Hard X-ray Focal Plane Detectors onboard the FOXSI-4 sounding rocket for solar flare observation

The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket experiment enables hard-X-ray solar observations with high angular resolution, high energy resolution, and high sensitivity using the direct imaging method. The fourth flight, FOXSI-4, aimed for the first-ever focusing imaging spectroscopic observation of a medium to large solar flare in hard X-rays and launched in spring 2024 as part of NASA's sounding rocket solar flare campaign. For resolving structures in the footpoints and the loop-top of a solar flare, the hard X-ray telescopes, which consist of focal plane detectors and Wolter-I mirrors, are required for high angular resolution of 3 arcseconds with high-count-rate photon detection. We developed wide-gap CdTe Double-sided Strip Detectors (CdTe-DSDs) for the hard X-ray focal plane detector, which achieved an energy resolution of 1 keV (FWHM) and a high position resolution of 30 µm with high detection efficiency. We also developed a new onboard data acquisition (DAQ) system with a Raspberry Pi, an FPGA board SPMU-001, and a SpaceWire interface for controlling all CdTe-DSDs and realizing fast readout of the observation data whose counting rate is estimated to be 5000 counts per second. The observation data is written in a 128 MB data ring buffer region for temporary storage. The software in the Raspberry Pi controls each detector by the commands from a ground-based computer and simultaneously reads the data at 0.6 Mbps for storage in the DAQ. Some essential data for operation, for example, light curves, energy spectrum, and the status of the DAQ system, is sent to the ground-based computer through the onboard control system.