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Collimation Issue

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#1 Guydive

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Posted 04 December 2020 - 02:18 AM

Well I don't know if to call that an issue, but something bothers me with my 10" SW flextube collimation.

I collimated it yesterday with c collimation cap I improvised from an old film canister. 

it generally looks ok, but if you take a second look, you can see that although the cap hole is inside the primary doughnut, they both aren't centered. also, on the bottom side of the secondary reflaction, I can see the edge of the drawtube and focuser. what can it be?

 

WhatsApp Image 2020 12 03 At 4.26.14 PM
 
WhatsApp Image 2020 12 04 At 9.06.27 AM

 

 
 

 



#2 otocycle

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Posted 04 December 2020 - 02:26 AM

Some offset of the reflected primary image is normal due to secondary geometry.   The drawtube shadow is also perfectly normal and depends on the focuser position.   See where it is when an eyepiece is in focus.

 

The final check for collimation is the star test to see if things are right.



#3 silios

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Posted 04 December 2020 - 02:57 AM

The secondary is a bit off based on your image through collimation cap.

So is primary alignment, but you are mostly there.

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#4 Asbytec

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Posted 04 December 2020 - 05:33 AM

 

...also, on the bottom side of the secondary reflection, I can see the edge of the drawtube and focuser. what can it be?

 

It is normal, but sometimes difficult, to see the bottom edge of the focuser draw tube reflected back to you. If the focuser is aligned to the primary center, i.e., collimated, then the reflection of the draw tube bottom edge will be centered on the reflection of the primary center mark. This whole assembly will be offset in the dark secondary reflection away from the focuser. The secondary reflection will appear to be offset the other way, toward the primary mirror. This is due to the secondary offset away from the focuser, it appears lower in the primary reflection because it is a bit lower in the tube to capture the thicker light cone opposite the focuser. 

 

One thing I notice, too, is your star image appears to be clipped between noon and 3 o'clock. This is probably caused by the reflection of the primary not being centered in the secondary and where the primary reflection grazes the secondary edge. At that focuser position, you are slightly vignetted by the secondary. Your secondary may be a tad too small for that application, or need to be more precisely rotated to better center the primary reflection. Your image above shows the entire primary reflection in the secondary, albeit it near the apex. I bet you are focusing a bit further out and currently losing a tiny sliver of the primary aperture at that grazing edge. 


Edited by Asbytec, 04 December 2020 - 05:45 AM.


#5 Guydive

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Posted 04 December 2020 - 08:30 AM

 I bet you are focusing a bit further out and currently losing a tiny sliver of the primary aperture at that grazing edge. 

How do you suggest I solve it?



#6 Vic Menard

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Posted 04 December 2020 - 08:56 AM

One thing I notice, too, is your star image appears to be clipped between noon and 3 o'clock. This is probably caused by the reflection of the primary not being centered in the secondary and where the primary reflection grazes the secondary edge. At that focuser position, you are slightly vignetted by the secondary. Your secondary may be a tad too small for that application, or need to be more precisely rotated to better center the primary reflection. Your image above shows the entire primary reflection in the secondary, albeit it near the apex. I bet you are focusing a bit further out and currently losing a tiny sliver of the primary aperture at that grazing edge. 

 waytogo.gif

 

I agree with your observations. The misplaced secondary mirror is effectively offset toward 7 o'clock (green circle is optimal), and the focuser axis is misaligned about the diameter of the primary mirror center marker/donut--maybe 1/2-inch? Together, with a smaller secondary and out focused image, I would expect some clipping. 

 

Clean up the axial alignments and the scope should be fine for visual work...

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#7 Asbytec

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Posted 04 December 2020 - 09:01 AM

How do you suggest I solve it?

Well, you can rotate the secondary a tiny bit in a direction that moves the primary reflection toward the upper edge. So that there is a more uniform dark ring around the primary reflection. This will move the focuser axis from the primary center and cause you to do another tilt collimation of the focuser axis. But, that's okay. We normally want the secondary and primary reflection to be concentric.

From what I can tell, your secondary looks well centered under the focuser. So, how I would do it is loosen only one secondary adjustment screw inline with the focuser axis just enough so the secondary will rotate. This will definitely decollimate the focuser axis, but we'll put it back.

Rotate the secondary so the primary reflection is centered on the secondary major axis, the primary center mark will also be on the secondary major axis. Then tighten the secondary adjustment screw inline with the focuser axis so the primary center mark moves back onto the cross hair. If you miss the cross hair, back off a little, refine the secondary rotation, and try again.

Edit: I see Vic replied. Rotating the secondary so his small red circle moves onto or very near the blue cross hair should clean up the focuser alignment a bit and better center the primary reflection. The cross hair is close to the secondary major axis. It may need a little tilt to realign the focuser axis onto the blue cross hair, but the primary reflection will be centered in the secondary. Then clean up the focuser axis, of course.

Edited by Asbytec, 04 December 2020 - 10:03 AM.

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#8 Guydive

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Posted 06 December 2020 - 10:40 AM

ok, I think I'm getting closer... what do you guys use to place the crosshair / reticle layer over the photo? 

 

collmation 6.12.jpeg



#9 gene 4181

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Posted 06 December 2020 - 11:32 AM

 Check out the catseye collimation website or google it and read the 7 steps too collimation of your newtonian. Also i'd recommmend the  booklet , New Perspectives on Newtonian Collimation , its a great read . I've    used a separate  sight tube and separate cheshire through out my use of newtonians  and have always been satisfied with the results.  The sight tube  aligns your secondary underneath the focuser  tube properly equally  and points the secondary  exactly at the center of the primary , cross hairs intersecting the primary donut .  THE beauty of it it is center the secondary under the focuser thru the sight  tube  and offset is taken care of automatically . .


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#10 Guydive

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Posted 06 December 2020 - 12:14 PM

ok, I think I'm getting closer... what do you guys use to place the crosshair / reticle layer over the photo? 

 

attachicon.gifcollmation 6.12.jpeg

just did a star test on this specific collimation:

 

collmation 6.12.jpeg

 

WhatsApp Image 2020-12-06 at 7.07.17 PM.jpeg


Edited by Guydive, 06 December 2020 - 12:16 PM.

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#11 Vic Menard

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Posted 06 December 2020 - 12:50 PM

just did a star test on this specific collimation:

 

Your focuser axial alignment is still off a bit (still about 1/2-inch--blue cross hairs relative to primary mirror center marker/small red circle). Your secondary mirror still needs to be tilted up towards about 1 or 2 o'clock, and maybe moved a bit closer to the primary mirror end of the tube assembly (green circle is optimal). Your primary mirror axial alignment looks good (yellow circle and red circle are concentric). The primary mirror reflection is centered a little better in this alignment iteration, which probably explains your "un" clipped star image. Your focuser axial error is only slightly outside the "high magnification" tolerance, and combined with your good primary mirror alignment, your scope should deliver good moderate to high magnification performance. There's no reason not to use this alignment "as is" for visual work.

 

(FYI--The software I'm using to annotate (and adjust exposure/shadow detail) images is the "Preview" application included in MacOS. I use the GraphicConverter app (also MacOS) for cropping, resizing, and adjusting file size.)

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#12 Asbytec

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Posted 06 December 2020 - 06:47 PM

The star image looks a lot better.

As Vic explained, you can tilt to secondary up a little and maybe down a tad. The primary reflection will move to the right and off the secondary. (You may have to adjust rotation a bit). But, that's okay for two good reasons.

First, when the focuser axis is collimated, the primary reflection will be centered under the focuser. That is two of the three collimation signatures. When their centers coincide with the donut under the cross hair, so must their edges be concentric.

Second, once the primary reflection is centered under the focuser, it can be used as a reference to center the secondary under the focuser, too. The secondary edge is the third collimation signature responsible for field illumination, ie, clipped images.

Again, when the focuser axis is collimated to the primary center, the primary reflection is necessarily centered under the focuser. Then the secondary centered on the primary reflection, again, also centers it under the focuser. This completes the three concentric signatures.

Edited by Asbytec, 06 December 2020 - 06:53 PM.


#13 Guydive

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Posted 24 December 2020 - 01:38 PM

The best I could get so far

4550CB21-F1C8-4A2B-A06D-784B2B31CFFD.jpeg


Edited by Guydive, 24 December 2020 - 01:41 PM.


#14 SteveG

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Posted 24 December 2020 - 06:37 PM

The best I could get so far

attachicon.gif4550CB21-F1C8-4A2B-A06D-784B2B31CFFD.jpeg

What are we looking at or looking through?



#15 Guydive

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Posted 25 December 2020 - 02:49 AM

What are we looking at or looking through?

Deneb via zwo183mm on my 10" flextube

#16 SteveG

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Posted 25 December 2020 - 03:22 PM

Deneb via zwo183mm on my 10" flextube

I think it's too far out of focus to determine anything. Can you take one through a collimating tool, or at least like the star test in post #10?


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#17 Asbytec

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Posted 25 December 2020 - 05:55 PM

I think it's too far out of focus to determine anything. Can you take one through a collimating tool, or at least like the star test in post #10?

I agree. Though, it does seem the clipping of his defocused image is much improved or non existent. That tells us his secondary placement is much better as it is capturing the entire on axis light cone. Unfortunately, it does not tell us much about the focuser alignment. And, as you say, it's a bit too far from focus to see coma. Usually in focus or close to focus is better. Using the star test, we want to see concentric rings with the Poisson spot centered in them (more like #10). We really cannot estimate collimation by looking at images of the secondary shadow, it will be offset anyway because the secondary is offset. So, that offset will be apparent in the largely defocused image, but not (not really) diffraction closer to focus. 


Edited by Asbytec, 25 December 2020 - 05:59 PM.



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