I think the trick is that the glass actually doesn't need to bend much. Notice that the caption here: https://en.wikipedia.org/wiki/Schmidt_corrector_plate#/me

I think the trick is that the glass actually doesn't need to bend much. Notice that the caption here: https://en.wikipedia.org/wiki/Schmidt_corrector_plate#/media/File:Schema_lame_de_Schmidt.svg https://en.wikipedia.org/wiki/Schmidt_corrector_plate#/media... describes the plate as being hard to tell apart from a flat disk visually.