The Focusing Optics X-ray Solar Imager (FOXSI) is a NASA Low Cost Access to Space sounding rocket payload for studying faint hard X-ray (HXR) emission from the quiet Sun and flares.

Solar HXR measurements are important for investigating flare particle acceleration. However, imaging faint nonthermal HXR requires a larger sensitivity and dynamic range than is currently available. The focusing optics of FOXSI will achieve a sensitivity 100 times better than that of current solar X-ray instruments (i.e. RHESSI) at energies around 10 keV.  FOXSI uses nested-shell, grazing-angle optics and fine-pitch silicon strip detectors to achieve a 0.5 keV energy resolution, an angular resolution of 10” (FWHM), and an energy range of 4-15 keV.  As a sounding rocket, FOXSI investigates the nonthermal energy content in quiet-Sun nanoflares, which are thought to play an important role in coronal heating.  FOXSI is a pathfinder for future solar HXR observing missions, particularly those interested in imaging faint HXR emission from particle acceleration regions in the corona.

FOXSI flew for the first time on November 2, 2012, successfully imaging a solar microflare and searching for faint HXR emission from the quiet Sun. On December 9 FOXSI will have its second flight. Some upgrades on the optics as well as a couple of CdTe detector have been included in its payload.