Improvement of Yohkoh hard x-ray imaging

The Hard X-ray Telescope (HXT) on board Yohkoh is a Fourier-synthesis-type imager for solar flare observations; it measures a set of spatially modulat ed photon counts with sixty-four, independent, bigrid modulation collimator s and an image is reconstructed from these data using sophisticated image s ynthesis procedures, such as the Maximum Entropy Method (MEM) and PIXON. Th e HXT-MEM imaging has been greatly improved by incorporating the following three improvements which we have recently achieved: (1) Accurately estimate d instrumental response functions (X-ray transmission/modulation patterns), which are made available through self-calibration using solar flares as ca libration sources; (2) Improvement of the MEM algorithm, which includes the total flux estimate in its iteration loop; and (3) Accurate error estimate and its proper inclusion in the MEM. Various problems seen in previous HXT -MEM imaging are drastically reduced and high-quality images are obtained.