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The fabrication and metrology of astronomical optics are very demanding tasks. In particular, the large sizes needed for
astronomical optics and mirrors present significant manufacturing challenges. One of the long-lead aspects (and primary
cost drivers) of this process has traditionally been the final polishing and metrology steps. Furthermore, traditional
polishing becomes increasingly difficult if the optics are aspheric and/or lightweight.
QED Technologies (QED(r)) has developed two novel technologies that have had a significant impact on the production
of precision optics. Magnetorheological Finishing (MRF(r)) is a deterministic, production proven, sub-aperture polishing
process that can enable significant reductions in cost and lead-time in the production of large optics. MRF routinely
achieves surface figure accuracy of better than 30 nm peak-to-valley (better than 5 nm rms) and microroughness better
than 1 nm rms on a variety of glasses, glass ceramics and ceramic materials. Unique characteristics of MRF such as a
comparatively high, stable removal rate, the conformal nature of the sub-aperture tool and a shear-mode material
removal mechanism give it advantages in finishing large and lightweight optics. QED has, for instance, developed the
Q22-950F MRF platform which is capable of finishing meter-class optics and the fundamental technology is scalable to
even larger apertures. Using MRF for large optics is ideally partnered by a flexible metrology system that provides full
aperture metrology of the surface to be finished. A method that provides significant advantages for mirror manufacturing
is to characterize the full surface by stitching an array of sub-aperture measurements. Such a technique inherently
enables the testing of larger apertures with higher resolution and typically higher accuracy. Furthermore, stitching lends
itself to a greater range of optical surfaces that can be measured in a single setup. QED's Subaperture Stitching
Interferometer (SSI(r)) complements MRF by extending the effective aperture, accuracy, resolution, and dynamic range of
a standard phase-shifting interferometer. This paper will describe these novel approaches to large optics finishing, and
present a variety of examples.
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