  Stray light in X-ray telescopes is a well-known issue. Unlike rays focused
via a double reflection by usual grazing-incidence geometries such as the
Wolter-I, stray rays coming from off-axis sources are reflected only once by
either the parabolic or the hyperbolic segment. Although not focused, stray
light may represent a major source of background and ghost images especially
when observing a field of faint sources in the vicinities of another, more
intense, just outside the field of view of the telescope. The stray light
problem is faced by mounting a pre-collimator in front of the mirror module, in
order to shade a part of the reflective surfaces that may give rise to
singly-reflected rays. Studying the expected stray light impact, and
consequently designing a pre-collimator, is a typical ray-tracing problem,
usually time and computation consuming, especially if we consider that rays
propagate throughout a densely nested structure. This in turn requires one to
pay attention to all the possible obstructions, increasing the complexity of
the simulation. In contrast, approaching the problems of stray light
calculation from an analytical viewpoint largely simplifies the problem, and
may also ease the task of designing an effective pre-collimator. In this work
we expose an analytical formalism that can be used to compute the stray light
in a nested optical module in a fast and effective way, accounting for
obstruction effects.
