Fold Mirror Testing

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

AstroKeith 2014