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Equipment Discussions >> Reflectors

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alonzo
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Reged: 06/06/12

different views thru collimation cap and cheshire
      #6211171 - 11/23/13 11:38 AM

...When I go to center my secondary with a cheshire things look as they should but when I replace the cheshire with a collimation cap the secondary appears too far forward. Which one should I believe.

Thanks
Alonzo


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Jason D
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Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6211265 - 11/23/13 12:18 PM

Did you mean cheshire/sight-tube combo? I would believe it over the collimation cap. However, when both are used properly with a well-collimated scope they should agree -- assuming the tools are machined accurately.
Jason


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Pinbout
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Re: different views thru collimation cap and cheshire new [Re: Jason D]
      #6211442 - 11/23/13 01:48 PM

Maybe he needs to sq up the focuser.

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Jason D
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Re: different views thru collimation cap and cheshire new [Re: Pinbout]
      #6211479 - 11/23/13 02:07 PM

Quote:

Maybe he needs to sq up the focuser.



Squaring the focuser has nothing to do with the discrepancy the OP described. If the secondary looks centered using a cheshire/sight-tube then it should also look centered with the collimation cap.
With the collimation cap, the eye axis need to be coincident with the drawtube axis which could be difficult. If the eye is placed at an angle then the secondary mirror might appear off. This is why the collimation cap is not a good tool to center/round the secondary mirror.
Jason


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Vic Menard
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Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6211580 - 11/23/13 02:51 PM

Quote:

...When I go to center my secondary with a cheshire things look as they should...



Did you place a sheet of white paper behind the secondary mirror? You need to be able to see the edge of the secondary mirror centered relative to the bottom edge of the Cheshire combo tool.


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alonzo
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Re: different views thru collimation cap and cheshire new [Re: Vic Menard]
      #6211957 - 11/23/13 06:33 PM

...Thanks for the comeback and to let you know my cheshire somehow got cocked in the focuser tube and at the time I didn't notice it. All is well now and thanks. Now another problem has emerged. With the secondary centered I can get the three mirror clips in view but when I start to tweak my secondary I start to loose one of the mirror clips. I am going nuts over this and about ready to call it quits.

Vic yes I had a white piece of paper inserted in the tube at that time but as I said I totally wasn't aware at the time the cheshire was in a serious tilt in the focuser.

Alonzo

Edited by alonzo (11/23/13 06:37 PM)


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Jason D
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Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6211982 - 11/23/13 06:48 PM

Try the following steps:

1- Get the cross-hairs to line up with the primary mirror center spot. Now the primary mirror reflection is centered under the focuser. Note I said "the primary mirror reflection is centered" -- not the secondary mirror.

2- Now imagine the secondary mirror is a window overlooking the primary mirror. You need to move the window "secondary" to get the whole primary mirror into view. Should you lower the window down the tube? Should you raise it up the tube?

3- Make the movement without looking down the sight-tube.

4 Repeat above steps until you are done. Refer to the animations below to help you visualize my description:





Jason


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alonzo
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Re: different views thru collimation cap and cheshire new [Re: Jason D]
      #6212198 - 11/23/13 09:16 PM

....Jason I am puzzled over step three.

Alonzo


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Jason D
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Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6212217 - 11/23/13 09:27 PM Attachment (25 downloads)

Assuming you end up with this view

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Vic Menard
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Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6212220 - 11/23/13 09:28 PM

Quote:

...With the secondary centered I can get the three mirror clips in view but when I start to tweak my secondary I start to loose one of the mirror clips...



If the secondary is already centered (concentric with bottom edge of the sight tube and cross hairs centered on primary mirror center spot), I don't understand why you need to "tweak" your secondary?

If, OTOH, you mean the primary mirror reflection is centered in the secondary mirror, but the cross hairs in the sight tube are not centered on the primary mirror center spot, then I suggest these two steps:

1.) Use the secondary mirror tilt adjustment screws to align the cross hairs with the primary mirror center spot.

2.) Now, to center the primary mirror reflection on the face of the secondary mirror, use either the secondary mirror rotation adjustment and/or the position of the secondary mirror closer to or farther from the primary mirror to make the alignment. Do NOT use the secondary mirror tilt adjustment.

Repeat steps 1 and 2 until the two alignments are corrected.


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Jason D
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Re: different views thru collimation cap and cheshire new [Re: Jason D]
      #6212224 - 11/23/13 09:31 PM Attachment (20 downloads)

It means the secondary mirror is too low in the tube.
I would tighten the central bolt of the secondary holder clockwise to raise the secondary mirror. I would do this without worrying about the sight-tube. After completing this step, I would start all over again to adjust the rotation/tilt of the secondary mirror to align the cross-hairs again on the top of the primary mirror center spot.
Hopefully you will end up with the following. If not, repeat the steps.

Edit: Spelling correction

Edited by Jason D (11/24/13 12:20 PM)


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alonzo
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Re: different views thru collimation cap and cheshire new [Re: Vic Menard]
      #6212233 - 11/23/13 09:37 PM

...Vic you guessed it correctly. When all looks well and I attempt to center the cross hairs on the center mark things go askew.

Alonzo

Edited by alonzo (11/23/13 09:47 PM)


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Vic Menard
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Re: different views thru collimation cap and cheshire new [Re: Jason D]
      #6212239 - 11/23/13 09:40 PM

Quote:

Assuming you end up with this view



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). 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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alonzo
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Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6212245 - 11/23/13 09:45 PM

..Jason are you refering to a fore and aft adjustment when you use the word tie and ifso I am to understand the secondary mirror is a little to close to the primary and needs to go forward towards the front of the tube.

Alonzo


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Vic Menard
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Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6212247 - 11/23/13 09:47 PM

Quote:

...you guessed it correctly. When all looks well and I attempt to center the cross hairs on the center mark things go askew.



Try repeating the two steps I suggested.
If your best alignment leaves the secondary mirror looking like a skewed ellipse (tilted toward two o'clock or four o'clock in Jason's graphic) with the reflection of the primary mirror centered in the secondary mirror, your focuser will need to be realigned...

If your best alignment leaves the secondary mirror looking like a skewed ellipse (tilted toward two o'clock or four o'clock as in Jason's first animation) with the reflection of the primary mirror OFF-center in the secondary mirror (like in Jason's animation), you have an unresolved tilt/rotation error.


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Vic Menard
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Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6212250 - 11/23/13 09:50 PM

Quote:

..Jason are you refering to a fore and aft adjustment when you use the word tie and ifso I am to understand the secondary mirror is a little to close to the primary and needs to go forward towards the front of the tube.



I believe he meant "tighten" (spell checker?)
And yes, he meant fore and aft.


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Vic Menard
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Re: different views thru collimation cap and cheshire new [Re: Vic Menard]
      #6212256 - 11/23/13 09:59 PM

FTR, I call the "fore and aft" adjustment the "offset" adjustment--when it's set correctly (centered under the focuser with the primary mirror reflection centered in the secondary mirror) the alignment is "offset". If you continue to move the secondary mirror away from the primary mirror in small increments, you will eventually achieve a "centered" alignment (the secondary mirror is NOT centered under the focuser but the primary mirror reflection IS centered under the focuser and the silhouette reflection of the secondary mirror that normally looks offset toward the primary mirror will instead be centered in the primary mirror reflection).

Offset alignment is the optimal alignment for visual work.
Centered alignment can be useful (when the secondary mirror is large enough to accommodate the unbalanced illumination caused by the lack of offset) to improve DSC accuracy (assuming the spider/secondary mirror is centered).


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alonzo
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Re: different views thru collimation cap and cheshire new [Re: Vic Menard]
      #6212262 - 11/23/13 10:08 PM

....Vic,Jason I am going retire for the night for its been a hectic day. In the morning I am going to neutralize the secondarys tilt screws and the primarys adjusting screws and start this mess all over again. Please don't give up on me for I really need some help. Just keep an eye out on this post for I will be getting back to you tommorrow.

Thanks
Alonzo


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Vic Menard
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Re: different views thru collimation cap and cheshire new [Re: alonzo]
      #6212276 - 11/23/13 10:20 PM



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alonzo
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Re: different views thru collimation cap and cheshire new [Re: Vic Menard]
      #6212999 - 11/24/13 11:05 AM

....Today I am going to neutralize my primary adjusters and tilt screws on my secondary. 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 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. After the procedure I just described I will use a collimation cap to view the results. 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. This is were I think things will go wrong and I will report back on my post with the results.If you guys wish to comment on the procedure I just outlined or if I need to do something different please comment. As always I appreciate the time and help you guys are giving me.

Alonzo

Edited by alonzo (11/24/13 11:08 AM)


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Vic Menard
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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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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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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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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
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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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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
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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
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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
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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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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
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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
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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
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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
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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
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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
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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
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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
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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
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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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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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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
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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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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
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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
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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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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
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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
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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
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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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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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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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Re: different views thru collimation cap and cheshire new [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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