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Form measurement of large, slightly curved freeform surfaces with diameters up to 1.5 meters

PTB is currently developing and setting up a new form measurement system for large optical surfaces with diameters up to 1.5 m. This is being done to meet the metrological requirements of the optical industry (e.g. for flatness or freeform reference standards) and of certain research areas (such as synchrotron and telescope mirrors).

One objective is to measure flatness with uncertainties of a few ten nanometers. The absolute form of a surface with a curvature radius of 10 m or greater should then be realized with uncertainties in the sub-micrometer range. For the measurement of the deviation of a shape from the best-fit sphere, the uncertainty should be in the range of a few ten nanometers.

The new system measures different sections of a specimen's topography in so-called subaperture measurements. The sections are stitched together to form the complete topography of the test specimen. Without further information, the measurement errors of individual subaperture measurements can lead to larger errors in the global topography. To prevent this, the distance and the relative tilt angles between the sensor head and the specimen's surface are measured and taken into account in the evaluation.

The system is designed to be flexible, so that different sensor heads can be used, e.g. Fizeau interferometers with apertures of 10 mm, 100 mm, and 150 mm. A sensor head with a small aperture can measure more curved surfaces with a higher lateral resolution, whereas a sensor head with a larger aperture shortens the measuring time and thus reduces the influences of the ambient conditions. The new system is currently being set up. The first measurements are expected to take place in 2021.

The figure below shows the principle of the measurement system's setup.

Figure: Design drawing of the mechanical setup for the optical form measurement of specimens with diameters up to 1.5 m.

Selected publications

[Spi20b] Jan Spichtinger, Michael Schulz, Gerd Ehret: Optomechanical simulations of an advanced stitching interferometer set-up for large freeform optics. Optical Metrology and Inspection for Industrial Applications VII ; (Proceedings of SPIE: 11552) (2020), https://doi.org/10.1117/12.2573704

[Spi20a] Jan Spichtinger, Michael Schulz, Gerd Ehret: Absolute distance interferometry for subaperture stitching of large freeform optics. EPJ Web of Conferences, 238 (2020), 1-2, https://doi.org/10.1051/epjconf/202023806011

 [Ehr20] Gerd Ehret, Jan Spichtinger, Manuel Stavridis Michael Schulz: Setup of a new form measurement system for flat and slightly curved optics with diameters up to 1.5 metres. Proceedings Volume 11478, Seventh European Seminar on Precision Optics Manufacturing; 1147808 (2020), https://doi.org/10.1117/12.2564912

[Spi19] Jan Spichtinger, Gerd Ehret, Michael Schulz, Manuel Stavridis, Clemes Elster: Konzept für eine hochgenaue und rückgeführte optische Formmessung an großen Optiken bis 1,5 Meter. DGaO-Proceedings (2019), www.dgao-proceedings.de/download/120/120_p1.pdf


Jan Michael Spichtinger
Phone: +49 531 592-4208
E-Mail: jan.spichtinger(at)ptb.de

Dr.-Ing. Gerd Ehret

Phone: +49 531 592-4220
E-Mail: gerd.ehret(at)ptb.de