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


From: "Dwight K. Elvey" 

>  I assume you are talking about a Newton. If so, the shadow
> should be off center for both inside and outside of focus.
> This is called offset. An F/4 will have noticeable offset.
>  Can you be more specific? What type of telescope are you talking
> about......

This is a 4.25" f4 Newt.  It has a 1.8" diagonal and I am trying
to fully illuminate the chip on my MX716 camera which is about .5".

I have positioned all the components as they should be including a
.14" offset to compensate for the foreshortening effect.  Looking
through the peep hole it looks exactly like the drawing in Suiter's book.

Problem is that when I tweek the collimation on a star centered in the
field, it is not collimated when I go to the other side of focus.  I ended
up sort of offsetting the view by the same amount on each side but this is
rather disconcerting.

A short image of the a star field slightly out of focus shows the central
stars with the diagonal in the center and all the rest sort of point to the
center.  This is as it should be but I do understand why a central star
does not look collimated on each side of focus visually.

js

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