Deflectometric Measurement of Astronomical Mirrors: Comparison of Different Calibration and Evaluation Methods

Deflectometry is a well-established technique for the precise measurement of specular optical surfaces. When measuring astronomical mirrors, it is advantageous to place the sensor in the center of curvature of the specimen at 2f. In this contribution, the calibrationrelated challenges for this specific application are investigated. The main focus lies on the comparison of different grayscale calibration strategies: A passive camera-side approach compensates “a posteriori” for system non-linearities and inhomogeneities, while an active approach directly drives the display to achieve the desired grayscale response. If aperture effects (circle of confusion on the screen) and gray-level quantization artifacts are also taken into account, intricate interactions emerge. Their impact on phase evaluation accuracy is investigated. In addition, the optical performance is assessed via a direct PSF estimation based on the histogram of the reconstructed screen coordinates, without requiring a full geometric system calibration. The different approaches are compared experimentally using measurements of the parabolic primary mirror of a 400 mm Newton telescope at the university observatory. 10 che Messtechnik I T Andreas W. Stark