Get the NEWT program from the web (freeware) and run it to find the size you want to use.
The secondary needs to be at least a 1/4 wave flat surface on the front - window glass usually isn't that flat but it's not hard to make it that flat.
The cutting of the oval can be done by either chipping it into the ellipse or doing the trepanning cut at 45deg. angle. It's wise to back up the glass in this cutting operation with plaster so that it doesn't break up a lot.

• 6" f/5 primary questions: - lachrimae - October 8, 2002 - 16:37 UTC