Fabrication of high precision metallic freeform mirrors with magnetorheological finishing (MRF)

Abstract

The fabrication of complex shaped metal mirrors for optical imaging is a classical application area of diamond machining techniques. Aspherical and freeform shaped optical components up to several 100 mm in diameter can be manufactured with high precision in an acceptable amount of time. However, applications are naturally limited to the infrared spectral region due to scatter losses for shorter wavelengths as a result of the remaining periodic diamond turning structure. Achieving diffraction limited performance in the visible spectrum demands for the application of additional polishing steps. Magnetorheological Finishing (MRF) is a powerful tool to improve figure and finish of complex shaped optics at the same time in a single processing step. The application of MRF as a figuring tool for precise metal mirrors is a nontrivial task since the technology was primarily developed for figuring and finishing a variety of other optical materials, such as glasses or glass ceramics. In the presented work, MRF is used as a figuring tool for diamond turned aluminum lightweight mirrors with electroless nickel plating. It is applied as a direct follow-up process after diamond machining of the mirrors. A high precision measurement setup, composed of an interferometer and an advanced Computer Generated Hologram with additional alignment features, allows for precise metrology of the freeform shaped optics in short measuring cycles. Shape deviations less than 150 nm PV / 20 nm rms are achieved reliably for freeform mirrors with apertures of more than 300 mm. Characterization of removable and induced spatial frequencies is carried out by investigating the Power Spectral Density

Dimensional stability of metal optics on nickel plated AlSi40

“…Applying computer-aided polishing with sub-aperture tools, a roughness lower than 1 nm RMS is achievable [2]. Furthermore, local figuring techniques like Magnetorheological Finishing (MRF) enable the deterministic figure correction of NiP-layered metal surfaces [16].…”

Precision manufacturing of a lightweight mirror body made by selective laser melting

“…In a next step the optical surface is processed by Magnetorheological Finishing (MRF), which is capable of performing local shape corrections with high accuracy [33]. The mechanism of local shape correction on electroless nickel plated metal mirrors as well as further process details can be found in [34].…”

Ultra-Precision Polishing of N-Bk7 Using an Innovative Self-Propelled Abrasive Fluid Multi-Jet Polishing Tool

“…Currently, as stated by Rhorer and Evans (2010), high-precision diamond turning is one of the processes used to generate high-quality optical surfaces. However, Beier et al (2013) found that diamond turning induces erratic jagged features. These imperfections could have an adverse effect on the performance of optical glasses, such as light scattering (Beier et al, 2013;Rubin, 2002;Trost et al, 2013).…”

“…However, Beier et al (2013) found that diamond turning induces erratic jagged features. These imperfections could have an adverse effect on the performance of optical glasses, such as light scattering (Beier et al, 2013;Rubin, 2002;Trost et al, 2013). Rogov et al (2001) demonstrated that poor surface roughness adversely affected the light scattering factor.…”