  We present estimates of the response to polarized signals by the PLANCK
telescope. These estimates are based on a set of simulations, using a physical
optics code (GRASP8), for linearly polarized detectors at different frequencies
and located in different positions of an early design of the PLANCK focal
plane. We show how the optical aberration introduced by the studied focal plane
configuration affects absolute and relative orientation of the sky signals
polarization planes. In addition, we compute the spurious signal introduced by
the telescope optics into a PLANCK-type measurement of the sky polarization.
Our analysis shows that the spurious polarization expected in a PLANCK-like
experiment is typically of the order of 0.2 % of the unpolarized intensity.
