization values and to understand their nature, however small their effects may be at the present stage. In this paper we derive a procedure for estimating such instrumental polarization values and also calculate them in the actual case of a 2.3 m telescope having beam sizes f/3.23 and f/13 at the prime and Cassegrain focii respectively. For the simplicity of calculations we shall limit our discussions to the effects on linear polarization only.