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SW 10" flextube - secondary shadow offset or no offset?

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#1 Baatar

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Posted 13 September 2019 - 06:04 AM

Need a quick advice on the collimation of SW 10" F4.7 Dob.

 

I have been using my DOB for past few months, and I believe with a pretty good collimation.  I don't have collimation caps, cheshire or expensive high precision laser tools.  I only use my own eyes and a cheapish TS (Telescope Service, Germany) LA2 simple laser + barlow for return laser.

 

I know you will grill me that this is not a good tool or a good way to collimate, and not precise at all.  But I think I have been managing, reasonably, to collimate my DOB after reading many collimation-related posts here.  And I don't think I had or have any image quality/sharpness issues if my DOB was indeed mis-collimated.  What do I know, but I am still more or less happy with the way I collimate.  Btw, the TS laser seems to be well collimated too, checked.

 

I read a few days ago that for a properly collimated fast DOB (like mine with F4.7), the focuser field, edges of the secondary and the primary should be concentric, with all centres (including the primary donut) overlapping in one spot when looked through the focuser, except for a slight offset of the secondary shadow slightly towards the primary mirror.  For a slow DOB, everything should be concentric without secondary shadow offset. 

 

After reading this, I checked and everything was concentric without secondary shadow offset (like the 2nd situation above for a slow DOB).  Then I loosened everything and started the collimation from scratch several times, and every time I was getting consistently similar results (everything concentric without secondary shadow offset).

 

I read further through posts, and some people are saying that for recent DOBs you don't need secondary shadow offset, as this has been already taken into account when designing and/or fixing the secondary mirror on its base.  But I don't seem to find many similar accounts from many people.

 

I am confused now.  I did acquire the DOB three months ago new, but the model is old one and was available for many years I guess.

 

So which one is it?  Is it true that new models do not display secondary shadow offset (and my DOB happens to be one), or am I doing the collimation wrong?

 

Any similar experiences with recently acquired SW 10" flextubes?

 

Two photos attached, ideal collimation for fast and slow DOBs.  Mine look like for slow DOB at the moment.  But I don't have any perceptible image quality issues.  So just leave it like that?

Attached Thumbnails

  • fast-collimation.jpg
  • slow-collimation.jpg

Edited by Baatar, 13 September 2019 - 06:14 AM.


#2 Garyth64

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Posted 13 September 2019 - 08:17 AM

"But I don't have any perceptible image quality issues.  So just leave it like that?"

 

If you're getting good images, leave it alone.


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#3 Asbytec

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Posted 13 September 2019 - 09:36 AM

"I read further through posts, and some people are saying that for recent DOBs you don't need secondary shadow offset, as this has been already taken into account when designing and/or fixing the secondary mirror on its base."

 

Offset has nothing to do with collimation involving alignment of optical axes (eyepiece and primary). The flat secondary has no optical axis of it's own. So, it's possible to achieve great collimation without offset of any kind. Offset is designed to bring the fully illuminated field into the center of the eyepiece FOV. This has to do with the geometry of the converging light cone from the primary mirror being intercepted at an angle. Hence we use an elliptical secondary to slice the primary cone at an angle. Offset simply ensures the elliptical diagonal intercepts the entire light cone correctly. The reason you see the dark offset being wider toward the primary is because the light cone is a little larger closer to the primary mirror and a little more narrow farther from the primary mirror. These are the points where the light cone strikes the secondary, and by the way, allows you to see the entire primary from the focuser peep hole. If you can see the entire primary mirror reflection from top dead center of the focuser, the diagonal flat is large enough to capture the on axis light cone form the primary and fully illuminate the field at that point in the focuser travel. If it's well collimated and fully illuminated, yes, you can leave that alone if you want. 

 

There are two models of collimation using offset. The classic model requires bidirectional offset of the secondary to intercept the light cone. The diagonal is offset toward the primary and away from the focuser so that it corresponds to the incoming light cone's varying dimensions. When it's offset this way, you will see the reflection of the focuser to one side of the secondary shadow. If there was no offset, the bottom of the light cone would miss the lower part of the secondary altogether and you would not be receiving all the light from the primary mirror. In other words, vignetting the primary and reducing the effective aperture by reducing some of the volume of the light cone and area of the primary mirror reflected to the eyepiece. Bi directional offset ensures the geometric center of the elliptical secondary and the optical axes of the primary light cone coincide. (Edit: Ooops, misspoke. Got confused visually slicing a cone in my head). The faster the primary, the more steeply the light cone converges and the more offset is needed to capture it. 

 

The new model most folks are talking about these days only relies on only one offset direction toward the primary. I believe folks are trying to coin this offset as unidirectional offset. This happens automatically when you ensure the secondary is concentric under the bottom of the focuser or draw tube. You do not need the offset away from the focuser. However, the optical axes and the secondary geometric center no longer coincide. That's okay. A simple adjustment in secondary and primary tilt will capture the light cone and reflect it toward the focal plane entirely (just not right at at 90 degrees like bi directional offset). The nice thing about it is, we only need concern ourselves with the secondary position directly under the focuser. This is why this is the first step in collimation. So, yes, in this case unidirectional offset is automatically achieved and you will receive full illumination regardless. Aligning the axes at this point, then, is achieving collimation with the secondary properly placed for full illumination at the center of the FOV.

 

In unidirectional offset, and since the secondary still needs to intercept some of the wider light cone closer to the fast primary, you should see some offset toward the primary as indicated by the (opposite) offset reflection of the focuser inside the secondary shadow. If the focuser reflection is not offset, but you can see the entire primary reflection through focuser travel, then you are still fully illuminated on axis somewhere in the field of view, anyway. To fix this, you may want to ensure the secondary edge is actually concentric under the focuser. My f/6 uses bi directional offset, so I cannot look at it to see how unidirectional offset appears. Your scope may well have the diagonal flat mounted on the stalk with offset built in, too. You can tell because the lower lip of the diagonal mirror is much larger than the upper lip (which is what we see as an offset secondary shadow). The longitudinal axis of the diagonal stalk does not go through the geometric center of the diagonal's reflective aluminium face - because the diagonal mirror is offset away from the focuser when it was mounted. If so, then you still center it under the focuser as before and you should still see an offset focuser reflection. It's more pronounced at f/4.7 than slower focal ratios. 

 

Two images of classical and the new model collimation you might find interesting:

https://www.cloudyni...73825239401.png

https://www.cloudyni...23135_thumb.png

The thread: https://www.cloudyni...rror-attaching/

 

And this: https://www.cloudyni...xt10/?p=8168213


Edited by Asbytec, 13 September 2019 - 10:16 AM.

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#4 Baatar

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Posted 13 September 2019 - 10:11 AM

Norme,

 

Many thanks for taking your time any for your great and detailed explanation!  All makes sense.

 

My spider, secondary, primary, the focuser tube are all well positioned and balanced.  As far as I can tell with my eyes (I don't claim to have eagle sharp eyes) and with some measurements.

 

I do really see the entire primary with all its clips in my secondary (everything well aligned and concentric), and believed that my Dob was well collimated.

 

Thus, I was confused reading those posts, and was wondering if I was doing something wrong and that I should see the secondary offset shadow.

 

Anyway, as Garyth suggested, I will just leave it as it is.  But was interested to know and learn about this, and why this is.

 

Thanks for the links, will read them with interest.



#5 Asbytec

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Posted 13 September 2019 - 10:50 AM

Yes, I believe you. Not sure either. Getting some popcorn. :)

#6 Vic Menard

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Posted 13 September 2019 - 01:04 PM

Need a quick advice on the collimation of SW 10" F4.7 Dob.

 

1.)...I read a few days ago that for a properly collimated fast DOB (like mine with F4.7), the focuser field, edges of the secondary and the primary should be concentric, with all centres (including the primary donut) overlapping in one spot when looked through the focuser, except for a slight offset of the secondary shadow slightly towards the primary mirror.  

 

2.) For a slow DOB, everything should be concentric without secondary shadow offset. 

 

3.) After reading this, I checked and everything was concentric without secondary shadow offset (like the 2nd situation above for a slow DOB).  Then I loosened everything and started the collimation from scratch several times, and every time I was getting consistently similar results (everything concentric without secondary shadow offset).

 

4.) I read further through posts, and some people are saying that for recent DOBs you don't need secondary shadow offset, as this has been already taken into account when designing and/or fixing the secondary mirror on its base.  But I don't seem to find many similar accounts from many people.

 

5.) So which one is it?  Is it true that new models do not display secondary shadow offset (and my DOB happens to be one), or am I doing the collimation wrong?

 

6.) Two photos attached, ideal collimation for fast and slow DOBs.  Mine look like for slow DOB at the moment.  But I don't have any perceptible image quality issues.  So just leave it like that?

Lots of questions here, let's sort them out.

 

1.) Is correct.

2.) Is incorrect (unless the Dob is very slow, maybe f/8 or f/10), and even then, probably not.

3.) Is impossible for f/4.7. If the silhouette reflection of the secondary mirror was concentric with the reflection of the bottom edge of the focuser, your secondary mirror placement was not offset, which also means the actual edge of the secondary mirror was not concentric with the bottom edge of the focuser. 

4.) I assume you mean that you don't need to have the secondary mirror mechanically offset away from the focuser--that's correct. When you center the secondary mirror under the focuser, you have effectively offset the secondary mirror closer to the primary mirror. Then, when you align the primary mirror (if you're using a laser, when you make the return laser coincide with the outgoing laser), you effectively tilt the optical axis closer to the focuser side of the OTA, completing the secondary mirror offset.

5.) The New Model (mechanically centered spider/secondary mirror, and the actual edge of the secondary mirror concentric with the bottom edge of the focuser drawtube), is an optically full offset model. This means everything is concentric except for the silhouette reflection of the secondary mirror, which appears offset toward the primary mirror end of the OTA.

6.) Again, Jason's second picture (the one you're calling "ideal collimation for slow DOBs") isn't possible for  most production Dobsonians today. If I recall correctly, it was originally intended to show what not to expect when collimating a Newtonian. To see what a "centered" secondary mirror alignment should look like for your scope, check Jason's illustration here:  https://www.cloudyni...ment/?p=3040728  (note that to get the secondary mirror silhouette concentric with the reflection of the underside of the focuser, the actual edge of the secondary mirror has to be moved away from the primary mirror end of the OTA). FYI, for actual images of a 10-inch f/5 Dob secondary mirror alignment, go to the beginning of the discussion linked above.

 

Finally, whether your secondary mirror placement is centered or offset, or unidirectionally or bidirectionally offset, or even somewhere in between either of the above, as long as the axial alignments are correct, your image performance will be unimpaired.


Edited by Vic Menard, 13 September 2019 - 01:05 PM.

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#7 Baatar

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Posted 16 September 2019 - 04:48 AM

Hi Vic and others, many thanks again for your responses.

 

After tweaking the collimation a bit lately based on your responses, I think I am getting now the secondary shadow offset visible (but not too evidently).

 

I have another question in relation to collimation.  I posted this question in my response to another person (in the beginners forum) with a similar issue, but want to post it here again to get your further views.

 

The issue is -- I can't really center the laser dot on the primary donut.  It seems to be the design feature of this (or SW dobs in general) where the laser will not appear as a dot in the center of the donut, but the entire white donut circle (not sure of what material it is made of) illuminates.  So my approach was (to my logic) to have an even illumination of the entire donut.  By having this uniform illumination, one can assume that the laser dot hits the center of the donut.  If half of the donut (or any portion of it) is illuminated, I assume the laser is not precisely centered and I tweak accordingly. Then, I barlow the laser (to get the primary collimated) and I could see the donut reflection in the laser return window and I try to center the window central hole in the center of the donut reflection.  This is my standard procedure.

 

Of course, when I remove the barlow the return laser dot does not hit the center of collimator window hole (usually the return laser dot is off by a few millimeters).  Also with barlow, the laser is dispersed and it does not hit (or cover) the primary donut, and the barlowed laser is off from the primary donut by several mm (even by a few centimeters).  I understand that both of these issues are not really "issues" as long as two conditions are met:

1. Laser (without the barlow) hits the centre of the donut (in my case, the laser illuminates the primary donut)

2. Barlowed return laser shows the donut reflection around the central hole of the collimator window.

 

Would be really good to hear other experiences with this particular Dob whether this is indeed the case, and that primary donut is illuminated instead of having a laser dot in the center of the donut.


Edited by Baatar, 16 September 2019 - 05:05 AM.


#8 Asbytec

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Posted 16 September 2019 - 05:46 AM

Of course, when I remove the barlow the return laser dot does not hit the center of collimator window hole (usually the return laser dot is off by a few millimeters).  Also with barlow, the laser is dispersed and it does not hit (or cover) the primary donut, and the barlowed laser is off from the primary donut by several mm (even by a few centimeters).

 

I have not used a Barlowed laser so never experienced this, but I have to ask if your laser is collimated and if it's possible to collimate it? I am sure you have thought if this, so I'm not sure if my reply is correct. Seems to me, even though the brighter area is centered on the primary center mark, where a collimated thin beam laser might show it as being off. A small error is also indicated by the return beam without the Barlow. A few millimeters at double pass might not be significant focuser axial error. I'd be curious to know, too, so waiting for another response to see what folks say. 



#9 scotsman328i

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Posted 16 September 2019 - 05:54 AM

I’ve had truss Dobs that had secondary offset and no secondary offset. As long as the focuser was fully reflected on the secondary mirror either center or offset from the center, I never had issues with amazing views when everything was laser collimated. Clear skies!


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#10 Baatar

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Posted 16 September 2019 - 06:00 AM

Norme, my laser seems to be well collimated.  I tried rotating the laser in the focuser and used "X" shaped stand to put the laser on and to rotate it.  So far, I don't see any circles or circular pattern created by the laser with both methods.  And the collimator does have adjustment screws if I would need to collimate the laser, but so far haven't had a need to do this. 

 

What I am reading is that barlowed laser reflection slightly off from the primary mirror center donut, as well as the unbarlowed return laser offset from the collimator diagonal central spot (given that barlowed reflection of the donut is centered) are not major sources of concern.  So, this is what I read and understand.

 

On my question if the SW dob primary donut is supposed to illuminate (instead of having the laser dot in the centre of the donut) is what interests me to know too.  Just another point to explore if this will impact how well the Dob will be collimated, if this affect the latter at all.


Edited by Baatar, 16 September 2019 - 06:01 AM.

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#11 Asbytec

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Posted 16 September 2019 - 06:21 AM

I recall the stock laser accessory that came with my 12" Dob years ago also scattered a lot when it struck the center mark. However, it was a tiny dot and could fit inside the inner hole once aligned. So, it did not present a constant glare off the center marker. I imagine when you shine the laser on a wall or something, it presents as a red dot? 

 

Well, I dunno...if the laser is collimated, then your focuser axis is within tolerance (I think) when the laser glare is symmetrical on the center mark. In any case, it's probably not off by more than the diameter of the center mark. It can deviate by as much as 3% of the primary mirror diameter, or 0.03 * 250 ~ 7.5mm. So, anywhere in or on the center mark should be fine for focuser axial alignment. This is probably what you are seeing on the un Barlowed return laser to the target (I think twice the error as at the primary mirror, need to play with some triangles to confirm this). If so, and you're only off by about 3mm on the return beam, then you are likely within 1.5mm at the primary center mark. 

 

As to why the out going beam is not centered on the primary center mark, there /may be/ registration issues with the Barlow. The beam hitting the Barlow lens at a slight off axis angle should cast the dispersed beam slightly off axis, too. I am trying to reason this in my head, I'd like to hear other responses, too, in case I am missing something. 



#12 Baatar

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Posted 16 September 2019 - 06:47 AM

Hi Norme

 

Yes, it is a red dot, the intensity / brightness of which can also be selected on the collimator.  But, you raise some good points too with regard to the registration with or without the barlow.

 

I am still using the supplied focuser, its 2" extension, 2" to 1.25 reducer etc.  Thus, all in all, there are 4 registration points using lock screws.  Not sure how these connection points, or any mix of these, affect the on-axis registration of the laser, but I assume these might result in some minor misalignments.

 

Actually, I ordered some baader clicklocks and extensions (still being shipped) to replaced supplied screw extensions.  This is mainly to benefit from secure locking and easiness in changing extensions/EPs.  Thus, if clicklocks are better in "centering" things, I would assume this would help in eliminating any existing on-axis misalignments.  But this will not be the case if using barlow, which has only one lock screw, thus I would expect some deviations from axis center.  But at least I would have hopefully eliminated other axial deviations from other registration points / lockscrews.

 

I will check how these affect collimation and laser projections once I will have my clicklocks delivered and fitted on my scope.


Edited by Baatar, 16 September 2019 - 07:17 AM.

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#13 Baatar

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Posted 05 November 2019 - 11:21 AM

Ok, I have another collimation question.

 

After using the cheshire combination tool and barlowed laser, and fiddling with secondary and primary positioning, I think I am getting more or less acceptable collimation with a secondary shadow off-set.

 

I also read that it is ideal if the secondary edge and the primary reflection edge are not only concentric but also overlapping. 

 

But the collimation process was not easy.  The combination tool tube is quite long, thus when I insert it fully in the focuser, the tube edge was smaller than secondary edge, thus I can't check the concentricity.  I could pull out the combination tool from the focuser quite a bit to have the secondary edge fully visible, but there is too much play and wobble of the combination tool in the focuser, thus is not accurate to check concentricity.  So what I did is that I removed lenses from a 10mm plossls EP (which I don't use) and used the EP's larger hole, and it seems to work well to compare the EP field stop and secondary/primary edges.  Needless to say that it was difficult to align the combination tool's blurry crosshairs with the primary donut, let alone the small dark center of cheshire.  Anyway, I am getting there.

 

However, I also installed GSO coma corrector recently, which moves the focal plane backwards.  I can barely reach focus with the ES62 32mm EP, where the focuser tube is racked out all the way out.  This is the only EP that has focusing issue.

 

To mitigate this, I moved the primary mirror all the way back (increasing distance between the secondary and primary mirrors), maybe by 5-6 mm or so to move the focus plane towards the secondary.  This is not a huge gain, but enough to focus the ES62 32mm.

 

Because of this, I did re-collimate the scope, and as a result, the primary reflected edge is slightly smaller than secondary edge, and primary reflection is concentric to but fully fits within secondary edge.  I could actually see a dark even bezel (OTA inner wall reflection) between the secondary and primary reflection edges.  I hope you get what I mean. 

 

Sorry for this long post, but the question is does the latter make any difference (on image quality, aberrations, illumination etc.)?

 

I know the two edges are not overlapping (which is said to be ideal), but as long as I can see the full reflection of primary in the secondary mirror, it is good no?  I have an intuitive feeling that it doesn't make any difference, as long as primary reflection is not clipped, and primary dot aligned behind crosshairs and with the cheshire black area.

 

I might be wrong, thus please educate me.


Edited by Baatar, 05 November 2019 - 11:27 AM.


#14 Vic Menard

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Posted 05 November 2019 - 03:07 PM

...does the latter make any difference (on image quality, aberrations, illumination etc.)?

 

I know the two edges are not overlapping (which is said to be ideal), but as long as I can see the full reflection of primary in the secondary mirror, it is good no?  I have an intuitive feeling that it doesn't make any difference, as long as primary reflection is not clipped, and primary dot aligned behind crosshairs and with the cheshire black area...

Here's what the three alignments accomplish:

The corrected primary mirror alignment centers the coma free field at the eyepiece field stop--this is the most important alignment for high magnification performance.

The corrected focuser axis (via the secondary mirror tilt) alignment makes the focal plane perpendicular to the optical axis (and coplanar) with the eyepiece field stop--in conjunction with the fully corrected primary mirror alignment, this makes focus "snap".

And finally, the corrected secondary mirror placement (three circles concentric) balances the optimal field illumination at the eyepiece field stop. This can be difficult (or impossible) to asses visually (unless the placement is significantly misaligned), but may show up as unbalanced vignetting in an imaging scope. So, although this misalignment is easy to see (and hard to fix!), small errors usually have little or no impact on image performance.

 

Finally, the "unclipped" visibility of the "full reflection of the primary" is not a valid alignment signature unless the focuser axial alignment is also fully corrected! This is why it's important that all three circles should be concentric. Given the choice, the focuser axial alignment (thin beam laser or sight tube crosshairs aligned to the primary mirror center spot, usually via secondary mirror tilt) is more important than an unclipped primary. You'll notice the loss of snap focus long before you'll notice a shift in the field illumination.


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#15 Baatar

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Posted 06 November 2019 - 04:33 AM

 

Finally, the "unclipped" visibility of the "full reflection of the primary" is not a valid alignment signature unless the focuser axial alignment is also fully corrected! This is why it's important that all three circles should be concentric. Given the choice, the focuser axial alignment (thin beam laser or sight tube crosshairs aligned to the primary mirror center spot, usually via secondary mirror tilt) is more important than an unclipped primary. You'll notice the loss of snap focus long before you'll notice a shift in the field illumination.

 

Hi Vic, thanks.

 

I think I am following you, and trying to digest what you said.  Yes, I fully recognize the importance of three alignment process.

 

However, I am not sure if I understood your reply to my question whether the primary reflection edge concentric to but smaller (i.e. two edges are not overlapping) than the secondary edge makes any difference. 

 

I fiddled again with the collimation today.  And what I seem to notice (was not the case before, or I might not have noticed carefully) is that when I reach concentricity of secondary and primary edges, the primary center spot is no longer aligned with crosshairs.  When I align center spot with combination tool crosshairs, then I lose concentricity slightly (no clipping and the primary edge is still fully within the secondary edge).

 

In the latter case, I still need to fiddle with the position of the secondary?  If I do this, the secondary seem no longer be in the central position when looked through empty focuser and/or the sight tube. 

 

This is getting complicated indeed, the more I fiddle the more it gets out of alignment.  I gather the SW dob mechanics are not perfectly aligned or symmetric, and such minor deviations might have resulted from any of these issues and/or any combination of them:

1.  focuser tube is not on-axis, and not perpendicular to optical axis

2.  misshapen secondary (not a true ellipse), or spider is not centered, or secondary holder is not perpendicular to primary mirror (or not parallel to primary axis)

3.  shaky registration of tools, extensions with the focuser.

4.  primary is not centered in its mirror base

5.  primary central dot is not in its true central position

6.  any other misalignments, reasons that I don't know of.

 

I didn't check all the above, but things seem not to be overly out of alignment to my eyes.

 

So I have two possible scenarios, and with which I need to go (or rely upon)?

 

A.  If primary dot, combination tool crosshairs and cheshire black spots are all aligned and overlapping (checked with barlowed laser too), BUT, the primary/secondary/site tube edges are not perfectly concentric 

 

B.  if the primary/secondary/site tube edges are concentric, BUT, the primary dot, crosshairs and cheshire black spot are not perfectly overlapping.

 

I am not going to give up, and am determined to reach to the bottom of these issues.  I am sensing the secondary positioning, rotation, tilting are the culprit (as the secondary is most moveable and adjustable piece in the whole optical chain).

 

Happy to hear any reactions whether I am doing anything that I am not supposed to do.


Edited by Baatar, 06 November 2019 - 04:39 AM.


#16 Asbytec

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Posted 06 November 2019 - 08:26 AM

OP, "...when I reach concentricity of secondary and primary edges, the primary center spot is no longer aligned with crosshairs."

Vic, "Finally, the "unclipped" visibility of the "full reflection of the primary" is not a valid alignment signature unless the focuser axial alignment is also fully corrected!"

Op, "If I do this, the secondary seem no longer be in the central position when looked through empty focuser and/or the sight tube."

Vic, "So, although this misalignment is easy to see (and hard to fix!), small errors usually have little or no impact on image performance."

Vic, "The corrected primary mirror alignment centers the coma free field at the eyepiece field stop--this is the most important alignment for high magnification performance."

Centering the secondary under the focuser only fixes the fully illuminated field at or near the center of the eyepeice FOV. It is, technically, not collimation as much as it is getting an unvignetted, fully illuminated FOV. It does not affect collimation. As long as you can see the entire prmary reflection, your field will be fully illuminated at that focuser position on axis at least. The point where the primary reflection and secondary "overlap" or are the same size is called the apex. If, with inward focuser travel, you see the primary reflection and some space around it, your scope will be fully illuminated on axis and proportionally off axis, too, at that focuser position.

Collimation of the respective axes, focuser/eyepeice and primary, begins with centering the cross hairs or thin beam laser on the primary center mark. As Vic explains, this focuser axial collimation makes the eyepeice focal plane coincide, more or less, with the collimated primary focal plane. When the cross hairs are in the center mark, the focuser axis is pointing at the primary center and the reflection of the primary will be centered in the focuser/site tube even if it's not centered in the secondary. The former is collimation, the latter is field illumination. The only time the concentric primary reflection and secondary are a valid collimation signature is when the cross hairs are on the center mark. However, the axes can be collimated without the concentric secondary which, again, only affects the location of the fully illuminated field of view. Not axial collimation which affects perfomance, ie, snap to focus.

Bringing the primary center mark into the Cheshire ring by primary tilt completes axial collimation bringing the primary's coma free point and diffraction limited (for coma) field of view to the center of the eyepeice FOV. This is the most critical collimation for high power viewing where centering the primary mirror's diffraction limited field to center of the small high power FOV requires some accuracy.

You can "fiddle" with secondary position (illuminated FOV) if you want. But so long as you can see the primary reflection through most or all of your focuser travel, you should be fine and fully illuminated over that range of focuser travel. Focus more on centering the cross hairs on the center mark and the center mark to the center of the Cheshire ring. Both are axial collimation and affect performance.

Thanks to Don (Starman), I found it's much easier to refine secondary rotation by rotating (only) the secondary so the reflection of the primary is centered on the major axis of the elliptical secondary. And not to rely solely on the circular appearance of the secondary under the focuser (some folks use colored paper to hide the primary and illuminate the secondary surface to estimate secondary rotation). That's fine, but don't rely on the rotation you attain, remove the paper and refine secondary rotation (only) using the primary reflection. The former rotation is not accurate and gives random results, the latter method is accurate and repeatable. Centering the primary reflection will make collimation easier and more accurate. And your offset will "point" to the primary mirror.

When rotation is set correctly, you only need secondary tilt to bring the cross hairs directly onto the center mark using one secondary tilt screw (and adjusting the other two accordingly). For example, tighten one and loosen the other two, or visa versa, tighten two and loosen one, as needed. Approach the center mark directly from one direction, not haphazardly from three different directions. I think you'll hit the center mark more easily and accurately and improve your focuser alignment.

Once you're familiar with the movement, you can check (at the apex) and repeat this secondary adjustment without fear and do it in very short order. It's liberating to tune the secondary quickly and without fear because we begin to understand it. :)

Edited by Asbytec, 06 November 2019 - 09:00 AM.

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#17 Baatar

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Posted 06 November 2019 - 09:20 AM

...

Centering the secondary under the focuser only fixes the fully illuminated field at or near the center of the eyepeice FOV. It is, technically, not collimation as much as it is getting an unvignetted, fully illuminated FOV. It does not affect collimation. As long as you can see the entire prmary reflection, your field will be fully illuminated at that focuser position on axis at least. The point where the primary reflection and secondary "overlap" or are the same size is called the apex. If, with inward focuser travel, you see the primary reflection and some space around it, your scope will be fully illuminated on axis and proportionally off axis, too, at that focuser position.

Collimation of the respective axes, focuser/eyepeice and primary, begins with centering the cross hairs or thin beam laser on the primary center mark. As Vic explains, this focuser axial collimation makes the eyepeice focal plane coincide, more or less, with the collimated primary focal plane. When the cross hairs are in the center mark, the focuser axis is pointing at the primary center and the reflection of the primary will be centered in the focuser/site tube even if it's not centered in the secondary. The former is collimation, the latter is field illumination. The only time the concentric primary reflection and secondary are a valid collimation signature is when the cross hairs are on the center mark. However, the axes can be collimated without the concentric secondary which, again, only affects the location of the fully illuminated field of view. Not axial collimation which affects perfomance, ie, snap to focus.

 

 

Thanks Norme!

 

This is it.  I had the gut feeling too that it is more important to align the crosshairs / primary dot / and cheshire dark spot.

 

Vic, no offence really, but I had a hard time relating your technical explanation to my questions.

 

So in layman terms, is it simpler to say that "I shouldn't be concerned with centering and concentricity of three edges" only if I properly aligned/overlapped the three axes?  As long as I achieve three axial alignment, then the centering/concentricity is only desirable (given that I should maintain the axial alignment and have full illumination of the primary)?

 

This is really clear now, and where I should focus.

 

Thank you to you and Vic !


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#18 Asbytec

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Posted 06 November 2019 - 10:14 AM

Sure. It is possible to achieve and hold all three collimation signatures concentric. You only have to "fiddle" with the secondary position toward or away from the primary, then refine secondary rotation (only) using the primary reflection as a reference.

Once secondary position is set and properly rotated, tilt only using the secondary adjustment screws "correctly" (to minimize unwanted tilt and rotation) will bring the primary reflection to the center of the secondary at the same time the cross hairs move onto the center mark. That is the valid secondary and primary reflection collimation signature Vic mentioned above...when they are concentric with the cross hairs on the center mark at the same time. This is a collimation signature because the cross hairs are also collimated.

The apex is a great focuser position to see how accurate you are with secondary position, tilt, and rotation and to make minor tweaks as needed. But by themselves, simply seeing a concentric primary reflection in the secondary does not guarantee the focuser is pointing at the primary center just as you asked about above. The cross hairs ensure the focuser axis is aligned, ideally with a closely centered primary reflection, too, for optimal field illumination. We shoot for this optimal position in step 1 when we position the secondary under the focuser.

Thus, there are three steps we use to collimate our Newt.

1. Position the secondary for field illumination.

2. Rotate and tilt the secondary to align the focuser axis (eyepeice focal plane tilt) to the soon to be collimated primary axis at the primary center.

3. Bring the coma free parabola (primary) optical axis and surrounding diffraction limited primary focal plane to the center of the eyepeice field of view retracing the focuser axis from the primary center back to the eyepeice.

Done.

Edited by Asbytec, 06 November 2019 - 10:38 AM.

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Posted 06 November 2019 - 10:37 AM

Sure. It is possible to achieve and hold all three collimation signatures concentric. You only have to "fiddle" with the secondary position toward or away from the primary, then refine secondary rotation (only) using the primary reflection as a reference.

Once secondary position is set and properly rotated, tilt only using the secondary adjustment screws "correctly" (to minimize unwanted tilt and rotation) will bring the primary reflection to the center of the secondary at the same time the cross hairs move onto the center mark. That is the valid secondary and primary reflection collimation signature Vic mentioned above...when they are concentric with the cross hairs on the center mark at the same time. This is a collimation signature because the cross hairs are also collimated.

But by themselves, simply seeing a concentric primary reflection in the secondary does not guarantee the focuser is pointing at the primary center just as you asked about above. The cross hairs ensure the focuser axis is aligned, ideally with a closely centered primary reflection, too, for optimal field illumination. We shoot for this optimal position in step 1 when we position the secondary under the focuser.

 

Ok, so if I am to devise a collimation plan for myself aiming for "perfection", the plan would be:

 

1.  center secondary as best as I could under focuser  (something I am doing)

 

2.  proceed with 3-axis alignment using cheshire combination tool and barlowed laser (only for primary alignment)

 

3.  if after collimation I see a slight misalignment in terms of centering/concentricity (which is what is happening at the moment), note down how much and in which direction/rotation the secondary needs to be moved (if at all and if I am not satisfied)

 

4.  slightly and equally loosen secondary collimation screws and make the necessary secondary adjustments as per the notes in step 3

 

5.  repeat step 2, and check again step 3 (by which time centering/concentricity might hopefully improve)

 

Repeat all the steps again, if required, one-by-one until such a time when the 3-axial alignment and centering/concentricity becomes near "perfect", where the latter is subject to any mechanical and/or manufacturing imperfections.

 

And if I see that balancing out everything will not be achievable with my particular scope, then the 3-axial alignment should always be a priority.

 

I think I am getting it now, as well as the inherently re-iterative process in and of itself.



#20 Asbytec

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Posted 06 November 2019 - 10:58 AM

Yes, it takes hearing it a few times. And doing it. Vic rightfully uses the proper vernacular to describe collimation in technical terms. We should be versed in the terms, it takes time.

I'm not sure what you mean by three axes. There are only two axes. The first is aligning the focuser axis (actually the eyepeice optical axis when we put an eyepiece in the focuser) to the primary center as seen with the cross hairs or thin beam laser. The second axis is the primary parabolic optical axis which contains the diffraction limited focal plane. We bring that to the eyepeice center as seen using a Cheshire or Barlow laser. Collimation is aligning these two axes, in that order, so they coincide to within tolerance.

The three steps get us there in the proper sequence, to include optimal field illumination. Position the secondary and (importantly) rotate it. Use tilt to bring the cross hairs or laser to the primary center mark. Then bring the primary center mark to the Cheshire ring or Barlow laser center. Conceptually it's easy to see, the trick is in the execution.

It all starts with proper rotation of the secondary to begin with, use the reflection of the primary, and use the secondary adjustment screws as described above. You'll better hold your collimation signatures. Check them at the apex of your focuser travel where the primary reflection "overlaps", or very nearly so, the secondary and refine as needed (as you noted). Then bring the primary into collimation. Done. It really is that easy once it clicks. :lol:

Edit: when your checking at the apex, only release tension on one secondary adjustment screw in line with the focuser so the secondary can rotate. Then tighten that same screw to bring the focuser axis (more accurately) back on the center mark. You can do tiny tilt adjustments, as needed, too. You'll note them when you see them.

Edited by Asbytec, 06 November 2019 - 11:09 AM.

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#21 Vic Menard

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Posted 06 November 2019 - 11:07 AM

...my question whether the primary reflection edge concentric to but smaller (i.e. two edges are not overlapping) than the secondary edge makes any difference. 

 

So I have two possible scenarios, and with which I need to go (or rely upon)?

 

A.  If primary dot, combination tool crosshairs and cheshire black spots are all aligned and overlapping (checked with barlowed laser too), BUT, the primary/secondary/site tube edges are not perfectly concentric 

 

B.  if the primary/secondary/site tube edges are concentric, BUT, the primary dot, crosshairs and cheshire black spot are not perfectly overlapping.

First--to answer your question, "whether the primary mirror reflection edge (is) concentric to but smaller than the secondary mirror edge makes any difference"? Ideally, to achieve precise offset, the three circles should be the same (apparent) size/diameter. In practice, it's easier to discern the three circles if they're in discrete steps. The difference in offset is inconsequential (typically a fraction of a millimeter). So, to answer your question, no--it doesn't make any difference.

 

Next, your two scenarios:

 

Scenario A is consistent with "correct" axial alignments and a slightly incorrect secondary mirror placement. This is acceptable.

 

Scenario B is impossible. If all three circles are concentric, and the sight tube cross hairs are actually centered relative to the bottom edge of the sight tube, and the primary mirror center spot is actually centered in the primary mirror, then the sight tube cross hairs must be aligned to the primary mirror center spot. (It's possible that the reflection of the Cheshire might be misaligned since that has nothing to do with secondary mirror placement or focuser axial alignment. In fact, some users prefer to leave the primary mirror misaligned to make it easier to see the sight tube cross hair/primary mirror center marker alignment.)


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Posted 06 November 2019 - 11:09 AM

I'm not sure what you mean by three axes. There are only two axes. The first is aligning the focuser axis (actually the eyepeice optical axis when we put an eyepiece in the focuser) to the primary center as seen with the cross hairs or thin beam laser. The second axis is the primary parabolic optical axis which contains the diffraction limited focal plane. We bring that to the eyepeice center as seen using a Cheshire or Barlow laser. Collimation is aligning these two axes, in that order, so they coincide to within tolerance.
 

 

Thanks for pointing out.  Pardon me, I don't know why I said it, but this is what I really meant and as you described.  My tongue slipped I guess.

 

I am now fully getting what you and Vic said.  Will try everything from scratch this evening.


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Posted 06 November 2019 - 11:16 AM

Scenario B is impossible. If all three circles are concentric, and the sight tube cross hairs are actually centered relative to the bottom edge of the sight tube, and the primary mirror center spot is actually centered in the primary mirror, then the sight tube cross hairs must be aligned to the primary mirror center spot. (It's possible that the reflection of the Cheshire might be misaligned since that has nothing to do with secondary mirror placement or focuser axial alignment. In fact, some users prefer to leave the primary mirror misaligned to make it easier to see the sight tube cross hair/primary mirror center marker alignment.)

 

Vic, this is really what baffles me.  Indeed, I thought that if primary reflection is concentric (at least in relation to the sight tube edge, and if not in relation to secondary edge), then logically the primary dot should also be aligned with crosshairs.  But this only assumes that primary dot is placed right in the physical center of primary, and that the crosshairs are also in the exact center of the sight tube.

 

I was thinking that if one of them, or both for that matter, are not the case then this resulted in the scenario B, which you said is impossible.

 

I will check and double check, measure everything tonight and see if any of the tools are at fault.  Have to admit that I didn't go expensive tools, thus quality may be compromised.



#24 Asbytec

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Posted 06 November 2019 - 11:25 AM

"In fact, some users prefer to leave the primary mirror misaligned to make it easier to see the sight tube cross hair/primary mirror center marker alignment."

Sure, with the primary grossly misaligned on purpose, it leaves the center mark way out from under the dark Cheshire reflection making it easy to see the center mark and cross hairs. We're not concerned with primary alignment at this point, anyway. Only focuser axial alignment onto the center mark. Primary alignment is done last and we'll bring the center mark to the Cheshire reflection, necessarily, when we complete collimation. So if you want, throw the primary out of whack and focus on the focuser axis only. Doing so won't(sic) displace the primary center mark or affect centering the primary reflection in the secondary. It only affects the primary optical axis, which gets fixed last, anyway.

Or just keep this in mind for later and focus on your current questions.

Edited by Asbytec, 06 November 2019 - 11:37 AM.

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#25 Vic Menard

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Posted 06 November 2019 - 11:36 AM

This is it.  I had the gut feeling too that it is more important to align the crosshairs / primary dot / and cheshire dark spot.

 

Vic, no offence really, but I had a hard time relating your technical explanation to my questions.

My apologies that my response was too technical. I'd like to say that it's not going to get worse, but sorting the secondary mirror placement (while maintaining the axial alignments) quickly gets complicated.

 

As I tried to explain (and you seem to have grasped), correct axial alignment delivers image performance, and correct secondary mirror placement delivers balanced illumination.

 

But, when you look into a collimation cap or a combo tool (in fact, any tool with a centered pupil), incorrect secondary mirror placement is easy to see! Understanding offset helps when you come to the realization that all of the circles/reflections you see in the pupil can't be concentric at the same time--either the actual edge of the secondary mirror, or the reflected edge of the secondary mirror, will be offset.

 

And this is where it gets complicated. When the secondary mirror is "centered" relative to the bottom edge of the focuser, you've already offset it toward the primary mirror! But that doesn't satisfy the secondary mirror placement as it must appear centered and rounded. If you look at the offset silhouette reflection of the secondary mirror surrounding the centered reflection of the underside of the focuser, the silhouette offset should look like it bulges toward the primary mirror end of the OTA. If the primary mirror end of the OTA is off to the left (9 o'clock) and the bulge is pointing above (10:30) or below (7:30) the secondary mirror has a combined tilt./rotation or skew error (and yes, you can have perfect axial alignment with a skewed secondary mirror). If the secondary mirror bulge is pointing above the primary mirror end of the OTA, the secondary mirror needs to be tilted downward (cross hairs or laser below the primary mirror center marker), and then the the secondary mirror needs to be rotated to bring the cross hairs back into alignment with the primary mirror center marker. Once you get comfortable correcting skew errors, you can move on to offset errors (combined secondary mirror position closer to or farther from the primary mirror/tilt errors). And, it's not uncommon to have both skew and offset errors present when the secondary mirror is centered and rounded under the focuser.

 

The good news is that, as long as the mechanicals (focuser "squared" to the OTA, spider "squared" to the OTA, secondary mirror "centered" relative to the OTA, primary mirror "centered" relative to the OTA, etc.) are reasonably close (within a few millimeters), sorting skew and offset errors is pretty easy (once you know what you're doing). If the mechanicals aren't reasonably close, that can make everything much more complicated.




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