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

Analytical computation of stray light in nested mirror modules for X-ray telescopes