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


>From: lifedata{at}vol.com
>
>
>>The Quality of the diagonal, if it is located at the objective(
>>mirror), needs to be the 'same' as the main mirror tolerance. This
>>tolerance is reduced to the same as the focus tolerance at focal plane.
>
>It seems to me that a given surface error near the focal plane would
>effect a greater "percentage" of the total image, therefore have a
>greater effect.  What's wrong with this logic?
>
>Jim L

Hi
 Just the opposite. A surface near the focal plane effects
a smaller percentage of the image for a particular part of the image. For
each spot at the focal plane, only the cone of light defined by the f/ratio
of the primary has an effect. I think it also needs to be made clear that
there is quite a difference between a secondary and a diagonal. The
secondary of a Newton needs to be good over almost the entire width. (
figure the size of the light cone ). A star diagonal that is close to the
focal point would require less since the cone is small. We are of course
talking about changes that are on the order of a wavelength of light and
not distortions that would be noticeable to the eye when looking at it with
a microscope. Gradual changes in the height of focal point of many wave
lengths of light are well within the eye's depth of field ( if it weren't
we'd be able to figure mirrors with a magnifying glass ).
 Ken's thaought about the light deviating more with distance
isn't totally corect either. Lets assume that we had some distortion that
was less then 1/4 wave for any one spot on the focal plane. If we where to
use a perfect relay lens to bring the focal plane to some other location,
it would be no worse than at the original. Here is a case where we are
farther away but no worse than when we started. Dwight

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