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different views thru collimation cap and cheshire

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#1 alonzo

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Posted 23 November 2013 - 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

#2 Jason D

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Posted 23 November 2013 - 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.
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#3 Pinbout

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Posted 23 November 2013 - 01:48 PM

Maybe he needs to sq up the focuser.

#4 Jason D

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Posted 23 November 2013 - 02:07 PM

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

#5 Vic Menard

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Posted 23 November 2013 - 02:51 PM

...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.

#6 alonzo

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Posted 23 November 2013 - 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

#7 Jason D

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Posted 23 November 2013 - 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:

Posted Image

Posted Image

Jason

#8 alonzo

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Posted 23 November 2013 - 09:16 PM

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

Alonzo

#9 Jason D

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Posted 23 November 2013 - 09:27 PM

Assuming you end up with this view

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#10 Vic Menard

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Posted 23 November 2013 - 09:28 PM

...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.

#11 Jason D

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Posted 23 November 2013 - 09:31 PM

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

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#12 alonzo

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Posted 23 November 2013 - 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

#13 Vic Menard

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Posted 23 November 2013 - 09:40 PM

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...

#14 alonzo

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Posted 23 November 2013 - 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

#15 Vic Menard

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Posted 23 November 2013 - 09:47 PM

...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.

#16 Vic Menard

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Posted 23 November 2013 - 09:50 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.

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

#17 Vic Menard

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Posted 23 November 2013 - 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).

#18 alonzo

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Posted 23 November 2013 - 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

#19 Vic Menard

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Posted 23 November 2013 - 10:20 PM

:waytogo:

#20 alonzo

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Posted 24 November 2013 - 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

#21 Vic Menard

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Posted 24 November 2013 - 12:17 PM

...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?

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.

...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.

#22 Jason D

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Posted 24 November 2013 - 12:59 PM

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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#23 Jason D

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Posted 24 November 2013 - 01:00 PM

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...

Attached Files



#24 Vic Menard

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Posted 24 November 2013 - 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! :bow:

#25 alonzo

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Posted 24 November 2013 - 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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