Development of precision Wolter mirrors for future solar x-ray observations

Abstract

High resolution imagery of the solar X-ray corona provides a crucial key to understand dynamics and heating processes of plasma particles there. However, X-ray imagery of the Sun with sub-arcsecond resolution has yet to be conducted due to severe technical difficulty in fabricating precision Wolter mirrors. For future X-ray observations of the Sun's corona, we are attempting to realize precision Wolter mirrors with sub-arcsecond resolution by adopting advanced surface polish and metrology methods based on nano-technology to sector mirrors which consist of a portion of an entire annulus. Following fabrication of the first engineering mirror and subsequent evaluation on the X-ray focusing performance in 2013, the second engineering mirror was made with improvements in both precision polish and metrology introduced. Measurement of focusing performance on the second mirror at SPring-8 synchrotron facility with 8 keV X-rays has demonstrated that the FWHM size of the PSF core reached down to 0.2" while its HPD (Half Power Diameter) size remained at ~3" due to the presence of small-angle scatter just outside of the core. Also, there was notable difference in the focal length between sagittal and meridional focusing which could have been caused by an error in the sag in the meridional direction of <10 nm in the mirror area. Further improvements to overcome these issues have been planned for the next engineering mirror.

Development of precision Wolter mirrors for solar x-ray observations

“…Based on the extensive evaluation of the metrology measurements during the precision polish and X-ray measurements of the focusing performance on the Engineering Mirror #2, we have identified areas of improvement for fabricating the next Engineering Mirror #3 17 . Through improvement in the precision polish by adopting a smaller polishing head for the deterministic MRF polish and refined smoothing, as well as the optimized order in combining the deterministic polish and smoothing, we have achieved a precision Wolter surface whose figure error PSD comparable to, or even exceeding in many spatial frequencies, that of the Chadra mirror (Figure 3).…”

“…Furthermore, this beam line is designed to provide X-rays of high spatial coherence thus wave-optical calculations can be applied in interpreting and predicting focused beam profiles from surface figure profiles of mirrors. Details of the measurement setup is given in Sakao et al 2015 17 .…”

“…We identified that, through fabrication and subsequent X-ray measurement of the Engineering Mirror #2, the figure error correction around the spatial frequency of 1 mm -1 was crucial for achieving sub-arcsecond HPD. Hence we adopted a smaller polishing head for MRF than that used for the Engineering Mirror #2 with which we aimed to perform deterministic polish in spatial scales down to as small as ~0.5 mm as discussed in Sakao et al 2015 17 . However, it turned out that the MRF polish, in combination with the filtered figure measurement data based on which the MRF polish was to be performed, would not be much effective for spatial scales below ~2−1 mm while there was a risk of introducing unwanted scratch features (though did not severely affect imaging performance) on the mirror surface to be generated by the polish.…”

“…A vacuum pipe, sealed with a 75 µm-thick polyimide film on each end, was placed in between the mirror and the focal plane to minimize attenuation of the 8-keV X-rays with air. The measurement setup was almost identical to those for the Engineering Mirror #2 17 . It is noted there are several intensity enhancements along the meridional direction.…”

“…In section 2, target specification of Wolter mirrors for future solar observations is summarized. In section 3, we present key issues identified through the measurements on the X-ray focusing performance of the second engineering mirror 17 (precision-polished in 2014) that was made following the first attempt in 2013 16 , as well as design parameters for the second and third engineering mirrors. In section 4, we describe polishing approach adopted for the third mirror (fabricated in 2015−2016) as well as the result of the measurements on its X-ray focusing performance, followed by summary in section 5.…”