Any Optics gurus out there?

Apparently an optics guru made this post invisible.

Can anyone direct me to detailed instructions on haw to build a Camera Obscura ?

More precisely:

I know that the head contains a first-surface mirror with a condensor lens assembly below it, and that the viewing table is parabolic.

Now the questions:

How to determine the formula for the table surface: what do I need to know about the lenses to derive the formula?

and yes, a fixed-focus model would be nice.

I asked this some time last year, or something like it-- I’d search but my connection’s wussy. There are some sort of wimpy diagrams out on the web somewhere.