Observational Artifacts of NuSTAR: Ghost Rays and Stray Light

TL;DR

This paper analyzes the observational artifacts known as ghost rays and stray light in NuSTAR, detailing their characteristics, impact on data, and suggesting ways to mitigate these issues in future X-ray observatories.

Contribution

It provides a detailed analysis and characterization of ghost rays and stray light in NuSTAR, highlighting their effects and proposing prevention strategies for future missions.

Findings

Characterization of ghost rays and stray light in NuSTAR

Impact of these artifacts on observational data

Recommendations for future observatory designs

Abstract

The Nuclear Spectroscopic Telescope Array (NuSTAR), launched in June 2012, flies two conical approximation Wolter-I mirrors at the end of a 10.15m mast. The optics are coated with multilayers of Pt/C and W/Si that operate from 3--80 keV. Since the optical path is not shrouded, aperture stops are used to limit the field of view from background and sources outside the field of view. However, there is still a sliver of sky (~1.0--4.0 degrees) where photons may bypass the optics altogether and fall directly on the detector array. We term these photons Stray-light. Additionally, there are also photons that do not undergo the focused double reflections in the optics and we term these Ghost Rays. We present detailed analysis and characterization of these two components and discuss how they impact observations. Finally, we discuss how they could have been prevented and should be in future observatories.