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 modulated photon counts with sixty-four, independent, bigrid modulation collimators and an image is reconstructed from these data using sophisticated image synthesis procedures, such as the Maximum Entropy Method (MEM) and PIXON. The HXT-MEM imaging has been greatly improved by incorporating the following three improvements which we have recently achieved: (1) Accurately estimated instrumental response functions (X-ray transmission/modulation patterns), which are made available through self-calibration using solar flares as calibration 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.