Re: ATM 21" F/6: response about sagitta equations

["Peter D. Brooks" <pdbrooks@pacbell.net>]

> On Thu, 31 Aug 2000, Michael Koch wrote:
> Do not try to derive it using the angles between the light and the mirror
> surface. This will be difficult.

I did it (was deriving the caustic equations), and yes, it is tough.

To try to clear up the confusion on the parabola vs sphere equations, remember
that in rough and fine grinding, you are usually trying for a sphere, but you
want to end up with a parabola.  For fast mirrors, and especially fast and big
mirrors, the differences will get large.  Further complicating the issue is
that you (usually) know for sure what form you are grinding--we assume it's a
sphere, but as in roughing, we know it's really not.

Pete Brooks