[ATM] astigmatism in star test

[artbianconi@blast.net]

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.

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