Introducing the SHell misAlignment Detection for straylight Estimation (SHADE) algorithm: The case of XMM-Newton

¹ Como Lake Center for Astrophysics (CLAP), DiSAT, Università degli Studi dell’Insubria, via Valleggio 11, 22100 Como, Italy
² INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße 1, 85711 Garching, Germany

^★ Corresponding author: simone.piscitelli@inaf.it

Received: 28 April 2025
Accepted: 24 September 2025

Abstract

When performing X-ray observations with a Wolter-I telescope, the presence of bright off-axis sources can introduce unfocused rays, known as stray light, which contaminate the detector and compromise the scientific analysis. Among the different components of stray light, single reflections off the hyperboloid section of the mirror shells often produce arc-like patterns on the detector. These arcs depend on not only the off-axis angle of the source but also the geometrical alignment of the individual shells. In this paper, we introduce the SHell misAlignment Detection for stray light Estimation (SHADE) algorithm, a novel and flexible tool designed to infer the misalignment parameters of individual shells, reproduce the geometry of stray light arcs, and predict its pattern on the detector. SHADE allows us to model each shell displacement with two parameters, (γ, ξ), which represent the tilt amplitude and direction. While the algorithm is general and applicable to any Wolter-like telescope, we demonstrate its effectiveness using a set of XMM-Newton observations of the low-mass X-ray binary GX5-1. As a proof of concept, we recover the best-fit misalignment parameters for a selected shell, obtaining γ = 21.9′′_−9.02^+10.3 and ξ = 5.88_−0.97^+1.02 rad. SHADE represents a new approach to diagnosing mirror misalignments from stray light patterns and can support both pre- and postlaunch calibration efforts and future telescope designs.