Reconstruction of the NuSTAR PSF Using Single-laser Metrology

Abstract

This paper describes a method by which the metrology system of the Nuclear Spectroscopic Telescope Array (NuSTAR) X-ray space observatory, which uses two lasers to characterize the relative motion of the optics and focal plane benches, can be approximated if one laser fails. The optics and focal plane benches are separated by a ten-meter-long rigid mast that undergoes small amounts of thermal flexing depending on how the spacecraft is illuminated by the Sun. Compensation for this motion is required in order to produce an in-focus image. We analyze the trends of mast motion by observation parameters, using archival NuSTAR data between 2012 and 2021, in order to discover whether parameters such as the solar aspect angle (SAA) can be used to predict the mast motion in future observations. We find that, by using the SAA, observation date, and orbital phase, we can simulate the motion measured by one of the lasers by translating the track produced by the other laser and applying modifications to the mast aspect solution calculated during the NuSTAR data reduction pipeline. With this simulated mast solution, we are able to reconstruct a minimally distorted point spread function for all but two narrow ranges of SAA. We plan to implement the generation of simulated mast files alongside the usual NuSTAR data reduction pipeline for contingency purposes should a laser on the spacecraft fail in the future. This work also has implications for reducing the risk of implementing laser metrology systems on future missions that use deployable masts to achieve the long focal lengths required in high-energy astronomy.