From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 



>From: "Rick Maillet" 
>
>I will correct myself before the flames....
>It depends on if you are inside or outside of R.o.C
>not focus, sorry, my mistake...;)
>
>And yes, as Ken has stated, you got a fantastic
>looking surface on that glass, wish my first look had
>been like that.
>
>                                   Rick


Hi
 Looking at a Ronchegram can be confusing. It helps
to think about what you are looking at and getting used to thinking in
terms of where you are looking at things.
 The first thing to realize is that the lines get
broader as you get closer to ROC. This makes thinking about what patterns
mean easier to understand.
 I assume that David's picture was inside of ROC.
This would mean that since the outside ring had its lines closer together,
that surface must be farther from its ROC. This means that going towards
ROC, the first ROC will be the center of the mirror and then the outside
ring next.
 If we were inside of ROC to start, this would mean
that the outside ring of the mirror was long ( flatter ) and the inside of
the mirror was shorter ( more curved ). ( also known as TDE )
 I still get confused looking at these things, especially
if the problem is towards the center of the mirror ( I've gotten used to
looking at TDE ). When I stop and think about the distance that I am away
from ROC of each part of the surface, I can then make sense out of it as to
short or long. I can then change that into terms of surface.
 Go though the steps a few times and you'll not have
any more problems with the Ronche.
Dwight

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