Here is the fold mirror. Not particularly flat!
On reading Ellison's book, (page 74) he states that he never polished a flat - rather he cut them from plate glass after selecting a flat piece. He describes how he cut them out of the plate using a brass tube on a lathe.
He admits that this can lead to distortion, and perhaps he considered this particular example to be OK?
In practice, with contemporary eyepieces, a much smaller area in the centre would have been actually used. Plus being used at 45deg incidence the results would have been x(sq.root 2) better.
A plot for the central region equivalent to a 5mm pupil at the eyepiece can be seen on the
