Description
In the past decade the freeform optics technology has enabled new types of all-reflective imaging system designs with low F-number, wide field of view, and high image quality. However, in most of the cases, these imagers have the only pupil that coincides with one of the mirrors. In the meantime, in some cases in astronomical instrumentation, it would be desirable to have an additional intermediate pupil to place a deformable mirror or a wavefront sensor in adaptive optics systems, or a dispersing component in a spectrograph. In the current work, we present a few optical design options with accessible pupil. Essentially, they represent freeform versions of widely known designs as reflective Schmidt telescope, Yolo telescope, or three-mirror anastigmat. Besides, moving the pupil into an offset and accessible position has an impact on the aberration correction conditions and requires adding more boundary conditions to exclude the apertures overlapping and avoid too fast surface shapes for individual mirrors. The designs presented here may reach the F-number of 2.5 with the field of view up to 10 degrees. They can reach a diffraction-limited nominal image quality in J to K bands. We discuss scaling these solutions in the range of focal lengths between 300 and 1600 mm.
