Details of the polishing spot in magnetorheological finishing (MRF)

Magnetorheological Finishing (MRF) is a novel process for deterministic figure correction and polishing of optical materials that utilizes a sub-aperture lap created by moving a magnetic field-stiffened magnetorheological (MR) fluid ribbon against an optical surface. MRF has been successfully applied to a wide range of optical materials. A new research platform has been designed and built that is used to generate sub-aperture polishing profiles, i.e., polishing "spots," on optical flats under well-controlled conditions. This platform uses the same fluid circulation and conditioning system as the commercial computer numerically controlled MRF machine, thereby allowing fluid performance issues to be investigated. This new machine complements the capabilities of the original MRF research platform that has been in continuous use for over six years. These two machines have been used to generate polishing spots on a variety of optical materials. The spot profiles were measured to calculate material removal rates and the quality of the polished surfaces characterized by measuring the microroughness within the polishing spots. Examples are presented which illustrate how the evaluation of polishing spots was used to develop MR fluids and operating conditions for calcium fluoride, CaF2, and potassium dihydrogen phosphate, KDP.