HESSI is a Small Explorer (SMEX) mission focussed on hard X-/-ray radiation as a signature of particle acceleration processes in flare-related events. It has a temporal resolution of 10 ms, without imaging optics but with a grid modulation mechanism to provide image reconstruction.
HESSI will be very much in need of context imaging from other missions (SOHO, TRACE) and/or ground based observatories. Science data will be accessible to the community immediately after aquisition. Operations will be simple, running autonomous full sun imaging.
See also http://hesperia.gsfc.nasa.gov/hessi/.