GREGOR spot diagrams for different wavelengths and MTF for 500 nm with max. FOV = 120″. The circle indicates the Airy disk. Top row: original M12 and M15. Diffraction-limited performance is only achieved for the center of the FOV with a very fast drop in contrast off-center. This simulation was made for ideal M12 and M15 mirrors, and we suspect that their specifications were not met, which would further worsen their properties. Middle row: new IR setup. The lateral shift is due to the BS plate, and 500 nm (red) is displayed to show that even the ZIMPOL guest instrument setup, which uses VIS in the IR beam by using a custom beamsplitter plate, would still have a diffraction-limited performance. Bottom row: new visible setup with basically no loss of contrast for the whole field of 120″. All MTFs have good properties in the IR, and therefore 500 nm is shown here.
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