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


>From: "Richard Schwartz" 
>
>I am wondering if the quality required in a diagonal depends on the distance
from the diagonal to the focal plane.   If the diagonal is very close to the
focal plane, perhaps error in the diagonal surface can't do much damage....
>
>. . . Richard

Hi Richard
 I agree but a secondary is much farther into the cone and
any spot at the focal plane uses almost the entire mirror. I think one has
to take the cone defined by the f-ratio and expand that to the diagonal.
That cone should cover a smaller surface that is better than 1/4 wave. Of
course, the entire path from primary to secondary to diagonal needs to be
better than 1/4 wave. If one were to think in these terms, if the entire
diagonal was better than 1/4 wave, the smaller portion that was defined by
the cone would add little to the over all error. A perfect primary and
secondary could tolerate a larger error in the diagonal.
 That's the way I see it.
Dwight

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