Investigation of Stray‐Light Patterns in the eROSITA X‐Ray Cameras

ABSTRACT eROSITA (extended ROentgen Survey with an Imaging Telescope Array) is the soft X‐ray instrument aboard the Spectrum‐Roentgen‐Gamma (SRG) mission, developed under the responsibility of the Max Planck Institute for Extraterrestrial Physics (MPE). It was launched on 13 July 2019 and operates at the second Lagrangian point (L 2 ) of the Earth–Sun system, 1.5 million km from Earth in the anti‐Sun direction. Following commissioning and performance verification, eROSITA began its all‐sky survey in December 2019, completing four full scans. During early mission phases, an unexpected phenomenon appeared: visible‐light contamination in two of the seven telescope modules (TM5 and TM7). The observed patterns depended on spacecraft orientation relative to the Sun and occurred even with the filter wheel in the closed position, where the camera should be fully shielded. As no light leak was anticipated, an investigation was initiated to identify the source and propagation path of the contamination. This paper presents a systematic reverse‐engineering approach to determine the origin of the light leak. Theoretical modelling of potential light paths was complemented by vacuum testing with a spare camera under representative conditions. The results provide a plausible explanation for the observed patterns and offer recommendations to mitigate similar issues in future X‐ray instruments.