Improvements in large window and optics production

Abstract

Fabrication of large optics has been a topic of discussion for decades. As early as the late 1980s, computer-controlled equipment has been used to semi-deterministically correct the figure error of large optics over a number of process iterations. Magnetorheological Finishing, MRF®, was developed and commercialized in the late 1990's to predictably and reliably allow the user to achieve deterministic results on a variety of optical glasses, ceramics and other common optical materials. Large and small optics such as primary mirrors, conformal optics and off-axis components are efficiently fabricated using this approach. More recently, specific processes, MR Fluids and equipment have been developed and implemented to enhance results when finishing large aperture sapphire windows. MRF, by virtue of its unique removal process, overcomes many of the drawbacks of a conventional polishing process. For example, lightweighted optics often exhibit a quilted pattern coincident with their pocket cell structure following conventional pad-based polishing. MRF does not induce mid-frequency errors and is capable of removing existing quilt patterns. Further, odd aperture shapes and part geometries which can represent significant challenges to conventional polish processing are simply and easily corrected with MRF tools. Similarly, aspheric optics which can often present multiple obstacles-particularly when lightweighted and off-axis−typically have a departure from best-fit sphere that is not well matched with to static pad-based polishing tools resulting in pad misfit and associated variations in removal. The conformal subaperture polishing tool inherent to the QED process works as well on typical circular apertures as it does on irregular shapes such as rectangles, petals and trapezoids for example and matches the surface perfectly at all points. Flats, spheres, aspheres and off-axis sections are easily corrected. The schedule uncertainties driven by edge roll and edge control are virtually eliminated with the MRF process. This paper presents some recent results of the deterministic finishing typified by the QED product line and more specifically of its large-aperture machines, presently capable of finishing optics up to one meter in size. Examples of large sapphire windows and meter-class aspheric glass optics will be reviewed. Associated metrology concerns will also be discussed.