Re: [ATM] astigmatism as seen via ronchi?

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

This is for a standard dob-newt, not the Lurie-Houghton.
On the latter, I have found that with nearly-flat-but-not-quite-flat surfaces, it's really hard to get them right!!!

Bob May <rmay@nethere.com> wrote: Yeppie, it generally means a problem!
Run the Ronchi closer to the ROC and get it to show just one line
across half the mirror and you will see the S shape really bad.
Is this what you're seeing with that Lurie-Houghton???  If you
want to, I'd love to see the scope and see what the difficulties
are with your set of glass.
Right now, we're fighting a bit of astig. on the concave R3
surfacce, mostly due to the fact that the glass is on the bottom
of the stack of stuff on the machine.  It is now down under .2
waves and that is mostly at the outer edges of the surfacee.  We,
are, of course, assuming that the convex surface is perfect
although it may not be.    On the Ronchi test with one wave of
astig., the glass showed a nice reversal of the line with the
abovce check.  Bery interesting to see the line start from the
bottom, go around the surface to near the top and then go down
the central area of the surface, getting near the bottom and then
going around the other side of the surface to the top!
We got most of the astig. out by doing a lot more rotating of the
glass and by using a more compliant support material than the
drawer liner stuff (that screen shaped material that is now being
sold for draweer linings of a foam plastic) and all of that seems
to have helped.  Now to get rid of that last quarter of an inch
of TDE and we'll be done with the concave surface, hopefully.
The TDE is slight but it is there and needs to be eliminated for
the perfect lens.
I'll also note that astigmatism is something that is polished in,
not ground in and needs to be eliminated in polishing the
surface.
Checks that I'd also do on a lens is whether there is some wedge
in the glass.  This will make for all kinds of errors in the
incoming light.  Wedge needs to be measured to less than a thou
of an inch and needs to be done accurately!
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net


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Guy Brandenburg, Washington, DC
My home page on astronomy, mathematics, education:
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or else 
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