[ATM] Re: astigmatism in star test

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

The fellow that had the astigmatism in our lab had had
the unfortunate habit of not rotating his mirror and
lap hardly at all. I hadn't paid enough attention to
what he was doing, what with relatively large crowds
of people, and given that he didn't ask for advice and
seemed to be doing everything else correctly. Once we
identified the problem, it was relatively easy to
eliminate the vast majority of the astigmatism by
doing the normal/regular rotation of tool and mirror
for a couple of hours, and the final figure looked
quite decent.
This makes me think that avoiding astigmatism on a
normal mirror is not really very hard. He worked very
hard (in retrospect) to create a cylindrical mirror,
but it was not hard to eliminate the cylinder.

Guy
--- artbianconi@blast.net wrote:
> In all the time I have been focused on making
> mirrors I can never 
> remember having seen any conversation on
> asymmetrical 
> aberrations. Some combination of turned down edge, a
> hole in 
> the center or a mound always seemed to dominate the
> dialogue 
> and what followed was the inevitable offered
> solution of how to 
> hold to tool, the pattern, overlap and extent of the
> stroke to 
> correct the condition.
> 
> This is the first time I can remember seeing
> anything discussed 
> where the mirror had a problem in one quadrant or
> hemisphere 
> that wasn't uniformly present to a similar degree
> elsewhere 
> "around the clock".
> 
> Have I not been paying attention? Was I perhaps
> having a 
> "Senior Moment"? Or, is this perhaps as rare a
> problem as my 
> memory suggests?
> 
> Art Bianconi
> 
> 
> Guy  Brandenburg, Washington, wrote:
> 
> I have seen astigmatism in a ronchi bench test. It
> showed up 
> when one end of the lines was fatter than the other.
> We rotated 
> the mirror by 180 degrees, and you could see that
> the fatter end 
> of the lines had rotated by 180 degrees. Also, if
> you rotated the
> mirror by other increments, the ronchi lines looked
> a little bit  like 
> the letter S. With some serious work, the maker of
> the mirror was
> able to fix it and ended up with a very fine mirror
> indeed. 
> However, I was not able to do a startest on it.
> 
> Jerry B. Hillman wrote:
> 
> > Similar to Foucault, a Ronchi star test can't
> > see primary astigmatism.  If the Ronchi grid 
> > is parallel or perpendicular to the astigmatism
> > axis, the lines will be just as straight as a 
> > perfect mirror. If they are at an angle to the
> > astigmatism axis what you'll see is still nice
> > straight lines, but slightly tilted.
> 
> =====
> 
> 


=====
Guy  BrandenburgWashington, DChttp://home.earthlink.net/~gfbranden/GFB_Home_Page.html


	
		
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