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In broadband dielectric coatings, the wavefront of the reflected wave can change dramatically in a resonance-like manner as a function of wavelength. These wavefront errors can be a significant issue in high precision instruments. In the last years, effort has been undertaken to design and produce coatings to reduce these resonances. However, today there is still limited capability to characterize by measurement the spectral dependence of the wavefront error with high spectral resolution and accuracy. The goal of this paper is to present and analyze a design for a setup to measure the reflected wavefront from a coated flat component with high accuracy as a function of the wavelength. The proposed design is based on a passive system using high-precision off-axis parabolic mirrors. For sensing the wavefront error a Shack-Hartmann sensor is proposed, whose microlens array design is to be modified. According to error analysis and tolerance studies, the setup is capable of measuring wavefront distortion with sub-2 nm RMS accuracy within 510 nm to 950 nm. The angle of incidence and the polarization can also be varied without a loss of accuracy. In order to determine the point spread function (PSF) with high accuracy in addition to the wavefront measurement, the wavefront error of the setup itself needs to be below 50 nm RMS. The tolerancing performed in this study included the light source, shape errors of the mirrors, beam splitter, polarizers, and the sensors. Shape irregularities of the single elements were simulated by Zernike polynomials, and the residual wavefront error of the setup is estimated by Monte Carlo simulations, including uncertainties of the mechanical positioning. From these simulations, specifications for the mirrors have been worked out based on the goal of a system wavefront error lower than 50 nm RMS. The intended broad spectral range makes it challenging to identify a suitable Shack-Hartmann wavefront sensor. Different sensor configurations are evaluated experimentally, and a reproducible wavefront measurement can be achieved by adjusting the focal length of the microlens array. Thereby, the repeatability in wavefront measurements could be reduced from 3 nm to less than 1 nm RMS by modifying the microlens array parameters. Tilting the polarizer and beam splitter by 2° prevents ghost images and multiple reflections in the setup. Finally, considerations about the realization of a suitable reference measurement with an optical flat of sufficient surface quality are presented.
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