  The interest in using optical transition radiation (OTR) in high energy
(multiGeV) beam diagnostics has motivated theoretical and experimental
investigations on the limitations brought by diffraction on the attainable
resolution. This paper presents calculations of the diffraction effects in an
optical set-up using OTR. The OTR diffraction pattern in a telescopic system is
calculated taking into account the radial polarization of OTR. The obtained
diffraction pattern is compared to the patterns obtained by other authors and
the effects of different parameters on the shape and on the size of the OTR
diffraction pattern are studied. The major role played by the radial
polarization on the shape of the diffraction pattern is outlined. An
alternative method to calculate the OTR diffraction pattern is also sketched
