To: "Russell Jocoy" 
Cc: atm{at}shore.net, owner-atm{at}shore.net
From: Hugues.Laroche{at}ses-astra.com
Reply-To: Hugues.Laroche{at}ses-astra.com



Hi Russell,

>Looking inside of the ROC the ends of the ronchi lines hook in.
>I fear this is TDE am I correct?

Yes it is TDE, or rolled edge if it is wide.

>Also I have either a hole or hill in the center.
>It is not massive but I don't know if it goes in or out

The trick is that when you look at the Ronchi bands projected on any zone
of the mirror, the closer your grating is to the COC center of curvature
of this particular zone the more magnified the bands will appear.

When you are near the overall COC this all becomes confusing.

If you are inside ROC and the end of Ronchi lines appear hooked in, that
means that where the grating is, it is closer to the COC of central zone
than outer zone, in other words the edge focuses long. Note that the
paraboloid does that too, by a well defined amount, so it is not
automatically bad.

I have seen a mirror like yours recently. In that case there was a central
hole. When inside ROC, it appeared like an eye while the bands were still
more or less straight and numerous. Conversely if it was an hill the
straight lines would get closer together near the center.

Good luck and happy polishing,

Hugues Laroche



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