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Deformable, piezo bimorph mirrors are often used to expand X-ray beams to a continuous range of sizes. However,
optical polishing errors present on all X-ray mirrors introduce striations into the reflected beam. To counteract them, reentrant
surface modifications with alternating concave and convex curvature have been proposed and applied to mirrors
of fixed shape or bimorph mirrors. For the latter, a new method of constructing re-entrant surface modifications on
segments of unequal length is described. This allows the re-entrant modification required for a desired beam size at the
focal point to be matched to the bimorph mirror’s polishing errors, thus reducing the voltage variations. Optical
profilometry using the Diamond-NOM showed that a 5-segment and a 7-segment modification could be suitably applied
to a deformable bimorph mirror. X-ray tests showed that striations caused by the 5-segment modification in the beam at
the focus are concentrated at the beam edges, while the beam center is left clear. This is in contrast to simple defocusing,
in which a strong side shoulder appears. The 7-segment modification produces a pattern of evenly spaced striations. The
intensity spikes seen with the re-entrant modifications are caused chiefly by the finite curvature of the mirror at the
turning points. The question of whether deformable bimorph mirrors with different piezo response functions could
sharpen the curvature changes will be investigated. Optimal modifications of continuous curvature, which could more
realistically be applied, will be sought.
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