Today, aspherical elements have become an indispensable part of modern high-precision optical assemblies. Several kinds of defects arise during their manufacture. As far as very precise aspherical surfaces are concerned, mid-spatial frequencies are probably the most important issue. This type of imperfection fills the gap between shape (low-spatial frequencies) and microroughness (high-spatial frequencies).
A smaller part of these defects arise during polishing; however, more of them are generated during the grinding process. Due to the interference of different controlling frequencies in the machine and imperfections in the constructional solution of the grinding machine, defects occur on the optical surface, which deform it. The periods of these defects usually lie in an interval of 0.5 to 10 mm, depending on the parameters of the machining process. To prevent the generation of these structures, a comprehensive measurement of the sources and transmission of vibrations was realised using the measuring device VibXpert II. The measurement was made on the grinding machine Optotech MCG 100 CNC. Several simulations of different types of processes were realised and the measurement was also subsequently performed during a real grinding process of aspherical optical surfaces. The data acquired from the measurement of vibrations were mathematically processed in frequency space. The experiment revealed several reasons for these defects.
The presented paper aims to theoretically analyze the possibilities, advantages and drawbacks of standard methods used for the assessment of optical surface defects (the so-called Scratch and Dig analysis). Based on the acquired knowledge, we design and apply a process of SaD analysis suitable for the evaluation of optical surfaces of mirrors of the space coronagraph Metis, whose manufacturing was successfully implemented within the Centre Toptec in the past period.
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