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


From: "Nils Olof Carlin" 

> Using a sight tube of appropriate ratio length/inner dia (approx. the
> f/ of your primary, or a little less), you should see the circle of
> the secondary centered within the sight tube opening (at least if you
> temporarily hold a cardboard piece "below" the secondary, to get it
> fully illuminated). This way, you don't have to actually determine the
> optical center of the secondary - not so easy to do. But if this is
> done, and after the collimation of the primary, the optical axis (of
> the focuser as well as of the primary) will be at the optical center.

Ah so!

You all may recall that this all started out with my problems gluing an
eliptical secondary to an eliptical support of a significantly smaller
size. Getting the two lined up and centered (whatever that means in this
case) is maddening and totally baffles me.  Net result is, they are not on
the same centers and probably not orthogonal to each other.  This is
apparent when eyeballing from the front of the scope.

1.  May I presume that if centered in the long sight tube this problem is
compensated for?

2.  I have spent hours on end trying to understand this and getting it to
behave as you all say it should but it simply does not. No matter what I
do, I get a different location of the diagonal shadow on either side of
focus and the sight tube, while centered over the diagonal, is not centered
in the reflection of the diagonal.  It is offset to the front of the scope.

3.  Moving the diagonal up and down with reference to the eyepiece, has no
effect on it's position in the sight tube.  It stays centered exactly as my
humble brain thinks it should.  Moving it side to side and fore and aft
moves it relaative to the sight tube.

The bottom line is my images over a .5" field have coma that differs
from one side to the other.  If I focus it away on one side, it shows up on
the other and vice versa and this is driving me nuts.

So, where should I go from here?

js

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