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Equipment Discussions >> Reflectors

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Vic Menard
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6215547 - 11/25/13 04:13 PM

Quote:

...yesterday I had good mirror clips showing and was chasing the doughnut a little. I must have done something last night before I retired because today all I have been doing is chasing mirror clips. I can get them to the very edge of my secondary with only a tad of each showing and thats it.



What's the distance from the center of the spider to the top of the collimation cap? And how is your secondary mounted (glued to stalk or inside a split shell with a lip around the front edge)?

Quote:

If at that point if I try to use the tilt screws to try centering my doughnut I will definitly lose the mirror clips. This thing (I call Goliath )has got me headed to the funny farm.



Remember, the secondary mirror placement doesn't need to be perfect. Only the axial alignments have tolerances, and if you're not using coma correction, the tolerance for the focuser axial alignment (those confounding tilt screws your using to center the donut) is quite forgiving--about 3- to 4-percent of the primary mirror diameter.

So, take a breath, and no funny farm talk until after Thanksgiving!


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alonzo
super member


Reged: 06/06/12

Re: different views thru collimation cap and cheshire new [Re: Vic Menard]
      #6215618 - 11/25/13 04:43 PM

....Vic I am good with the mirror clips now. It was just as you said I had the collimation cap racked all the way out and now my clips are standing proud again. I also noticed on the very outer edge of my primary I can see a we3 bit of my rear telescope tube. Should I clear this condition up before I go any farther by moving the secondary down a little. After centering the secondary I noticed that I am almost collimated as it sits. I have mirror clips standing proud,spider vanes are equal,secondary centered and the primarys doughnut well within the collimation caps circle.
I am almost tempted to leave well enough alone and just use the primary adjusters to bring it home. It is so dang close the doughnuts outer edge is actually touching the peep holes dot ( black dot ) in the view of the collimation cap. My next move will be use the cheshire as you said earlier and bring it home. I want to mention that the outer edge I am seeing of the back of my telescope tube is just a little and its equally all around.

Thanks
Alonzo


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Vic Menard
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6215651 - 11/25/13 04:59 PM

Get the primary mirror center spot aligned with the collimation cap circle (fine adjust the primary mirror tilt) and see if the scope focuses on a star or Venus (something not terrestrial), and if it does, how far the focuser is racked in or out with the scope focused.

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alonzo
super member


Reged: 06/06/12

Re: different views thru collimation cap and cheshire new [Re: Vic Menard]
      #6215699 - 11/25/13 05:15 PM

.Vic I will continue tommorrow and post my results. My wife and I have guest coming over tonight so I must discontinue for now. I am asasauming the we bit of rear telescope tube that is showing is o.k. and will not impact collimation of the scope.

Alonzo


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alonzo
super member


Reged: 06/06/12

Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6217189 - 11/26/13 01:09 PM

....Vic,Jason,Nils and all others that chimed in with your help I am COLLIMATED ( I think ). My spider vanes are all equal,my mirror clips stand proud even when focuser is racked
at its highest position,the secondary is dead centered in the view and the doughnut is dead center on the back black dot created by the collimation caps peep hole. I did this with a few minor tweaks of the primary and with the collimation cap. Vic I will double check it with the cheshire to be sure. You had mention in an earlier post that for this step I do not need to use an extension. If all appears that in fact I accompolished collimation I want to thank each of you for hanging with me. Someday I hope I will gain enough knowledge to chime in on some newbie with collimation problems from my continous experience in our hobby. I cannot praise or thank you guys enough.

Thank you,thank you,thank you.
Alonzo

Edited by alonzo (11/26/13 01:12 PM)


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Vic Menard
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6217282 - 11/26/13 01:55 PM

Quote:

....My spider vanes are all equal,my mirror clips stand proud even when focuser is racked at its highest position,the secondary is dead centered in the view and the doughnut is dead center on the back black dot created by the collimation caps peep hole...I will double check it with the cheshire to be sure. You had mention in an earlier post that for this step I do not need to use an extension...



You don't need the extension...the Cheshire/sight tube cross hairs/center spot alignment will tell you whether or not the focuser axis is in tolerance.

Like most anything else, practice makes a big difference (remember, it doesn't have to be perfect!) Add persistence and patience, and you will soon be able to share your knowledge and expertise!


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john gabriel
sage
*****

Reged: 12/13/11

Loc: Southern California
Re: different views thru collimation cap and cheshire new [Re: Vic Menard]
      #6218531 - 11/27/13 01:24 AM

Please pardon my jumping in, but I have been following this conversation with interest, and I would like to ask the question in a different way - just to see if I really "have it"

If my newt 2ndary is perfectly centered (even from the standpoint of correct offset) and has a perfect (45*) tilt as well, are you (Vic and Jason) saying that the primary center spot will be centered under my cheshire cross-hairs - even if the 2ndary rotation is off? I have a hard time getting my head around that one.

Everything but the 2ndary rotation is "spot on", and the primary center will line up under the cheshire cross-hairs, even though the 2ndary rotation is off? If that is so, then only misalignment of the focuser axis itself would be the cause of any error between the cross-hairs and the primary center!

Thank you for entertaining my question! Your answer will probably not take the mystery out for me - it is just so hard to visualize the 2ndary rotating around this way or that, and yet leave the cross-hairs on the primary center!

Regards,
John


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Jason D
Postmaster
*****

Reged: 10/21/06

Loc: California
Re: different views thru collimation cap and cheshire new [Re: john gabriel]
      #6218559 - 11/27/13 02:11 AM

Let us start with a scope that has its secondary mirror perfectly positioned. Now rotate the secondary stalk by a little amount then use only the 3 tilt set screws to realign the cross hairs with the center spot reflection. The question is how can we consider the secondary mirror aligned when we applied little rotation? Well, mathematically speaking both setups have the planar angles of the secondary mirror surface "identical". See, there is an inherent planar rotation when you use the 3 tilt set screws placed at (120 degree angles) and this inherent rotation can undo the initial rotation. I know it is confusing.
If you only had two tilt set screws placed along the focuser axis then you can't correct for rotation errors using the tilt set screws. But having the set screws at both sides of the focuser axis will introduce a planar rotation that can be used to correct any small initial rotation errors.
Jason


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john gabriel
sage
*****

Reged: 12/13/11

Loc: Southern California
Re: different views thru collimation cap and cheshire new [Re: Jason D]
      #6218605 - 11/27/13 03:45 AM

Jason, please pardon my thick head. Your first two sentences are telling me that the answer to my question is "no" (which actually does make more sense).

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.

This stuff is certainly reworking some brain cells that once seemed to do better! Thank you!


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Vic Menard
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
Re: different views thru collimation cap and cheshire new [Re: john gabriel]
      #6218853 - 11/27/13 09:20 AM

Quote:

...If my newt 2ndary is perfectly centered (even from the standpoint of correct offset) and has a perfect (45*) tilt as well, are you (Vic and Jason) saying that the primary center spot will be centered under my cheshire cross-hairs - even if the 2ndary rotation is off?



There's a lot of perfects in there--I'm not sure if you intended to catch them all. But let's assume for this part of the question you did.

1.)If everything is perfect, full classical offset, intercept angle, secondary mirror elliptical shape, focuser "squaring", spider squaring, UTA to mirror box centering and squaring, all mechanicals precision made and perfectly fitted (e.g., secondary mirror holder, secondary holder mounting stalk, spider receptacle for mounting stalk, etc.)...

2.)...and the secondary mirror placement is visualized as a circle bounded closely on the outside by the bottom edge of the sight tube and on the inner edge by the reflection of the primary mirror (and focuser and primary mirror axial alignments are correct)...

3.)...then there's minimal chance that there is any residual secondary mirror combined tilt/rotation error.

But it's unlikely that you can count on this degree of precision with respect to paragraph 1, although you can still achieve the optimal alignments in paragraph 2, with the same paragraph 3 result.

Quote:

Everything but the 2ndary rotation is "spot on", and the primary center will line up under the cheshire cross-hairs, even though the 2ndary rotation is off? If that is so, then only misalignment of the focuser axis itself would be the cause of any error between the cross-hairs and the primary center!



No. If the secondary mirror rotation is off, so is the secondary tilt (to get the cross hairs aligned to the primary mirror center spot)--so "everything" is not "spot on". The better question is, can the error be easily corrected, or will correction reveal an underlying mechanical error that will be much more complicated to correct, and if so, is it worth the effort?

(The focuser axis is only one consideration. There are several other potential errors (mentioned above) that can create the same skewed alignment that often ends up being accommodated with a combined rotation/tilt error.)

Edited by Vic Menard (11/27/13 09:30 AM)


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Vic Menard
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
Re: different views thru collimation cap and cheshire new [Re: john gabriel]
      #6218877 - 11/27/13 09:29 AM

Quote:

...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.


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john gabriel
sage
*****

Reged: 12/13/11

Loc: Southern California
Re: different views thru collimation cap and cheshire [Re: Vic Menard]
      #6227965 - 12/01/13 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!
I've been out with family for several days, so just now getting back to this. Hope everyone had a great Thanksgiving!
John


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