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Session 13 - Image Processing, Chromosphere.
Oral session, Monday, June 30
Ballroom A, Chair: Thomas Metcalf

[13.01] Principles, Practical Problems, and the Performance of MEM Imaging for the Yohkoh Hard X-ray Telescope

T. Kosugi (Nobeyama Radio Observatory, NAOJ), J. Sato, T. Sakao (NAOJ), S. Masuda (STE Laboratory, Nagoya University)

The Hard X-ray Telescope (HXT) on-board Yohkoh is a Fourier-synthesis telescope with 64 modulation collimators. Solar flare images have been synthesized using the Maximum Entropy Method (MEM) or some other imaging techniques from sets of the 64 photon counts. This paper reports our recent success in improving the MEM imaging for HXT.

The key factors that led us to this success are as follows. First, modulation patterns for the 64 individual collimators were precisely re-calibrated in orbit using solar flares themselves as calibration sources. Note that the patterns we had used before were based on the pre-launch calibrations using artificial sources. Also the formulae that represent the patterns are renewed. Second, we re-formulated the MEM principle; now the total flux is a free parameter to be determined by the MEM principle. This new formulation was successfully incorporated into an iterative code. Third, the observation error terms were estimated and incorporated properly in the MEM imaging.

With these improvements, we have confirmed (i) that the so-called over-resolution problem, sometimes misunderstood as resulting from the MEM principle itself, disappears so that the stability of images improves drastically, (ii) that the image quality improves proportionally with increasing photon counts as naturally expected, (iii) that the number of events for which MEM imaging fails decreases (with a small number of exceptions, maybe due to large intrinsic source size), and (iv) that the similarity between HXT L-band images and SXT images becomes much clearer than before.

Program listing for Monday