Magnetorheological finishing of SiC aspheric mirrors

Many high-precision optical components are made of glass, but they can also be made of a wide variety of other materials. For example, reaction-bonded silicon carbide (RB-SiC) has recently been used to make optical parts for poor working conditions due to its distinct properties such as low density, high hardness, and low thermal expansion coefficient. Thus, in order to rapidly manufacture high-precision aspherical mirrors made from RB-SiC, a novel aspheric manufacturing system equipped with magnetorheological finishing (MRF) technology is described in this paper, which incorporates a host of features and has unprecedented processing versatility. The system has four degrees of freedom, and can realize manufacture or measurement based on the Cartesian coordinate or polar coordinate systems. To achieve good surface accuracy, the polishing wheel combines self-rotation movement with corotation. The prepolishing and fine polishing processes can be done on the same machine by changing different process parameters accordingly, and the on-machine profilometer measures the aspheric forms generated during prepolishing. Experiments were carried out on the features of magnetorheological (MR) fluids by polishing a parabolic SiC mirror with a 100 mm aperture. The results indicated that the required convergence speed, surface roughness, and figure accuracy can be achieved with high efficiency. The final surface roughness with an initial value of Ra = 34.39 nm reached 26.74 nm after 20 hours of prepolishing using MR fluids carrying oil (Al 2 O 3 as the nonmagnetic particle), and then reached 1.14 nm after 50 hours of fine polishing using the MR fluid with diamond power as the nonmagnetic particle. The figure accuracy also improved to λ /30 rms (λ = 632.8 nm) at an arbitrary 30 mm subaperture.