A two-stage adaptive optical system using four piezoelectric deformable mirrors was constructed at SPring-8 to form
collimated X-ray beams. The deformable mirrors were finely deformed to target shapes (elliptical for the upstream mirrors
and parabolic for the downstream mirrors) based on shape data measured with the X-ray pencil beam scanning method.
Ultraprecise control of the mirror shapes enables us to obtain various collimated beams with different beam sizes of 314
μm (358 μm) and 127 μm (65 μm) in the horizontal (vertical) directions, respectively, with parallelism accuracy of ~ 1
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.
A one-dimensional two-stage focusing system using two deformable mirrors was constructed. To realize the precise
elliptical shapes, the mirror deformations were finely adjusted using the pencil-beam scan, which is a method of
wavefront measurement. X-rays of 10 keV energy were one-dimensionally focused to a full width at half maximum of
90 nm, which agrees well with the diffraction limit.
To construct an adaptive X-ray focusing optical system, we developed an ultraprecise deformable mirror that consists of a substrate, piezoelectric actuators, and 18 electrodes. A one-dimensional focusing test was performed at SPring-8 at 15 keV. The mirror deformation was roughly adjusted by applying voltages determined by a deformation test with a Fizeau interferometer. The shape was then finely corrected based on the shape determined by the pencil-beam method and the phase retrieval method. A focused beam with a full width at half maximum of 120 nm was obtained.