Abstract
This paper addresses computer numerical control (CNC) polishing of aspheric or freeform optics. Prior CNC grinding of the asphere tends to produce mid-spatial frequencies (MSFs) at some level. Precessions polishing can rectify these, but the very ability of the bonnet tooling to adapt to the local asphere enables it to do so, at least in part, to similar spatial frequencies in the MSFs. To accelerate smoothing, hard tools can, in principle, be used, but aspheric misfit is often assumed to preclude this. In this paper, we explore new insight into the role of abrasive particle size in accommodating misfit. First, we report on a glass-bending rig to produce a continuous range of complex surfaces, while withstanding process forces. Then, we describe how this was used to evaluate the triangle of misfit, abrasive size, and MSFs produced for hard rotating tools. This has revealed a regime in which such tools can be used without introducing significant new MSFs, as evidenced by manufacture of prototype off-axis aspheric segments for the European Extremely Large Telescope project.
Highlights
As is well known, incorporating aspheric or freeform surfaces in an optical system can improve performance, using fewer optical surfaces, giving reduced system size and mass, or delivering functionality otherwise impossible
For low and medium values of tool misfit with the asphere, the mid-spatial frequencies (MSFs) content is dominated by the overlapping track-spacing effect, where increased track spacing leads to additional ripple on the surface
Using a novel glass-bending rig, both to create a variable complex surface form and to withstand process forces, we have quantified the relationship among tool misfit, abrasive grit size, and MSFs created on the surface
Summary
As is well known, incorporating aspheric or freeform surfaces in an optical system can improve performance, using fewer optical surfaces, giving reduced system size and mass, or delivering functionality otherwise impossible. When computer numerical control (CNC) grinding the base asphere or freeform in a brittle material, or CNC polishing thereafter, the tool traditionally contacts the part over a small area. The misfit of tool and surface would lead to gross mid-spatial frequency (MSF) surface features. The small tool progresses over the surface along the defined tool path, with position being precisely controlled (grinding) [1], or dwell time (polishing) [2,3]. MSFs arise from superposition of adjacent tool paths (“cusping”), positional errors, and dynamic effects. MSFs seen after polishing arise from various causes, including grinding print-through, misfit of polishing tools, the effects of overlapping tool paths, and the spatial frequencies inherent in the tool’s influence function
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