KEYWORDS: Sensors, Data acquisition, Solar processes, Clocks, Spatial resolution, Signal attenuation, Data storage, Control systems, Hard x-rays, Rockets
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.
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.
The FOXSI-4 sounding rocket will fly a significantly upgraded instrument in NASA's first solar are campaign. It will deploy direct X-ray focusing optics which have revolutionized our understanding of astrophysical phenomena. For example, they have allowed NuSTAR to provide X-ray imaging and IXPE (scheduled for launch in 2021) to provide X-ray polarization observations with detectors with higher photon rate capability and greater sensitivity than their predecessors. The FOXSI sounding rocket is the first solar dedicated mission using this method and has demonstrated high sensitivity and improved imaging dynamic range with its three successful flights. Although the building blocks are already in place for a FOXSI satellite instrument, further advances are needed to equip the next generation of solar X-ray explorers. FOXSI-4 will develop and implement higher angular resolution optics/detector pairs to investigate fine spatial structures (both bright and faint) in a solar are. FOXSI-4 will use highly polished electroformed Wolter-I mirrors fabricated at the NASA/Marshall Space Flight Center (MSFC), together with finely pixelated Si CMOS sensors and fine-pitch CdTe strip detectors provided by a collaboration with institutes in Japan. FOXSI-4 will also implement a set of novel perforated attenuators that will enable both the low and high energy spectral components to be observed simultaneously in each pixel, even at the high rates expected from a medium (or large) size solar are. The campaign will take place during one of the Parker Solar Probe (PSP) perihelia, allowing coordination between this spacecraft and other instruments which observe the Sun at different wavelengths.
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