  In this paper we analyze the image formation problem for undulator radiation
through an optical system, accounting for the influence of the electron beam
emittance. On the one hand, image formation with Synchrotron Radiation is
governed by the laws of Statistical Optics. On the other hand, the widely used
Gaussian-Shell model cannot be applied to describe the coherence properties of
X-ray beams from third generation Synchrotron Radiation sources. As a result, a
more rigorous analysis of coherence properties is required. We propose a
technique to explicitly calculate the cross-spectral density of an undulator
source, that we subsequently propagate through an optical imaging system. At
first we focus on the case of an ideal lens with a non-limiting pupil aperture.
Our theory, which makes consistent use of dimensionless analysis, also allows
treatment and physical understanding of many asymptotes of the parameter space,
together with their applicability region. Particular emphasis is given to the
asymptotic situation when the horizontal emittance is much larger than the
radiation wavelength, which is relevant for third generation Synchrotron
Radiation sources. First principle calculations of undulator radiation
characteristics (i.e. ten-dimensional integrals) are then reduced to
one-dimensional convolutions of analytical functions with universal functions
specific for undulator radiation sources. We also consider the imaging problem
for a non-ideal lens in presence of aberrations and a limiting pupil aperture,
which increases the dimension of the convolution from one to three. In
particular we give emphasis to cases when the intensity at the observation
plane can be presented as a convolution of an impulse response function and the
intensity from an ideal lens.
