Manufactureing of 720mm SiC Aspherical Mirror

1.Milling process: the back-side milling of the mirror can be achieved on a general three-axis CNC machining center, while the mirror surface milling requires a four-axis or even five-axis machining center. If a high-precision five-axis machining center is used and appropriate grinding tools are selected, the mirror surface milling accuracy within one meter diameter can reach 10um.

2.Grinding process: grinding mainly removes the tool traces and subsurface damage layer left by milling, and improves the surface roughness of the mirror. Early grinding equipment mainly used a five-axis CNC small grinding head, and in recent years, robot CNC small grinding heads have been widely used. The detection uses a three-coordinate system to collect mirror surface coordinate points, and the actual surface shape error can be obtained by fitting the aspherical surface and ideal aspherical surface based on the coordinates of each point on the mirror surface. After the mirror surface roughness meets the requirements of the grinding process, the surface accuracy of several microns can generally enter the next process.

3.Rough polishing: Rough polishing further improves the surface roughness and surface accuracy of the larger aperture SiC aspherical mirrors. CNC small grinding heads are still used for rough polishing, and the latter stage MRF-Magnetorheological Finishing is also involved. After rough polishing, the mirror surface enters the interference metrology stage, and can enter the Si coating stage after the surface accuracy RMS value is better than 1/10 λ.

4.Si Coating: The mirror blank material of a large aperture SiC aspherical mirror often uses reaction-sintered SiC material, which has slightly lower density compared to pressure-sintered SiC material. In order to improve the reflectivity of the larger aperture SiC aspherical mirror, a layer of Si is plated on the mirror surface to improve the surface density.

5.Fine polishing: After modification, the larger aperture SiC aspherical mirror enters the fine polishing stage. CNC small grinding heads, Stress Lap Polishing, and MRF-Magnetorheological Finishing are often used in the initial stage of fine polishing to remove the relatively loose layer on the modified surface. Later, CNC small grinding heads, together with Ion Beam Figuring, are used to refine the surface accuracy to the design requirements. At present, the surface accuracy of larger aperture SiC aspherical mirrors can be processed to a full-diameter RMS value better than 1/60 λ or even better.

After several years of technical accumulation, MG-Optics currently has the ability to develop the entire process of large-aperture SiC aspherical mirror. A set of 720mm diameter large-aperture SiC aspherical mirror has been completed with a final surface accuracy full-diameter RMS value better than 1/60 λ.