From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 



>From: "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?

Hi
 As I stated, I've shortened the sight tube so that I can see the entire
secondary. If you can't see the entire thing and you don't want to modify
the sight tube, some way of indicating the secondaries optical center makes
sense. Just don't mark the secondary's center.

>
>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.

 This is as it should be. You can either do the fore/aft and
right/left as is recommended by Chuck Dethloff or you can tilt the focuser
if it was only a small amount. Your up/down adjustment does not show
directly from the focuser. It only effects the path that the optically
centered light will follow into the tube ( after collimation is done ).
 The only way I know to check that offset is to use a ruler and
slide it in from the aperture's side, while looking into the peep sight.
When the ruler just becomes visible, at the edge of the primary, to the
peep sight, take that measurement. It should measure the same for the side
the focuser is on as the opposite. If they are not the same, the offset
toward and away from the focuser is not right. Dwight

>
>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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