Description
An undesired property of Wolter telescopes with a large field-of-view is that, in addition to the double reflection that leads to the actual image, also single reflections occur on the parabolic and in particular on the hyperbolic part of the mirror. These come from off-axis light sources whose off-axis angle is approximately twice the angle of inclination of the paraboloidal surface w.r.t the optical axis. In a nested mirror system, there is a variety of different inclination angles, so that the angular range of off-axis light sources that generate single reflections is correspondingly large. With eROSITA, this angular range extends approximately from 0.5 to 3 degrees off-axis. About 90% of the single reflections are shielded by the X-ray baffle. The remaining ones contribute to the background. However, bright sources create recognizable arcuate structures in the field-of-view. These structures are analyzed on the basis of selected pointing observations and compared with ray tracing simulations.
