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Tensioning screw changes focuser tube alignment

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#1 precaud

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Posted 04 January 2013 - 10:35 AM

I received the Agena Cheshire/sight tube yesterday, and am starting the collimation process on my AD10. If I rack in the focuser and loosen the Crayford's tensioning and locking screws, the secondary appears well-centered in the sight tube. (I'm guessing that is how it was aligned at the factory). But if I tighten the tensioning screw enough so that it engages the focuser for adjustment, which is how it will be in use, the drawtube alignment is changed and the secondary is no longer centered.

My inclination is to center the 2ndary with the focuser tube tensioned, as that reflects actual conditions of use. But I'm wondering, is there an adjustment so the tube alignment isn't affected by the tensioning screw?


#2 dpwoos

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Posted 04 January 2013 - 02:58 PM

In my thinking, it always makes sense to collimate with respect to your (high power) eyepieces, so however you hold them is how you should hold the sight tube.

#3 precaud

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Posted 05 January 2013 - 09:10 AM

Yes, that does make sense. And given the construction of the Crayford, I now see that it is essential that the tensioner be engaged exactly as it would be in use. This is my first exposure to this type of focuser, and I was confused by instructions that said to rack in the focuser completely to start the secondary centering. And I have never seen mentioned the interaction between tension and drawtube alignment. It's quite significant, from what I see.

#4 precaud

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Posted 06 January 2013 - 01:38 PM

Well this is proving to be a bigger problem than I first thought. Here's what happens.

: With the sight tube installed, release the tensioner screw so everything sits down on the shoulders.
: Now slowly tighten the tensioning screw and watch what happens to the axis of the focuser or, in my case, the sight tube.

On mine, the right side stays down against the shoulders but the left side (toward the OTA opening) "rides up" as the tensioner engages enough to be used. This tilts the focuser axis toward the front of the scope.

So, engaging the tensioner changes the focuser axis to such an extent that I can not adjust the secondary mirror enough to bring it entirely under the sight tube. It has the appearance of being be too far into the tube. But in fact, the real problem is that the tensioner tilts the focuser axis.

Any ideas? I don't see any adjustments on the outside.






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