From: "Dan Chaffee" 
To: 
Reply-To: "Dan Chaffee" 


Mark H.:

>Wavefront errors are additive, period.  It doesn't matter how close or far
>away an optical surface is.

I think this is important to remember, and shouldn't be confused with the
fact that shortening the distance form the diagonal to the eyepiece uses
less of its surface, and therefore relaxes the tolerence of the entire
surface. Once waves are out of phase by a certain amount, I don't see why
changing the distance of the aberration's origin to the image plane will
have any impact on the wavefront.

> If you are considering only a single "point" in the final
image, say the
>image of a single star, it is likely that your diagonal is somewhat larger
than
>the minimum needed to catch all the light from the primary headed for that
>point.

An easy way to determine how much of the diagonal is used to form a star
image on axis is to go to Mel's diagonal calculator and see at what size
the diagonal no longer gives 100% illumination for a 0 deg. off axis image.
For example, if I have a 16" f/6 mirror with a diagonal 11 inched from
the focal plane, the diagonal is 1.84" minor axis at its minimum size
for 100% illumination on axis.

Dan

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