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

TL;DR

This paper presents an analytical method to efficiently compute stray light in nested X-ray telescope mirror modules, accounting for obstructions, which simplifies the design of pre-collimators to reduce background noise.

Contribution

The work introduces a novel analytical formalism for calculating stray light in nested X-ray telescope mirrors, reducing reliance on time-consuming ray-tracing simulations.

Findings

Analytical formalism accurately predicts stray light levels.

Method accounts for obstruction effects in nested modules.

Simplifies the design process for stray light mitigation.

Abstract

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.