[ATM] Posible variation on the Cassegrain secondary

[Jim Burrows]

At 2006-09-14 07:20 -0400, vladimir sacek wrote:

>As someone with zero hands-on experience, I wonder
>what makes controlling the ROC to within a couple of percent during the
>figuring difficult?

The contrary hypothesis is easier to demonstrate.  When chasing the 
parabola, it is often helps a lot to realize it's not "the" parabola, but 
"a" parabola - there's a infinite number of 'em out there with different 
ROCs.  For example, looking at the current surface deviations from the 
best-fit parabola (the one minimizing the RMS surface error), it is quite 
often true that although the RMS error vs. a parabola with slightly shorter 
ROC is bigger, the center deviations will be higher and the edge deviations 
smaller - an easier situation to tackle.  When the goal is fixed R and b, 
it becomes a 2D problem - what's the best route from where I am to the goal?

         -- Jim Burrows
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