From: Mel Bartels 
cc: atm{at}shore.net
Reply-To: Mel Bartels 


> > As it should be - but relative to the optical axis that is centered in
> > the sight tube, it is the reflection of the diagonal that appears
> > offset towards the primary....
>
> Isn't this a contradiction?  What did I miss?

No - the diagonal appears centered under the focuser but the shadow of the
diagonal as seen from the centerline of the primary appears offset - two
different things.

>  But as a point of interest, a de-centered diagonal shadow always resolves
> itself into a coma when focused so it is a true symptom of the disease, in
> spite of what everyone says about having to be very close to focus and at
> high power.


No.  What you are saying is that any offset shadow or obstruction placed in
front of the tube would magically produce coma when there was none before. 
That's simply not the case - try it yourself and see.  Take a 1/3
obstruction made from cardboard and move it to and fro over the front of
the tube.  The defocused star image sure enough shows the moving shadow,
but the properly focused image will not change (except for very minor
effects relating to the change in the Modulation Transfer Function).

Coma does not result from obstructions whether centered or not.  Coma for
paraboloids is an off-axis issue, not an on-axis issue.

Mel Bartels

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