I have become addicted to building telescopes I think. I fixed this 10" dob that my local planetarium has in its care right now, and ever since then, I have become obsessed with designing a killer telescope. This bad boy is going to be a monstrosity. I just have one problem, I don't know how to figure the focal length of a primary mirror.

I don't suppose anyone knows the math behind figuring it out do they?

Diameter times the f number. 

If you are planning on grinding your own mirror, you will have one additional calculation to perform.  That is determining the radius of curviture.  This will allow you to cut a template to check the depth of the curve accross the mirror front.

But then again there are some excellant commerical mirrors available.

Cook

You can also measure it experimentally. Go to my web page (address in the signature) and look at the OTA page. I just clamped my primary to a portable work bench and measured it experimentally.

Good luck with your project,

Bill

Why not just go here : http://stellafane.org/