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different views thru collimation cap and cheshire

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#51 Vic Menard

Vic Menard


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Posted 27 November 2013 - 09:29 AM

...I'm going to have to cogitate on this and re-read this whole thread. Somewhere in the earlier comments led me to think (you or Vic?) were saying that the cross-hair alignment with primary center would not change even as the 2ndary is rotated on its stalk.

I'm pretty sure neither Jason nor I would make such a statement.

But Jason has provided some pretty cool animations that illustrate what I've been saying for many years...that there are an infinite number of secondary mirror placements for any fully corrected axial alignment, but only one is the optimal secondary mirror placement.

#52 john gabriel

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Posted 01 December 2013 - 09:07 PM

Thank you, Vic. You've answered my question. I think it might have been Jason's 1st picture in his 3rd post (page one). (These are great diagrammatic displays, Jason!) The 2ndary moving around in the upper illustration made me think that the cross hairs would stay centered on the primary, even as the 2ndary was moved from left to right (up and down in the digram). I see now that this was not Jason's intention with that particular graphic.

You've reaffirmed for me that my original intuition was correct - the 2ndary tilt angle relative to the primary would change - not if the 2ndary is moved Up and down the tube, but if it is moved from side to side (with stalk center remaining centered at the spider axis) or rotated on its stalk.

I've enjoyed this discussion very much, and am clear - at least for my scope - that the focuser alignment is my main problem. I have to go back and check that very carefully, before re-doing the main collimation steps.

Thanks again!
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