Influence of mounting on the optical surface figure in optical reference surfaces

The paper presents the effect of mechanical mounting of optical reference elements on their surface shape. Optical reference surfaces are key elements when traceable, highly accurate and precise optical surface measurements are required. In order to calibrate measuring instruments and compare the metrological capabilities of different metrology institutes, universities and other stakeholders, the reference artefacts were developed. Different measurement instruments require a different way of mounting and the reference artefacts are supposed to be useful for reliable and repeatable calibration of a great majority of the instruments worldwide. However, not only their shape was critical, but also the way of mounting was crucial. FEM analyses followed by experiments have revealed an unacceptable surface shape error in the order of hundreds of nanometres in the case of the commonly used screw mount, even for low applied torques. Other mounting options, such as the collet chuck or the Morse taper, are examined by means of FEM analysis and verified by interferometric measurements. It is shown that only the Morse taper can fulfil the strict criterion of less than 30 nm for surface shape deviation due to mounting, which is required in optical surface shape metrology.