High resolution imagery of solar X-ray corona provides a crucial key to understand dynamics and heating processes of plasmas there. However, imagery of the Sun with sub-arcsecond resolution in X-ray wavelengths has never been conducted due to severe technical difficulty in fabricating precision Wolter mirrors with a wide field of view exceeding several 100”.
For future X-ray observations of the Sun, we are attempting to realize precision Wolter mirrors with sub-arcsecond resolution by adopting state-of-the-art surface polish and measurement methods to segmented mirrors which consist of a portion of an entire circle.
Following evaluation of X-ray focusing performance of the first engineering Wolter mirror using BL29XUL coherent X-ray beam line at SPring-8 synchrotron facility, the second engineering mirror was fabricated with improvements in precision polish from the first mirror incorporated. X-ray evaluation of the second mirror at SPring-8 was conducted in February 2015, yielding FWHM size of ~0.2” for the PSF core at 8 keV while its HPD (half power diameter) size still remained at ~3” due to a large amount of small-angle scattering right outside the PSF core.
Further improvements in the precision polish for the second mirror, in particular in the spatial scale from 0.3 mm to 5 mm, is currently under way with another X-ray evaluation at SPring-8 planned in spring 2016. Progress in our development activities for precision Wolter mirrors will be reported including at-wavelength evaluation results.
|