We have been developing our original space X-ray optics by combining space- and ground-based technologies for a solar sounding rocket experiment, FOXSI-4. We completed two Flight Models (FM), soft and hard X-ray optics. Vibration tolerance is one of the most important launch environmental tolerance and thus we established our experimental setup for the two FMs. The imaging quality was evaluated before and after the vibration tests. As a result, no significant changes were detected. In addition, we checked the effect of shims needed for the angle alignment between the optics and detectors integrated into the sounding rocket. We confirmed that the shims do not affect the angle alignment significantly before and after vibration tests.
KEYWORDS: X-ray optics, X-rays, Rockets, Calibration, Solar processes, Mirrors, Solar radiation models, Point spread functions, Hard x-rays, X-ray imaging
We have been developing X-ray optics for a fourth FOXSI (Focusing Optics X-ray Solar Imager) sounding rocket experiment, FOXSI-4. We fabricated two types of X-ray optics, soft X-ray optics (SXR) and hard X-ray optics (HXR), specified for soft and hard X-ray observations. We conducted X-ray irradiation tests to evaluate X-ray performances such as angular resolution and effective area. For the Flight Model (FM) SXR, the resultant angular resolution in Half Power Diameter (HPD) is 16 ± 2 / 14 ± 2 arcseconds at 2.7 / 4.5 keV for the best images, respectively and no significant energy dependence in HPD is seen. The observed effective area was reproduced within an accuracy of 10% by using our original ray-tracing simulator considering all of the SXR components in the 1.5-16 keV range at the smallest off-axis angle. Subsequently, both FM mirror module assemblies were integrated into a sounding rocket in combination with detectors and the vibration tests were performed for the whole sounding rocket system. Finally, FOXSI-4 was successfully launched on April 17, 2024 (UTC), and the sounding rocket and the data were also recovered successfully.
We have been developing high-resolution space X-ray optics by using high-precision electroforming technology which has been refined through the development of ground-based X-ray focusing systems. Our optics have been on board the Japan-U.S. joint NASA sounding rocket experiment, the 4th Focusing Optics X-ray Solar Imager (FOXSI-4), which succeeded in the world's first X-ray imaging and spectroscopic observation of solar flares. So far, we have conducted ground calibration tests on these onboard items. We have been building our own ray-tracing simulator in order to model the data and finally construct a response function which is essential for obtaining physical parameters in imaging and spectroscopic analysis accurately. In this paper, we modeled the uncertainty of the reflection angle due to the surface shape error in the axial direction and implemented it in this simulator for the first time. As a result, we succeeded in reproducing the point spread function up to ~50 arcsec assuming a Cauchy distribution and confirmed the consistency between simulations and the data in terms of FWHM and HPD for the on-axis direction. Additionally, we optimized parameters of a Cauchy distribution also for the off-axis data and investigated the dependence of the parameters on the off-axis angles.
The first three flights of the Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket established the usefulness and feasibility of direct-focusing hard X-ray instruments optimized for the Sun. While the fundamental building blocks of this concept are ready for a spacecraft mission, concurrent development is required to prepare for a subsequent generation of high-energy solar explorers, which will require higher rates and even better angular resolution. The fourth flight of FOXSI features technological advances for high resolution and high rate capability. We are developing high-precision mirror production methods, substrip/subpixel resolution in fine-pitch CdTe sensors, and novel pixelated attenuators (that optimize energy coverage even at high rates). These technologies will be demonstrated in NASA’s first-ever solar flare campaign in March 2024. Multiple payloads will be launched during a solar flare, supporting Parker Solar Probe observations during one of its perihelia.
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