The third flight of the FOXSI rocket: Performance and results

The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket payload is scheduled to fly a third time on August 22, 2018 as part of the Low-Cost Access to Space (LCAS) program of NASA. FOXSI is the first solar dedicated telescope that utilizes direct focusing optics, together with high-sensitivity semiconductor detectors, to observe the Sun in hard X-rays. This combination makes FOXSI an instrument with higher sensitivity and better dynamic range than any other solar hard X-ray imager of the past.

FOXSI implements Wolter-I figured mirrors nested together in seven optical modules, each with 7-10 mirrors.. On-axis X-rays reflect twice on a Wolter-I mirror to then reach the focal plane in focus, where the FOXSI detectors are located. However, some off-axis rays can reflect only once and still have a path to the FOXSI detectors. These singly reflected rays are known as ghost rays, and if not addressed correctly, they can affect the sensitivity of the instrument for some scientific targets. For the third flight, a combination of honeycomb type collimators and optimized central blockers were attached to the FOXSI optics to reduce ghost rays. Also, a set of two brand new, cutting-edge CdTe strip detectors for hard X-rays and a CMOS detector customized for soft X-rays were included in the payload. These upgrades enhanced the overall performance of the instrument by reducing stray light reaching the focal plane and by raising the sensitivity of observations at higher and lower energies.

We present an overview of the performance of the FOXSI experiment during its third flight, as well as preliminary outcomes of the scientific analysis. We emphasize the ray tracing simulation conducted to characterize ghost rays and the effect of the upgrades mentioned above on observations of the targets we plan to aim during the third FOXSI campaign.