Metal mirror based VIS freeform telescope with smart integration approach

Modern optical telescopes for Earth observation and remote sensing operations often rely on off-axis mirror designs with aspheric or free-shaped surfaces in order to generate an unobscured image while at the same time covering a large field of view and maintaining an excellent system quality. Continuous improvements in manufacturing and test methods allow for the fabrication of freeform surfaces with low tolerances on figure and roughness. We describe the development, fabrication, and testing of an anamorphic imaging four mirror freeform telescope that operates diffraction limited at visible wavelengths. Based on fabrication and test techniques developed, figure errors of each optical freeform surface were reduced to < 12 nm rms. Roughness values < 0,5 nm rms have been realized based on the applied polishing processes. The optomechanical design of the telescope aims at a minimum of integration effort, reducing the relevant degrees of freedom during mirror alignment from 24 to 3 only and thus allowing for a total system integration within one single day. The telescope concept ensures a reproducible integration and therefore allows for a multi-stage integration after different manufacturing steps in order to correlate surface errors of different spatial frequency with observed wave aberrations on system level. The final experimentally obtained wave aberration is in excellent agreement to the optical design.