Precision measurement of large optics up to 850 mm in diameter by use of a scanning point multi-wavelength interferometer

Abstract

An established measurement system in asphere production, which is a promising approach in high precision freeform manufacturing as-well, is given by a scanning point interferometer based on a multi-wavelength approach. The scanning principle enables for a great flexibility, reduces setup time and costs, and has almost no limitations in spherical departure. Due to the absolute measurement capability, the utilized multi-wavelength approach is beneficial for segmented and interrupted surfaces, which are common apertures of modern application’s optical elements. This approach has been adapted to allow for large surface measurements of up to 850 mm in diameter with highest accuracy of down to 150 nm peak-to-valley on the maximum aperture. The combination of an optimized metrology frame setup with a unique antivibration system improves long-term stability e.g., enabling a 3σ RMSi repeatability on a hemispheric surface (with 90 kg of additional load) over 10 hours without recalibration of down to 1.7 nm only. This contribution gives a general overview of challenges when measuring complex surfaces, with a special focus on large objects and their requirements. Starting from the based scanning multi-wavelength approach, the newly developed and optimized system hardware will be highlighted, as-well as first measurement results presented. These also include consistency data for different workload e.g., simulating different tooling for the sensitive objects under test.