Re: ATM 8" f3.8 -- Worthy project?

[Dwight Elvey <elvey@hal.com>]

"Richard" <richas@idt.net> wrote:
> 
> Here is a radical idea not found in authoritative books....
> what if, instead of a wire, you use a small spherical bead?
> Then you get the circular shadow, but not the vertical bar
> shadow.   The bead can be moved from side to side to allow
> testing of tilted or astigmatic surfaces.   Hmmm.....
> 
> And if you replace the bead with a pinhole (the pinhole is
> an inverse bead), I guess you see an illuminated circle.
> Hmmm....
> 
> And if you use a slit, I guess you see an illuminated circle
> with a vertical illuminated bar....  Hmmm....
> 
> Please keep this idea secret, as it is not according to
> established orthodox dogma.

Hi Richard
 Of course, if you go back in the archives, you'll see that
I talked about this myself. There was a paper on the subject
but I've lost the pointer. The biggest issue is making a
round enough bead and a round enough hole in a thin material.
You could swap having the source as the bead or the hole
because of the inversion of the mirror. With your ideas
on supporting mirrors while testing, I would think that
it would work well for detecting astigmatism. In a standard
two pin support like is usually used, it would always show
some astigmatism. Still, it might be easier to see the
relative differences when doing the 90 degree rotation since
the only way to see this now is to look at the calculated
focal length as Jim B.'s program shows ( because you can't
put the mirror back at exactly the same mirror surface position
easily after rotating ). This is one level removed from
actually looking at it and subject to greater error.
 I was thinking about this idea to be used on a fast mirror
except that I was going to have it all coaxial by placing
the bead source closer to the mirror to get a magnification
factor as was shown in my previous post. My problem with this
was that along with the magnification would be an equal
reduction in contrast of the edge of the null. Still, it
is a fun idea to think about.
Dwight