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Equipment Discussions >> Reflectors

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Vic Menard
Post Laureate
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Reged: 07/21/04

Loc: Bradenton, FL
Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6213150 - 11/24/13 12:17 PM

Quote:

...I will try the best I can to center the secondary with the cheshire. This is done by extending the cheshire very carefully up the focuser with an extension tube.



I don't understand why you need to use an extension tube. What's the aperture and focal ratio of the scope you're collimating? I assume you have a 2-inch focuser and a 1.25-inch Cheshire/sight tube. What kind of 2- to 1.25-inch adapter are you using?

Quote:

I don't like doing it this way but if I can just get halfway close to collimation I can do a star test later and bring it in.



If you don't like doing it this way, then don't. If the fit is too sloppy (what I refer to as "registration"), your result will be inconclusive. And the alignment you're trying to accomplish (secondary mirror placement and focuser axial alignment) is not readily visible in a star test.

Quote:

...The collimation cap offers a much wider view of my efforts. Once this is done I will attempt to put the cheshires cross hair on the primarys center spot.



OK--this part of your procedure is wrong. Using your Cheshire/sight tube, you should be able to see three alignment circles: the bottom edge of the sight tube, the actual edge of the secondary mirror, and the reflected edge of the primary mirror. When these three circles are concentric, the sight tube cross hairs will already be aligned to the primary mirror center spot.

The collimation cap offers a similar view, except the three circles are of significantly different diameters: the much larger outer circle is the bottom edge of the focuser drawtube, the actual edge of the secondary mirror is still about the same, and the reflection of the primary mirror will be smaller if the collimation cap pupil is closer to the secondary mirror than the pupil of the Cheshire/sight tube. You can make the last two circles similar if you keep the pupils (Cheshire/sight tube and collimation cap) about the same height above the outside of the focuser/OTA.

After you've sorted out the secondary mirror placement (three circles aligned by adjusting secondary mirror rotation and fore and aft position) and the focuser axial alignment (sight tube cross hairs lined up with the primary mirror center spot by adjusting secondary mirror tilt)--then you can use the bright Cheshire ring in the combo tool or the collimation cap pupil to align the primary mirror center spot by adjusting the primary mirror tilt.

The first two adjustments--secondary mirror placement and focuser axial alignment--impact each other (as you've already found out). By limiting the alignments to specific adjustments as I've indicated above, and repeating the adjustments sequentially (secondary mirror placement--focuser axial alignment--secondary mirror placement--focuser axial alignment...), the errors will become progressively smaller until there is little or no noticeable improvement. Assuming the focuser "squaring" is correct, when you reach this point of "diminishing return", I suggest you finish with the best focuser axial alignment you can (sight tube cross hairs accurately aligned to the primary mirror center spot) and then proceed to the final primary mirror axial alignment.

Assuming the primary mirror center spot is reasonably well placed, once you've finished these three alignments your scope should easily pass a star alignment assessment.


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Jason D
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Reged: 10/21/06

Loc: California
Re: different views thru collimation cap and cheshire new [Re: Vic Menard]
      #6213254 - 11/24/13 12:59 PM Attachment (14 downloads)

Quote:


Jason, I wonder if you could create a graphic like this one showing the secondary mirror offset downward (toward the six o'clock position) to demonstrate when the user needs to consider realigning the focuser (assuming the spider/secondary mirror are properly centered).




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Jason D
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Reged: 10/21/06

Loc: California
Re: different views thru collimation cap and cheshire new [Re: Vic Menard]
      #6213258 - 11/24/13 01:00 PM Attachment (15 downloads)

Quote:

A second graphic, showing the secondary skewed toward the center of the fov (an ellipse with the centered primary mirror reflection) would demonstrate the other possible "best" alignment with a misaligned focuser.

This would help clear up what looks like rotation/offset/tilt combined errors when in fact, the focuser needs to be shimmed/leveled...




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Vic Menard
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Reged: 07/21/04

Loc: Bradenton, FL
Re: different views thru collimation cap and cheshire new [Re: Jason D]
      #6213285 - 11/24/13 01:16 PM

Jason,
Thank you!
I hope these graphics will prove useful when trying to diagnose secondary mirror alignment issues caused by mechanical focuser misalignment.
You rock!


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alonzo
super member


Reged: 06/06/12

Re: different views thru collimation cap and cheshire new [Re: Vic Menard]
      #6213576 - 11/24/13 04:26 PM

....Vic I am using an extension tube for my cheshire because the cheshire will not incorparate the view I need of the secondary. In order to fully see the secondary the cheshire needs to go up farther up my focuser in order to see the secondary completly. I am using a moonlite focuser with only .5" travel.Today I decided to try collimation with a collimation cap and things have began working out better for me. I know the collimation cap is the weakest link in collimation but so would the registration on the cheshire. Today I have been able to center the secondary under the focuser with good results. I am able to see all three primary mirror clips in good relief and center my primarys center doughnut on the collimation cap. The rest of this afternoon I have been chasing that same doughnut with the primarys adjusters. I have so far been able to get the doughnut in the center of the silver circle of the collimation cap and eclipsing the the black dot but its not quite there yet. I am handicapped in using the cheshire or sight tube because they are not long enough. I am building
a 12.5" f/4 telescope and using a 3.10" secondary and the tube diameter is 16". Its quite a beast especially with that heavy Coulter mirror of yesterday. Vic I guess I am stuck with the collimation cap until I can find a long sight tube or cheshire to accomadate my needs. Getting that big secondary mirror in the cheshire is just not possibe and thats why I opted later this morning on the collimation cap.


Alonzo


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Vic Menard
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Reged: 07/21/04

Loc: Bradenton, FL
Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6213901 - 11/24/13 07:28 PM

Quote:

...I am using an extension tube for my cheshire because the cheshire will not incorparate the view I need of the secondary.



OK--now we're getting somewhere.

When you make the adjustments for secondary mirror placement (make three circles concentric by adjusting secondary mirror rotation and moving secondary mirror fore and aft)--use the collimation cap.

When you make the adjustments for the focuser axis (align the cross hairs to the primary mirror center spot by adjusting the secondary mirror tilt)--use the Cheshire/sight tube combo tool. You won't need to use the extension tube to make this adjustment.

Quote:

...I am using a moonlite focuser with only .5" travel.



Why did you choose a focuser with such short travel?

Quote:

Today I decided to try collimation with a collimation cap and things have began working out better for me. I know the collimation cap is the weakest link in collimation but so would the registration on the cheshire.



The way you're using the collimation cap looks OK because it's not showing the focuser axial error. At f/4, I suspect you will be using a coma corrector (the Paracorr 2 works great with my f/4 scope). But that also means your focuser axial tolerance will be about 1/2 of 1-percent of the primary mirror diameter--or +/-0.06-inch. And that means you need to be working harder to get a good, consistent read with your sight tube. If your 2- to 1.25-inch adapter isn't getting the job done, I find a Glatter Parallizer delivers the goods.

Quote:

Today I have been able to center the secondary under the focuser with good results. I am able to see all three primary mirror clips in good relief and center my primary's center doughnut on the collimation cap.



Unfortunately, that still leaves the focuser axis out of your collimation procedure. For the best image performance, you need to be keeping an eye on your axial alignments.

Quote:

The rest of this afternoon I have been chasing that same doughnut with the primarys adjusters. I have so far been able to get the doughnut in the center of the silver circle of the collimation cap and eclipsing the the black dot but its not quite there yet.



Getting the primary mirror center spot (donut) aligned to the collimation cap's pupil is critical for an f/4 optic. The tolerance is very small, +/-0.014 inch. But the collimation cap, like a Cheshire, magnifies any residual error 2X, so the read tolerance is +/-0.03-inch, which is a pretty easy read and not too hard to manage (if your primary mirror tilt adjustments are precise).

Quote:

I am handicapped in using the cheshire or sight tube because they are not long enough.



As long as you can get a consistent read using the tools you have the way I've described, your Cheshire/sight tube combo tool should work.

Quote:

I am building a 12.5" f/4 telescope and using a 3.10" secondary and the tube diameter is 16". Its quite a beast especially with that heavy Coulter mirror of yesterday.



Let's see, maximum intercept distance is 3.1 X 4 = 12.4-inches. With a 1/2-inch fully illuminated image, your intercept distance is about 10-inches. You have a 16-inch tube, so, assuming your focuser height is 2-inches, your focal plane will end up close to the top of the fully racked in focuser. Even at minimum focus, the sight tube pupil shouldn't be much more than 3-inches above the top of the fully racked in focuser. I'm guessing your Cheshire/sight tube is about 5X longer than the inside diameter of the bottom aperture, which means it's too long to work correctly (the multiplier should be equal to or a little less than the focal ratio).

Quote:

...I guess I am stuck with the collimation cap until I can find a long sight tube or cheshire to accomadate my needs. Getting that big secondary mirror in the cheshire is just not possibe and thats why I opted later this morning on the collimation cap.



The Cheshire part of the combo tool (the bright Cheshire ring) and the cross hairs should work fine as is. You'll just need to use the collimation cap to assess and correct the secondary mirror placement.

If you want to use a sight tube or combo tool that will work with your scope aligning the secondary mirror placement you'll need to upgrade. To see all of the secondary mirror (and the full primary mirror reflection and mirror clips) in your f/4 scope you'll need a shorter sight tube (about 3.5X the inside diameter of the bottom aperture), preferably in 2-inch diameter to minimize registration errors. The only one I know of is the CatsEye TeleTube XL-S (or TeleCat XL-S combo tool).

Used correctly, the tools you currently own can be used to assess and correct the three Newtonian alignments. But a CatsEye TeleTube or TeleCat will make the job easier.


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alonzo
super member


Reged: 06/06/12

Re: different views thru collimation cap and cheshire new [Re: Vic Menard]
      #6213944 - 11/24/13 07:56 PM

....Vic I am sorry I am so dense but what do mean by axial alighments.

Alonzo


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Jason D
Postmaster
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Reged: 10/21/06

Loc: California
Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6213960 - 11/24/13 08:02 PM Attachment (13 downloads)

Quote:

....Vic I am sorry I am so dense but what do mean by axial alighments.

Alonzo




Axial alignment is achieved when both the primary mirror and eyepiece optical axes coincide.

That title of the following gif file should have been "What is axial alignment?"


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alonzo
super member


Reged: 06/06/12

Re: different views thru collimation cap and cheshire new [Re: Jason D]
      #6213967 - 11/24/13 08:08 PM

...Jason that just looks like centering the secondary to the focuser am I right. How do I know I have accompolished this correctly.

Alonzo

Edited by alonzo (11/24/13 08:10 PM)


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Jason D
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Reged: 10/21/06

Loc: California
Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6213984 - 11/24/13 08:20 PM

No it is not.
Axial alignment has nothing to do with centering the secondary mirror under the focuser. It has to do with the light path along the eyepiece/primary mirror optical axis. Think of a laser beam traversing the focuser axis, then deflecting off the secondary mirror towards the primary center, and finally reflecting off the primary mirror and retracing its path to the source. I just described axial alignment. The secondary mirror does not have to be centered to achieve axial alignment.
In the following animation, axial alignment has been achieved in each frame yet the secondary mirror is all over the place.
Jason



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alonzo
super member


Reged: 06/06/12

Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6214033 - 11/24/13 08:50 PM

...Vic,Jason tommorrow I plan on using a Helix laser to be sure my primarys optical axis is dead center of my tube. It has a hologram that projects a bullseye. By checking the shadow of the secondary in the bullseye I should be able to tweak the primarys adjuasters to center the primarys optical axis(not my idea as I read this somewere). Vic I will try and follow your procedures as you outlined in your last post
and report back sometime tommorrow afternoon. Guys thanks a million for your help,it really means a lot.

Alonzo


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alonzo
super member


Reged: 06/06/12

Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6214163 - 11/24/13 10:13 PM

...Vic,Jason when the secondary is centered in the tube both
fore and aft and laterally how much of the primary should be seen in the collimation cap. I see my mirror clips that stand proud and I assume I am at the very edge of the primary but to this point I have yet to see it completly in the secondary. Should I move the secondary up a tiny bit to encompass the primary and if so by about how much.

Thanks
Berry


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Nils Olof Carlin
Carpal Tunnel
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Reged: 07/26/04

Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6214448 - 11/25/13 04:13 AM

Quote:

...Vic,Jason tommorrow I plan on using a Helix laser to be sure my primarys optical axis is dead center of my tube. It has a hologram that projects a bullseye. By checking the shadow of the secondary in the bullseye I should be able to tweak the primarys adjuasters to center the primarys optical axis(not my idea as I read this somewere).




This idea, if I understand you rightly (i.e. the shadow of the secondary is centered in the bullseye pattern, as projected on a wall or roof), has been touted by Helix (I believe, but it was along time ago I read it - not to mention anybody else ) as a means of checking the collimation of the primary.
Trouble it is false. The shadow of the secondary is offset if collimation is correct in all respects. (Centering can only be achieved by moving the secondary away from the primary, cancelling the offset that centers the secondary in the sight tube and thus the fully illuminated field).

Better use one of the tried and true methods - you have a collimation cap (they may be poorly centered, though) and a Cheshire (likely more accurate).

If your objective is just to center the optical axis in the tube you can put a semi-transparent sheet across the tube opening and see it the bullseye pattern is centered. A squares pattern is likely better, though.

In this case, the secondary must be offset correctly away from the focuser.

Nils Olof


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alonzo
super member


Reged: 06/06/12

Re: different views thru collimation cap and cheshire new [Re: Nils Olof Carlin]
      #6214820 - 11/25/13 10:43 AM

....Nils what your saying makes sense so I will leave well enough alone.

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]
      #6215055 - 11/25/13 12:40 PM

Quote:

...what do mean by axial alignments.



Your scope has two axial alignments:
The focuser axial alignment, which is usually corrected by adjusting the secondary mirror tilt to align the sight tube cross hairs (or thin beam laser) to the primary mirror center spot, and
The primary mirror axial alignment, which is corrected by adjusting the primary mirror tilt to align the collimation cap pupil (or bright Cheshire ring) to the primary mirror center spot.

When both axes are corrected (within tolerance), the image "sweet spot" will be centered in the eyepiece fov and focus will "snap".


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michaeldurban
member


Reged: 03/06/12

Loc: Durban, South Africa
Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6215070 - 11/25/13 12:50 PM

hmm..

My basic technique is to put a piece of white paper behind the secondary and block the reflection from the primary.

I have a truss so that makes things easier.

I then cut out a circle that precisely matches the lid for over the bottom OTA (I have a SW 10" truss).
I trace a crosshair with concentric circles.

Then I look through the cheshire combo..

I always make sure (by marking the tube and the focusser) that the cheshire is always aligned in the exact same way.

I even bought a self-centering adapter to make double sure.
When I insert the cheshire into the self-centering adapter I also make sure I keep the 'depth' the same every time.

Then I check if my secondary is (still) circular and centered. Again I check this by racking the focusser tube in and out.

When I'm happy I remove the bottom lid to expose the primary.
And often there is very little 'tilting' to do (regarding the secondary).

I use the barlowed laser technique (thank you Nils !!)to 'roughly' align the primary since my cheapie barlow reflects the laser light back onto the face plate, making it hard to see the shadow reflection and the center spot reflection.

Then again, I put in the cheshire and check the primary.
And every time I find that it's a little bit off, but that's because of my inacurate barlow method.

As a note : I cannot see the six primary mirror clips through the cheshire, not even when it's fully racked in.

I can only see them when I remove the tube and look through the bare focusser tube.
(as far as I know my secondary -2.48"- is just big enough)

So..there is my OCD collimation mambo...

I know i might be quartered for the next statement but..
a laser colli is kaka...
Only the barlowed method is reliable.

get a cheshire combo tube !!!


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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: michaeldurban]
      #6215245 - 11/25/13 02:17 PM

I don't find your collimation "mambo" OCD--it works for you, and that's what counts!

A couple observations:

Quote:

...When I'm happy I remove the bottom lid to expose the primary.
And often there is very little 'tilting' to do (regarding the secondary).



Unless the combo tool has shifted, this means your template on the bottom lid is a bit off with respect to the face of the primary mirror. Fixing the misalignment (focuser axis) by tweaking the secondary mirror adjustment screws is the best next step!

Quote:

...Then again, I put in the cheshire and check the primary.
And every time I find that it's a little bit off, but that's because of my inacurate barlow method.



While the Cheshire is sensitive to parallax, it is relatively insensitive to tilt registration errors. Your Barlowed laser (which is insensitive to parallax) method places the target (in the laser window) outside of the OTA, which means the alignment read is sensitive to tilt registration errors. If the difference is significant, I usually defer to the Cheshire.

Quote:

As a note : I cannot see the six primary mirror clips through the cheshire, not even when it's fully racked in.



I suspect fully racked in, the bottom edge of the combo tool will obscure the outer edge of the secondary mirror, and since the primary mirror will appear larger, you won't be able to see the primary mirror clips. The solution is a shorter sight tube.

Quote:

...I know i might be quartered for the next statement but..
a laser colli is ( )...



I don't think you'll be quartered in this forum for calling out poorly made economy laser collimators.

Quote:

...Only the barlowed method is reliable.



When it's done properly.

Quote:

...get a cheshire combo tube !!!



And a good 2- to 1.25-inch adapter if you're going to use a 1.25-inch combo tool in a 2-inch focuser.


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Jason D
Postmaster
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Reged: 10/21/06

Loc: California
Re: different views thru collimation cap and cheshire new [Re: Nils Olof Carlin]
      #6215275 - 11/25/13 02:30 PM

Quote:

Trouble it is false. The shadow of the secondary is offset if collimation is correct in all respects. (Centering can only be achieved by moving the secondary away from the primary, cancelling the offset that centers the secondary in the sight tube and thus the fully illuminated field).




Illustrating Nils Olof's points. Below are images for a well-collimated scope


[image
]http://www.cloudynights.com/photopost/data/500/19446front2.jpg[/image]





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alonzo
super member


Reged: 06/06/12

Re: different views thru collimation cap and cheshire new [Re: Jason D]
      #6215406 - 11/25/13 03:17 PM

...Vic and Jason 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. 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.

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]
      #6215509 - 11/25/13 04:02 PM

Quote:

...Vic,Jason when the secondary is centered in the tube both
fore and aft and laterally how much of the primary should be seen in the collimation cap.



If you rack the focuser in, you should easily see the bright reflection of the primary mirror, the primary mirror clips, and a dark ring around the primary mirror reflection that is the surrounding surface of the secondary mirror.

If you can't see the surrounding dark annulus with your focuser racked in, your secondary mirror may be slightly undersized...

Quote:

I see my mirror clips that stand proud and I assume I am at the very edge of the primary but to this point I have yet to see it completly in the secondary. Should I move the secondary up a tiny bit to encompass the primary and if so by about how much.



Moving the secondary mirror "up" (away from the primary mirror) might work if you also move the focuser (let's not go there)--why not move the primary mirror "down". If you have long collimation screws in your primary mirror mount and there's enough space, you may have a 1/2-inch or more to play with.

But with your limited focuser drawtube range--you don't have much room to move your focal plane (I'm assuming you've actually used the scope to verify eyepieces, Paracorrs, etc., will focus with your current alignment).


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