From: "Jack Schmidling" 
To: 
Reply-To: "Jack Schmidling" 


From: "Dwight K. Elvey" 

>  Something is wrong here. When focused, the centering of obstructions
> does not effect things like coma. Coma is caused by unequal light
> paths from off axis use of the parabola.....

We seem to have a cause and effect problem here.  I never suggested that
coma was caused by decentering obstructions. I am saying that improper
colimation produces coma and this can also be seen as a decentered diagonal
shadow.

I do not doubt that somewhere out there is the on axis star and there will
be no coma and the diagonal will be centered when defocused.

The point of collimation is to get that star or position in the center of
the field...no?

All I ever claimed is that if the diagonal shadow is not in the center of
of an out of focus star in the center of the field, the scope is not
colimated. When brought to focus, this star will exhibit a
"flair" if you do not want to call it coma.

> I still think you are misunderstanding something about the alignment.....

No doubt but that is the point of this discussion.... to under stand it.

>  Still, the fact that you see the secondary shadow not being
> offset when sufficiently out of focus that the shadow is not
> dominated by diffraction, tells me that something is wrong.

You lost me on this one.  I see the shadow offset in different directions
on either side off focus.

js

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