[ATM] mirror cells and parabola...

[Mark Holm]

Norm Prince wrote:
>  it
> occurs to me that i might need a pretty fancy cell involving some fancy math
> for what i'm seeing as a pretty darn thin chunk of what appears to be a
> pretty soft chunk of glass.

It's a problem that has a pretty good answer, at least as far as the most basic
design question is concerned.  We need to know the glass type (plate, Pyrex,
BVC, etc.), the edge thickness (accurate to 1 mm), the diameter (accurate to a
couple mm), the intended focal length (accurate to a few cm) and the intended
secondary diameter (we can make a reasonable guess on that an be fine).  I'll
run the numbers through Plop and give you a support geometry that will work.
Plop does the fancy math.  As to how fancy the cell needs to be, we let Plop
tell us that.

If you don't know what Plop is, and want to have some idea what I am talking
about, start here
http://www.atmsite.org/contrib/Holm/Plop_optimized_cells/cellfaq.html
and here
http://www.atmsite.org/contrib/Holm/Plop_optimized_cells/index.html

> 
> I've been thinking about figuring too, and running some numbers...it seems
> to me that parabolizing an f/5 doesn't really buy me much.

Yes, at f/5 and 12 inches, you do need to parabolize.


> Does
> the correction help in other ways??? chromatically...etc.?

No, it doesn't involve the color of light at all, only what is called spherical
aberration.  With as much SA as a 12 inch f/5 sphere would have, you would never
get a sharp image.  Everything would always look fuzzy.


-- 
Mark Holm
mdholm at telerama.com