To: "A. Suykerbuyk" ,atm{at}shore.net
From: Mark Suchting 
Reply-To: Mark Suchting 


At 10:00 PM 8/11/2003 +0200, A. Suykerbuyk wrote:


>I wonder if someone could explain the phase map to me. Is this map showing
>the wavefront?
>Is a turned down edge visible? Thanks for your help.


The phase map is the wavefront.

I suspect the sharply rolled edge visible in the star test, has been has
been masked off the analyzed data and doesn't appear on this map.

The outer zones , beyond the 70% zone, only _appear to be rolled_ , because
the mirror is overcorrected with reference to a best fit Parabolaid of -1
Conic Constant. an overcorrected mirror always shows a best fit wavefront
to ideal that has  a high 70 % zone ( and hence a low edge) . Thats why it
is always corrected by polishing down the 70 % zone.

If this  mirror were actually undercorrected ( low 70 % zone ) by the same
amount  there would be some _finite_  distance away that an artificial 
`star' would come to a focus with a negligable  wavefront figure error . If
that `star' distance brought as close as to equal the  Radius of Curvature
of the mirror, the aspheric correction required would become nil, ( meaning
a Conic Constant of 0..compared to that of a parabaloid at -1 ) of  and a
spherical mirror would produce a perfect image with  a perfect ( nil )
wavefront error . This is why a spherical mirror tested at a distance of
the Radius of Curvature produces an Optical `Null' with a perfect flat
cut-off in the foucault test and perfectly straight ronchi bands  in the
Ronchi test.

Hope this helps to give some clarity to  the issues of relative wavefront
shape , at  best focus.

~Mark Suchting

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