Direct Polishing of Full-Shell, High-Resolution X-Ray Optics

Abstract: Future x-ray telescopes will likely require lightweight mirrors to attain the largecollecting areas needed to accomplish the science objectives. Understanding anddemonstrating processes now is critical to achieving sub-arcsecond performance in thefuture. Consequently, designs not only of the mirrors but of fixtures for supporting themduring fabrication, metrology, handling, assembly, and testing must be adequately modeledand verified. To this end, MSFC is using finite-element modeling to study the effects ofmounting on thin, full-shell grazing-incidence mirrors, during all processes leading to a flightmirror assemblies. Here we report initial results of this study. • 6mm cylinder mounted into hardware to attach to the Zeekopolisher• Top center: finite-element model of cylinder (height=0.300m, diameter=0.610m, thickness= 0.002m and 0.006m) held at base with polishing force applied• Middle center: displacement results• Bottom center: von Mises stress results• Left: stress and displacement results for finite-element model of polishing forces on a 6mm-thick shell, varying the contact area of polisher• Right: results for finite-element model of 2mm-thick shell, varying contact areaThe Vertical Long-Trace Profilometer (VLT P, from Continental Optics) vertically scans the surface under test (sitting on a rotary table) and is best suited for metrology of full-cylinder mirror sh ells and large-diameter mandrels• Top: using FEM shown in Fabrication, distortion due to gravity in the axial direction, mounted at base. Left to right: 3-point, 6-point, 9-point, and 12-point mount.• Bottom: stress due to gravity in axial direction, mounted at base. Left to right: 3-point, 6-point, 9-point, and 12-point mount.Displacement and stress results from above parametric studyWhiffletree design based on parametric stud y, where 6 points is minimum number of points necessary• Because lightweight mirrors are flimsy, they are susceptible to significant distortion due to mounting errors, including gravitational sag, machining errors and assembly distortions• Blocking fixtures are necessary to fabricate sub-arcsecond grazing-incidence optics on the order of 2mm thick, however, with the added strength comes added error in the optic• Metrology in a “free form” is required before adding a blocking fixture• A subtraction between the blocking error and the “free form” is applied to the corrective polishing• Left: grazing-incidence optic mounted in blocking fixture• Right: stiffening ring to hold optic for metrology• Fabrication of thin, lightweight, sub-arcsecond optics requires innovative technology• NASA MSFC procured a Zeeko polisher to aide in technical advancement; its capabilities include corrective polishing on the order of nanometers.run# #pts disp(microns) stress(psi)1 3 8.35 22726 0.16 833 9 0.02 54412 0.01 41