Institute of Space and Astronautical Science (ISAS), Sagamihara, Kanagawa 229-8510, Japan

The Hard X-ray Telescope (HXT) onboard Yohkoh, with approximately 5-arcsec spatial and 0.5-s temporal resolution in four energy bands (14 - 23 - 33 - 53 - 93 keV), has been observing the Sun for approximately eight years since October 1991. More than 1300 flares have so far been detected with Yohkoh HXT and several key characteristics of flare hard X-rays have been obtained, which include the magnetically-conjugate footpoint sources and the over-the-loop-top source. However, flares that can be imaged over its four energy bands with its 0.5-s maximum temporal resolution are rare; the imaging capability of Yohkoh HXT is sensitivity-limited. On the other hand, Yohkoh HXT data are usually analyzed together with supplemental soft X-ray images simultaneously obtained with the Soft X-ray Telescope (SXT) co-carried onboard the same satellite, which has been proven crucial in unveiling where and how flare energy is released and energetic electrons accelerated in complex magnetic fields involved in a flaring active region. Based upon such an experience, we will discuss (1) the expected number of flares to be imaged with HESSI as a function of photon energy and GOES X-ray class of flares, and also (2) image quality that is inevitably sensitivity-limited. In addition we'll discuss (3) what types of context observations are crucially needed for HESSI.