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Why use the CDP procedure?

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#1 bokemon

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Posted 26 January 2021 - 03:44 AM

Hello folks,

I finally got an XLKP collimator, and first things first - I screwed up and ordered 0.5" too short.  But how can I tell if I am "close enough" to the focal plane?  At some point right before the focuser reaches the end of the travel, the "hall of mirrors" multiple reflections in the offset pupil view goes away.  Is that good enough?  (P & #2 reflections looks about the same size anyway)  I know that if I get the focal plane obviously wrong, then #2 offsets relative to P in the offset pupil.  Since the AC axis is defined by the top lip of the focuser, I can just lathe a ring to raise the AC up a little more if needed.

On to the main question, which is "why use the carefully decollimated primary procedure"?  It has me look thru the center hole to line up P and #3, but #3 is really dim and fuzzy, and its movement seems relatively insensitive to the secondary collimation knobs.  I would much rather jump to the last step with the well-collimated primary and just use P and #2 in the offset pupil to adjust the secondary.  At least #2 is bright and sharp, and I can carefully align based on slivers of gaps between the nuclear symbols.  And then alternate back and forth between this and the Cheshire.



#2 Vic Menard

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Posted 26 January 2021 - 07:13 AM

...On to the main question, which is "why use the carefully decollimated primary procedure"?  It has me look thru the center hole to line up P and #3, but #3 is really dim and fuzzy, and its movement seems relatively insensitive to the secondary collimation knobs.  I would much rather jump to the last step with the well-collimated primary and just use P and #2 in the offset pupil to adjust the secondary.  At least #2 is bright and sharp, and I can carefully align based on slivers of gaps between the nuclear symbols.  And then alternate back and forth between this and the Cheshire.

When you use the CDP procedure with the center pupil, P-3 is only sensitive to focuser axial error (secondary mirror adjustment). Primary mirror adjustment will not change the P-3 alignment. P-2 in the offset pupil (parallelism) is sensitive to both secondary and primary mirror adjustments, so you'll have to iterate between the offset pupil (P-2) alignment and a calibrated Cheshire to reduce the focuser axial error to the equivalent of the primary mirror error.

 

As far as the pupil placement relative to the focal plane, as long as the reflections appear to be the same size as P, you should be fine (you can check using the P-2 alignment while rotating the autocollimator).



#3 bokemon

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Posted 26 January 2021 - 08:52 AM

Thanks for explaining.  But every time I do the CDP and then go back for the final check, there is always a little offset because #3 is just too fuzzy for some reason.  Anyway, I give up doing CDP.

 

Yes, I found out that rotating the AC while you are not at the focal plane causes the p-2 offset to change, and I assume there will be no more changes at the exact focal plane.  (The "correct" alignment of P and 2 should have the offsets be equal and opposite upon rotation)

 

Welll, I got another problem which is that my focuser drawtube actually moves in a very slight arc instead of a straight line.  So I guess that means I need to do the collimation at the same focuser extension that I use when imaging, and either add "rotate AC" as part of my procedure, or space the AC outwards to be at the focal plane.

 

Maybe it won't matter that much since the focuser will droop anyway once I stick on the imaging train.  Last few times I did a collimation without the AC and I got perfect starts over 95% of the image with only one corner elongated by one pixel worth.  So that's pretty close. 


Edited by bokemon, 26 January 2021 - 09:04 AM.


#4 Vic Menard

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Posted 26 January 2021 - 11:39 AM

For the record, P-2 magnifies both axial errors 4X (and the alignment is parallax free), but any shift more than about 0.01-inch will cause reflection 2 to oscillate a noticeable amount behind the center marker (P). P-3 magnifies the focuser axial error 2X (and the alignment can also oscillate a bit due to parallax), and as you've noted, it can be difficult to assess the alignment with shorter (less than about 50- to 60-inches) focal length scopes due to the odd focus.

 

I find that with careful attention to my thin beam laser alignment and subsequent calibrated Cheshire alignment (88-inch focal length), I can usually get a "perfect" stack in the autocollimator center pupil (only P is visible) and a nearly perfect stack of P-2 in the off center pupil. And I find that's good enough for my coma corrected f/4 primary mirror.

 

I suspect for your imaging scope with a tiny amount of focuser "droop", you'll find that the autocollimator precision is overkill. But it's still nice to know you started with that level of precision...



#5 bokemon

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Posted 26 January 2021 - 08:25 PM

For the record, P-2 magnifies both axial errors 4X (and the alignment is parallax free),

Sorry, I don't understand this part about being parallax free.  If the AC is below the focal plane, and I stack P & 2 in the offset pupil, then rotate the AC by 180 degrees, P & 2 aren't stacked any more.



#6 Vic Menard

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Posted 27 January 2021 - 07:31 AM

Sorry, I don't understand this part about being parallax free.

The position of your pupil.




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