Re: ATM : Re: Astigmatism

[Hugues.Laroche@ses-astra.com]

Hi Dan,

>What do you consider horribly severe? Can you give us some rms values
>based on your findings in terms of the threshhold of detectability with
>foucault or ronchi?

And they kept saying one should take notes while figuring & testing!
And I did not do it! Bad guy.

OK, from memory then, hopefully it is faithful: my 26" f/4.25 had an
overall
astig causing the focii of both axis being separated about 2-3 mm.
Actually such a value is quite inferior to the longitudinal aberration to
be
created for 100% correction, so I guess
it could have been polished out (I did not attempt for other reasons), and
should not be classified as "horribly severe".

On the Ronchi test, with the astig axes 45 deg from the Ronchi bands,
the astig was slightly visible as ying/yang on the central band when close
to ROC. That is, the central band had to be rather magnified (1/5 diam
of the mirror maybe) for that to be visible - but it was unmistakably
there when looking critically.

Hope it helps,

Hugues



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