18 August 2018 Model based error separation of power spectral density artefacts in wavefront measurement
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Standard interferometers are not suitable for measuring highly aspherical lenses due to the obligatory null configuration. In a tilted wave interferometer (TWI), however, the surface under test is illuminated from multiple directions by a point light source array 1,2. Thus, there is at least one source for each location on the test object, which illuminates the specimen perpendicular so an evaluable measuring signal is generated. In combination with the stitching approach, even large convex apertures can be evaluated. However, the system must be calibrated before measuring with a TWI. During this calibration, a known sphere is measured in several positions. The obtained information describes the entire imaging system with a finite number of Zernike polynomials. High-frequency structures in the system components are not covered and can lead to calibration errors. For this reason, they must be reduced to a minimum. In order to control high-frequency aberrations, the wavefront of each individual lens and the entire system must be determined. In classical methods with a return mirror, many error effects are encountered, such as air turbulences at large distances. The proposed model for the mathematical determination of wavefront aberration uses single measurements of the front and back surface. This reduces measurement uncertainties and simplifies the measuring effort. In addition, the costs for otherwise necessary return mirrors are also omitted. However, the lenses used must be verified additional for impurities or streaks3. Different measurement configurations are compared with the proposed method.
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Alexander Haberl, Alexander Haberl, Antonia Harsch, Antonia Harsch, Gerald Fütterer, Gerald Fütterer, Johannes Liebl, Johannes Liebl, Christof Pruß, Christof Pruß, Rolf Rascher, Rolf Rascher, Wolfgang Osten, Wolfgang Osten, "Model based error separation of power spectral density artefacts in wavefront measurement", Proc. SPIE 10749, Interferometry XIX, 107490T (18 August 2018); doi: 10.1117/12.2321106; https://doi.org/10.1117/12.2321106

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