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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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