Ehhh, I feel like I have to mention that optics like a Schmidt–Väisälä camera exist, and notably have their focal plane right in the middle of the tube. They r

Ehhh, I feel like I have to mention that optics like a Schmidt–Väisälä camera exist, and notably have their focal plane right in the middle of the tube. They readily go as fast as e.g. 400mm f/2.0 with 2 tame (iirc literally just spherical) optical surfaces (mirror and near-sensor plano-convex lens) and 1 more elaborate wavy surface (back side of front glass), naturally achromatic for a 20~30 MP large image circle. Modern technology (ability to manufacture of array waveguides that act like a bundle of fiber optics, to de-flatten the sensor's focal plane) also allows making better use of objective designs that perform well in every way but field flatness.