Re: ATM Mirror Evaluation (was: Getting close - figuring 21"f/6)

[Aplanatic@aol.com]

Hi Nils Olof:

> I'm not sure I know how you mean - a 
> *diffraction ring* will always be
> present in the situation, but it may be 
> more or less bright. 

Yes, of course.  In practice, under the Foucault test, one can see a clear diffraction ring around the entire mirror when the edge measures well by interference.  As the TDE increases this ring disappears, again in practice.  As a case in point, my little 5" f/4.5 does not show any diffraction ring on the KE side when the outer zone is nulled.  It has about 3 mm of turned edge and the edge is down about 1/2 wave at the wavefront.  

I suspect, but have not shown, that the ring becomes too dim to easily disern under a "normal" brightness slit when the edge is bad enough to make a difference in the star test.  All conjecture based on experiences with my own and other ATM's mirrors.  

Let's say that the mirror has a quadratic turned down edge 3mm in width.  (Very typical.)  At what surface height error, at the edge, does the ring reduce in intensity by, say, a factor of five over the ring's intensity for a perfect edge?

Dave Rowe