Development and evaluation of a metal 3D printed pre-collimator for FOXSI-4 sounding rocket experiment

FOXSI-4, a US-Japan joint-sounding rocket experiment, will be launched in spring 2024, targeting the world's first X-ray focusing-imaging spectroscopy of a large solar flare. FOXSI employs Wolter type-I optics for high dynamic range. However, it reluctantly collects light even from outside of the field of view (FOV). To remove it, we designed a metal 3D-printed pre-collimator with a porous honeycomb structure. The holes are 0.7 mm in diameter, and 190mm deep, whose aspect ratio is 1:270. The wall thickness is 0.09 mm, ensuring an aperture ratio of more than 60% as a goal. According to evaluation tests, the pre-collimator exhibited a 42% aperture ratio and a 1:367 aspect ratio. This aperture ratio was still comparable to the previous successful mission of FOXSI-3, and the FOV adequately covers the expected flare size. Thus, this collimator has enough capability for a flare observation and is adopted as a flight product.