Michael, I hadn't forgotten your reply to this thread. Yes, the focal plane will be altered, and I had been expecting that, but it can be dealt with.
I had posed this question too in Cloudy Nights. It turns out that there is a Newtonian design that does incorporate a correcting lens system in a Newtonian that uses a spherical mirror - the Bird-Jones Newtonian. It sees a "barlow" corrector lens placed between the primary and the secondary. Fake Bird-Jones scopes actually just have a barlow lens stuffed into the focuser's drawtube and offers no correction at all for spherical aberration. In fact, a Newt using a spherical mirror with a natural f/8 figure shows less SA than a fake barlowed Bird-Jones with a synthesised f/8.8.
The best known true Bird-Jones Newtonian is the fabulous old Tasco 8V produced by Vixen. I believe Vixen produced other true Bird-Jones Newts.
One of the replies on CN gave a fantastic link to the sort of thing I have in mind in removing the barlow from the drawtube. The link also show how to
force a spherical mirror into a parabolic shape by pulling down the centre of the mirror down from behind! Just ingenious:
How to convert a hybrid short tube Newtonian into an enjoyable RFT
Just the sort of thing that appeals to me too,
