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FOXSI-4, the fourth flight of a US-Japan joint-sounding rocket experiment FOXSI, launched successfully in April 2024 and conducted the world’s first X-ray focusing-imaging spectroscopy of a large solar flare. It aims to reveal the mechanism of releasing and conversing magnetic energy caused by magnetic reconnection. FOXSI used high-precision Wolter type-I optics to directly focus X-rays, thus ensuring a high dynamic range. However, light entering from outside the field of view (13’ away from the on-axis position in FOXSI-4) can reach the focal plane, called stray light. For removing the stray light, FOXSI was equipped with a pre-collimator. FOXSI-4’s one has a porous honeycomb structure with a hole diameter of 0.7 mm and hole depth of 190 mm, whose aspect ratio is 1:270, thereby removing stray light. The wall thickness of the honeycomb structure is set at 0.09 mm, ensuring an aperture ratio of more than 60 % as a goal. This performance is very important, especially for analyzing dark regions (e.g., magnetic diffusion regions). Since it is hard to manufacture the required high aspect ratio of 1:270 by a machining process, we used a metal 3D printer. The performance of the pre-collimator mounted on the FOXSI-4 optics was evaluated with the parallel X-ray light at the Straylight Test Facility in NASA/MSFC. The evaluated aperture ratio and aspect ratio were 42 % and 1:370, respectively. Though the aperture ratio was smaller than the designed one, it was almost the same as that of FOXSI-3 which collected enough X-ray photons (more than 10M photons). While the aspect ratio was higher than the designed one, the corresponding field of view such as 550 arcsec could adequately cover the entire flaring region. Thus, this collimator had enough capability for a flare observation and was adopted as a flight product for FOXSI-4.
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