Optical image quality testing and correction of a 250-mm freeform telescope

Designing with freeform optics makes it possible to correct imaging systems to high-performance levels in a compact format. To justify the extra challenges inherent in the design, fabrication, metrology, and assembly of systems utilizing freeform optics, engineers need a realistic understanding of the performance that can be expected. The real-world performance of these systems has remained under-explored in the literature, a factor that may dissuade the adoption of freeform optics. In this work, the MTF of an F/3, 3° diagonal full FOV, 250-mm aperture freeform telescope is characterized at multiple field angles using an off-axis parabolic collimator. The freeform optic components required minimal alignment steps and no post-polishing. Twenty-seven sub-aperture tilted knife-edge tests were conducted to evaluate the correlation between form error and image quality. The telescope’s image quality was determined to be consistent with the design specification, which was 30% MTF at 76 lp/mm (70% of the sensor-array Nyquist frequency) across the entire field.