[ATM] Foucault blind to astigmatism?

[atmpob at yahoo.com (Dale Eason)]

Concerning Foucault testing and astigmatism
you should find comments about it in the archives.  It
has been discussed many times on the list I think. 
Maybe I'm confused about what list.  In particular it
was involved in the discussion of "do slings cause
astigmatism and what other test stand methods cause it
and if they do then why do mirror manufactures still
use those stands to test their mirrors...."

Texereau writes about testing for Defects other than
Figures of Revolution:

"The Foucault test is not recommended for detecting
these defects; the shadows are difficult to interpret
unless the worker is quite familar with them,
especially for certian orentations of the defect with
respect to the knife edge."

Test stand and cell induced astig when in the
horicontal position can create an astig that is 90 deg
to the knife and thus not measurable by the knife.

What I meant by blind is that it does not show up in
the numbers when you reduce the data to surface error.
Humans can see the coma shape if the astigmatism is
severe in both the Ronchi and Foucault.  It probably
shows up better in the Ronchi.  A star test or
simulated star test as described by Texereau is much
more sensitive and can see much smaller astig than the
knife edge test.

Texereau's method shows up astigmatism that I cannot
see at all in the Foucault image nor in the Ronchi
image. 


Dale Eason 

--- Bill Kelley <flex@commspeed.net> wrote:
> Dale,
> In skimming thro the recent posts I didn't notice
> any
> comments on this. I look for astigmatism both with
> Foucault and Ronchi and find it easy to see. One of
> my 
> students found astigmatism in his mirror recently
> with 
> Ronchi set at 2 bars outside of focus. The tilt
> alerted him.
> At focus it was easy to see. We switched to Foucault
> to observe the typical "comma" shape.
> Bill
> 
> 
> 
> > Thanks Don,
> > The reason I asked was that much of the distortion
> for
> > a mirror in the horizontal position as you
> described
> > would be due to astigmatism.  The Foucault test is
> > blind to astigmatism.  So saying that the mirror
> > tested fine under these conditions does not
> confirm
> > that the cell did not induce astigmatism
> distortion.
> > 
> > 
> > Dale
>  
> _______________________________________________
> ATM mailing list http://www.atmlist.net/


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