Magnetorheological finishing of large and lightweight optics

Magnetorheological finishing (MRF) is a production proven, sub-aperture polishing process for flat, spherical, aspherical, and cylindrical optics in the size range of 10 - 400 mm. Surface figure accuracy of better than 30 nm peak-to-valley (better than 5 nm rms), and microroughness better than 1 nm rms is routinely achieved on a variety of glasses, glass ceramics and single crystal materials. Recent work has demonstrated the applicability of MRF for larger apertures and lightweight optics. A platform capable of finishing 1000 mm apertures has already been built. Engineering studies for extending the aperture size further are underway. Finishing of large, lightweight mirrors has additional challenges because the non-uniform support of the face-sheet requires special efforts to avoid quilting errors caused by print-through of the cell structure due to fabrication processes, gravity and/or temperature effects. Unique characteristics of MRF such as a competitively high, stable removal rate, the conformal nature of the sub-aperture tool and a shear mode of material removal give it advantages in finishing this class of optics. Specifically, MRF avoids generating print-through errors and has a high rate of convergence in correcting quilting errors created by other processes, gravity or temperature effects. An additional important quality is that it has been shown that inserting MRF into a manufacturing process can substantially reduce the subsurface damage (SSD), increasing the laser damage threshold of a surface, providing advantages for use in mirror fabrication for high-energy applications. Supporting results will be given in this paper.