Improving low, mid and high-spatial frequency errors on advanced aspherical and freeform optics with MRF

Advanced optical systems are more and more demanding in terms of resolution, imaging quality and speed of capture. Controlling mid-spatial frequencies and surface roughness on these optics is critical for mitigating scattering effects such as flare and energy loss. By improving these two frequency regimes, higher contrast images can be collected with improved efficiency and lower distortion. Classically, Magnetorheological Finishing (MRF) is implemented in production to correct low order errors generated by conventional polishing techniques on planos, spheres, on- and off-axis aspheres and freeform optics achieving figure errors as low as 1nm RMS while using careful metrology setups. MRF is also used routinely to turn a sphere into an asphere or freeform, or to print high resolution wavefront corrective patterns on optical surfaces to compensate for system errors or bulk material inhomogeneity. Recent advancements enable correction of mid-spatial wavelengths as small as ∼1mm and smoothing of surface roughness to ∼1Å RMS. Using these new developments combined with correction of low order form error have improved MRF performance to manufacture higher precision optics with respect to the mid- and high-spatial frequency regimes. Efficient mid-spatial frequency corrections utilize optimized process conditions; raster polishing with a small tool size. Furthermore, a novel MRF fluid, called C30, can finish surfaces to ultra-low roughness (ULR) and its low removal rate is optimal for fine figure correction of mid-spatial frequency errors. C30 MRF fluid is able to achieve <1.5Å RMS roughness on Silicon, CaF₂, Fused Silica, glass and other materials. It is best utilized within a fine figure correction process to target mid-spatial frequency errors as well as smooth surface roughness 'for free' all in one step. These expanded capabilities of MRF technology are well suited for producing high precision optics to be used for industrial, medical or semiconductor optics.