I guess there were enough Newtonians and Newtonian primaries discussed here, I thought of adding a bit more spice ...
A few pics of the progress of my 12" CDK project. Making a secondary of any Cassegrain is quite involving, as one needs to finish TWO surfaces before even starting with the secondary itself. You need to grind and polish a "window" (in my case back of the reference concave tool) so you can see interference patterns later. I used the mirror itself to grind both of the backs really flat (to within one micron saggita). There was of course no need to get that precise, but I guess why not
Polishing went quickly - I stuck polishing pads to the back of the secondary itself (now very flat) and got the required transparency in roughly one hour. You can still see some remaining pits towards the edge, but it will do.
Getting the curve right was another matter. Even having a spherometer that could read one micron, there is still enough guessing to make things er - interesting. I did manage to hit the required ROC to within 3mm in the end with polishing pads, which is close enough as I'm yet to cut the glass with pitch (my plaster tools are still drying out). I used first a convex curve on the secondary as a tool (stuck the pads onto it) to polish the concave side; then used polished concave reference to stick the pads onto it and polish the convex mirror itself. Sounds brave, but I was confident enough after using the pads for many other projects before.
Reference sphere only got about an hour of polish before pads started to move so I had to abandon early. What looks like TDE in the Ronchigram below (snapped just outside ROC) is in fact the outer 5mm that is yet to be fully polished. The secondary itself (convex) got solid 3 hours of polish with pads and it is more or less fully polished. I used Zeiss type machine and 5kg weight on the pin which makes pads cut really fast.
Despite non-conforming pads, concave sphere only shows a minor bump in the center and some groovy zones, which I'm quite confident will disappear quickly once I attack it with proper pitch (Acculap).
More to follow.