The imaging capability of a thin foil x-ray mirror has been examined with optical light, using a laser beam and a wide optical parallel beam. These measurements reveal that (1) image broadening due to millimeter scale waviness (orange peel) of the aluminum substrate, partly intrinsic to the foil and partly caused during the foil treatment, is 1.2-min of arc half-power diameter (HPD) in two reflections; (2) slope errors due to foil shaping and misalignment cause broadening of 1.6–2.0-min of arc HPD; (3) total broadening is ~3-min of arc HPD, which is consistent with the broadening of 2.6-min of arc HPD measured with x rays.
Hideyo Kunieda and Peter J. Serlemitsos, "X-ray mirror assessment with optical light," Appl. Opt. 27, 1544-1547 (1988)