Does this look as decent collimation?

Reflectors Acquisition Deep Sky Collimation
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Christian Bennich avatar
Christian Bennich avatar
Hi all,

I use this laser collimator for my Skywatcher 200OPDS - https://www.baader-planetarium.com/en/laser-colli---mark-iii.html and have recently started looking at doing additional collimation with out of focus stars before my sessions. 

A few questions:
Does the enclosed image look as a "decently" collimated Newtonian?
I see the square in the bottom that cover a bit of the bottom. My first guess was that it was my OAG prism. I moved it back, without any changes seen, next guess was my focuser - but it's not entering into the tube when I'm "out of focus" - any other ideas....or should I reinvestigate the aforementioned 2 options?

Brian Puhl avatar
Brian Puhl avatar
That's your focuser.  Looks decent otherwise.  Maybe just a tad off.   Unless you're rolling with fast optics… You probably won't notice.
Christian Bennich avatar
Christian Bennich avatar
Thx @Brian P. 

So a tad off - would mean that I should try and move the primary mirror slightly towards the bottom left?
My newt is an F/5 and for this this project I use a Coma Corrector with no focal reduction. So I guess the optics are "fairly" fast?

I got a Starizona Nexus that I will try for my next project with a slightly larger target.
Brian Puhl avatar
Brian Puhl avatar
F/5 to me is more on the slow end.   I have a nexus with my GSO newtonian, and I'm going to give it one more shot this weekend, but I think im gonna have to sell the nexus.   I think my GSO mirror might be slightly warped.   I've never been able to get it collimated properly at F/4.      My other option is the paracorr which takes me to F/4.5 and I haven't had issues there.  

Anyways, yes, you've got a slight oval to it that you should probably correct.   Just make sure you're perfectly centered on your star after every adjustment or you will be chasing your tail.   Collimation is quick and easy, at least it always was until I got my Nexus lol.   Just be warned before you go down that road :-)
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Christian Bennich avatar
Christian Bennich avatar
HAHA, nothing in this hobby ever seems easy. 
I see 2 options if the Nexus will not work - sell it….or buy a better scope with which the Nexus works 🤪🥳🥳
Christian Bennich avatar
Christian Bennich avatar
I have seen posts from people on CN and here as well that have achieved great results with the 200 PDS in combination with the Nexus - so I hope my mirrors are not warpes.
Brian Puhl avatar
Brian Puhl avatar
Christian Bennich:
HAHA, nothing in this hobby ever seems easy. 
I see 2 options if the Nexus will not work - sell it….or buy a better scope with which the Nexus works 🤪🥳🥳



Meanwhile I'm sitting here eyeballing Espirits lol.   I really wished I could get the Nexus to work, in a bad way, but getting this scope straight has been a pain for me.   Then I go out and spend 20 minutes aligning my Meade refractor....  After days screwing with the newt...  It almost feels like a no brainer to go full frac lineup
Christian Bennich avatar
Christian Bennich avatar
I did some simple quick tests with my Starizona tonight. 
I think it will work out fine with my Skywatcher. 

It has not been fine-tuned yet, gotta give the collimation a bit more work, but I'm sure it will be great. 

Took some 60 sec exposures to measure in PI - https://drive.google.com/drive/folders/1QSfS0hbJszGslaX_ycgIiGXu8CaD5ep5?usp=sharing
It's not all too bad for a first go...I think 😉

andrea tasselli avatar
andrea tasselli avatar
I had a look and it doesn't look too good in my book. I don't think it's collimation, more like spacing and significant tilt. Me thinks you'll be better off with a GPU unit (if not the Paracorr which is really good).
Christian Bennich avatar
Christian Bennich avatar
andrea tasselli:
I had a look and it doesn't look too good in my book. I don't think it's collimation, more like spacing and significant tilt. Me thinks you'll be better off with a GPU unit (if not the Paracorr which is really good).

Thx @andrea tasselli - I am working my way towards understanding tilt and how to correct it. 

For my benefit - which indicators are you specifically looking at when you point to spacing?

Does the tilt part give itself away in the FWHM measurement?

I currently image with this GPU unit - https://www.teleskop-express.de/shop/product_info.php/info/p6706 but were looking for something that could also allow for a slightly bigger fov as well. 

I would assume that the GPU suffer from the same tilt as the Nexus. 
Until now I have postponed working on eliminating tilt as I am waiting for an upgrade to my focuser - but Baader is unable to deliver at the moment. 

Thank you for your feedback.
Aaron H. avatar
Aaron H. avatar
Assuming your secondary is offset (which is the norm for imaging Newts), then the secondary shadow shouldn't be centred when properly collimated. At f/5, you're fast enough that you should actually see a small offset of the secondary shadow. See the images under "Fast Slow Telescopes" in Step 4 here:
https://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/

An offset is the norm for imaging Newts as it maximizes field illumination without requiring an oversized secondary (and therefore an oversized secondary obstruction).

This offset means that relying on the secondary shadow as an indicator of collimation is misleading for most imaging Newts. If you want to do a star test with a Newt, you need to very slightly de-focus a bright star so you can resolve its airy disk. This will likely wobble about due to seeing, but it will give an accurate indication of collimation.

Also, that does indeed look like your focuser intruding into the light path. It doesn't need to protrude much for that to occur. It may simply be because you are so defocused.

Personally, when imaging, I simply collimate with a laser for the secondary and a Cheshire or Tublug for the primary.
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andrea tasselli avatar
andrea tasselli avatar
Christian Bennich:
Thx @andrea tasselli - I am working my way towards understanding tilt and how to correct it.

For my benefit - which indicators are you specifically looking at when you point to spacing?

Does the tilt part give itself away in the FWHM measurement?

I currently image with this GPU unit - https://www.teleskop-express.de/shop/product_info.php/info/p6706 but were looking for something that could also allow for a slightly bigger fov as well.

I would assume that the GPU suffer from the same tilt as the Nexus.
Until now I have postponed working on eliminating tilt as I am waiting for an upgrade to my focuser - but Baader is unable to deliver at the moment.

Thank you for your feedback.


Normally, if it is just tilt, some areas are in focus with no aberrations while opposite areas have them, mainly astigmatic coma. One typical situation is a tilt about one of the main diagonals of the camera, wherein you have good roundness  along one diagonal (and the center) while the two other opposite corners show elongation in opposite directions. In case is both spacing and tilt, than you should also see some residual coma just off the axis along one side showing less of it and one side showing more of it. In your case I think you also have some collimation issues (slight flares on stars about the centre). Issue with tilt are best righted using something like Gerd Neumann's CTU unit, not by changing the focuser (which carries risks in itself).
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Dale Penkala avatar
Dale Penkala avatar
Yes agree that is the focuser drawtube at the 6’oclock position in your image. For f5 your collimation is close but certainly can be tweaked. 

My suggestion from here is to maybe do another tweak or two with your laser in the out of focus defraction pattern and then move to an in focus star using the airy disc and the free video software called MetaGuide here: https://smallstarspot.com/metaguide/ MetaGuide is actually a guiding software but there is a collimation tool that is excellent and works great once you get past the learning curve.

I have used 6” - 12” newts that I’ve rebuild and use MetaGuide for my final collimation. While it does take some getting used to and there is a learning curve as I stated above, in the end its is well worth the effort!

Also wanted to point out that Brians comment about a secondary offset in an f5 optic set is correct. There would be a very slight offset in the secondary shadow and yes this is to have a more evenly illuminated field of view.

Dale
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Christian Bennich avatar
Christian Bennich avatar
Aaron H.:
Assuming your secondary is offset (which is the norm for imaging Newts), then the secondary shadow shouldn't be centred when properly collimated.


@Aaron H. - the mirror is glued in the offset position, so I "just" have to center it in the focuser. 
I have taken my secondary mirror apart, measured everything, gotten myself a "concenter eyepiece" to center the secondary - which is now deadcenter in the focuser. I then use a collimated Baader laser for the Primary mirror (I need to get my self a high quality cheshire....the one I have is not great).

I have not yet found the perfect off focus position to get the airy discs on camera to collimate on before my imaging sessions. I have seen videos of these narrow perfect airy star circles - mine are always "fatter" and therefore difficult to see how I should adjust my collimation - which is already considerably better than before.

Any suggestions to find the correct off focus position, other that just keep trying 😃
Christian Bennich avatar
Christian Bennich avatar
andrea tasselli:
Christian Bennich:
Thx @andrea tasselli - I am working my way towards understanding tilt and how to correct it.

For my benefit - which indicators are you specifically looking at when you point to spacing?

Does the tilt part give itself away in the FWHM measurement?

I currently image with this GPU unit - https://www.teleskop-express.de/shop/product_info.php/info/p6706 but were looking for something that could also allow for a slightly bigger fov as well.

I would assume that the GPU suffer from the same tilt as the Nexus.
Until now I have postponed working on eliminating tilt as I am waiting for an upgrade to my focuser - but Baader is unable to deliver at the moment.

Thank you for your feedback.


Normally, if it is just tilt, some areas are in focus with no aberrations while opposite areas have them, mainly astigmatic coma. One typical situation is a tilt about one of the main diagonals of the camera, wherein you have good roundness  along one diagonal (and the center) while the two other opposite corners show elongation in opposite directions. In case is both spacing and tilt, than you should also see some residual coma just off the axis along one side showing less of it and one side showing more of it. In your case I think you also have some collimation issues (slight flares on stars about the centre). Issue with tilt are best righted using something like Gerd Neumann's CTU unit, not by changing the focuser (which carries risks in itself).

@andrea tasselli - that CTU does indeed look very nice. I am somewhat limited in my imaging train. I have the ASI2600 + filter wheel + OAG-L + a 5mm tilt plate - that adds up to 55mm - I can't see any way to free up enough space to put in such a CTU....unless I remove my OAG and go back to a "normal" guide setup. 

After switching to an OAG my guiding has dropped to between 0,4 and 0,7 arcseconds - which is waaaaayyyy better than I could achieve before. 

So I guess I will have to fix my tilt with the current tilt plate....unless there are Astro Jedi tricks I have not yet learned?

Another follow-up question:
In your initial answer, you also mentioned spacing - if I "do the math" on my imaging train that look like this:


It does NOT consider the 2mm filter in the EFW - which means I must be 0,66mm off in my back focus - is that correctly understood????
To solve this - I would need to add 0,66mm of spacing between my tilt plate and my GPU.

If I understand it correctly - I would then need some M48 spacers - as the threaded connection between my CPU and the OAG as M48 - I can only seem to get 0,5mm and 1mm - so I can choose to add 0,5mm - which will still leave a "gap" of 0,167mm or add a 1mm spacer and introduce 0,33mm difference in the other direction. 

I would assume I should go for the solution closest to 55,66 mm back focus - meaning adding the 0,5mm spacer.

Long post - thank you in advance.
Aaron H. avatar
Aaron H. avatar
Christian Bennich:
Aaron H.:
Assuming your secondary is offset (which is the norm for imaging Newts), then the secondary shadow shouldn't be centred when properly collimated.


@Aaron H. - the mirror is glued in the offset position, so I "just" have to center it in the focuser. 
I have taken my secondary mirror apart, measured everything, gotten myself a "concenter eyepiece" to center the secondary - which is now deadcenter in the focuser. I then use a collimated Baader laser for the Primary mirror (I need to get my self a high quality cheshire....the one I have is not great).

No matter how the mirror is geometrically positioned in the tube, its shadow will appear offset if heavily defocused. The defocused image and the actual physical geometry won't match. There is no way to avoid this without shifting the primary mirror reflection towards the edge of the secondary mirror, which will lead to uneven illumination.

This offset is either a "full offset" (the mirror is pushed away from the focuser and down the tube), or a partial offset (the mirror is pushed down the tube from the focuser only). Which of these it is doesn't really matter unless you're actually designing a Newtonian. From a user perspective, as long as the reflection of the primary is centered in the secondary when looking through a sight tube (or collimation cap, or Cheshire), then all is well with the offset.

I've seen plenty of forum posts where users are trying to collimate a Newtonian with a heavily defocused "donut" as you would with a SCT. This won't work. The shadow shouldn't be centered, so trying to achieve that will actually put you out of collimation.

The only way to test through a defocused star is by resolving the actual airy disc. However, I find this tedious, fussy and error-prone with a camera. If trying to collimate with this approach, you're better off removing the camera, inserting your highest-powered eyepiece and doing it visually. I don't bother. When imaging, I just collimate with a laser for the secondary and Tublug or Cheshire for the primary. This is accurate enough for me.

Lasers are fantastic for collimating the secondary mirror, but are far too inaccurate for satisfactory primary mirror collimation because any registration error is doubled on the return path. Even a cheap Cheshire will do a much better job.
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Lorenzo Siciliano avatar
Lorenzo Siciliano avatar
Aaron H.:
Lasers are fantastic for collimating the secondary mirror, but are far too inaccurate for satisfactory primary mirror collimation because any registration error is doubled on the return path. Even a cheap Cheshire will do a much better job.

Using a barlowed laser will do the trick and will collimate the primary mirror in seconds.
Give it a try.
Ciao
Lorenzo
Christian Bennich avatar
Christian Bennich avatar
Lorenzo Siciliano:
Aaron H.:
Lasers are fantastic for collimating the secondary mirror, but are far too inaccurate for satisfactory primary mirror collimation because any registration error is doubled on the return path. Even a cheap Cheshire will do a much better job.

Using a barlowed laser will do the trick and will collimate the primary mirror in seconds.
Give it a try.
Ciao
Lorenzo

Hey @Lorenzo Siciliano 
I have seen barlowed lasers mentioned before. How does it work?
Christian Bennich avatar
Christian Bennich avatar
Let me google that for you Christian 🤣🤣
https://www.obsessiontelescopes.com/learning_center/collimation/index.php
Lorenzo Siciliano avatar
Lorenzo Siciliano avatar
Christian Bennich:
Let me google that for you Christian 🤣🤣
https://www.obsessiontelescopes.com/learning_center/collimation/index.php

 yeah, exactly!!! ​​​​​
Ciao
Lorenzo
Christian Bennich avatar
Christian Bennich avatar
It's challenging to see what's going on in the bottom of the barlow lense where the donut from the main mirror is projected to as, at least in my case, it's a few centimeters up in the focuser. 

Have you had similar experience?
Aaron H. avatar
Aaron H. avatar
Lorenzo Siciliano:
Aaron H.:
Lasers are fantastic for collimating the secondary mirror, but are far too inaccurate for satisfactory primary mirror collimation because any registration error is doubled on the return path. Even a cheap Cheshire will do a much better job.

Using a barlowed laser will do the trick and will collimate the primary mirror in seconds.
Give it a try.
Ciao
Lorenzo

Indeed. I usually use a Tublug for primary mirror collimation when imaging.

It’s more convenient than a Cheshire, or a traditional Barlowed laser (where the reflection is inside the tube).

Unfortunately, Tublugs are getting hard to find these days.
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Lorenzo Siciliano avatar
Lorenzo Siciliano avatar
Christian Bennich:
It's challenging to see what's going on in the bottom of the barlow lense where the donut from the main mirror is projected to as, at least in my case, it's a few centimeters up in the focuser. 

Have you had similar experience?

Yes.
I used to use a little mirror to spot the reflection of the donut, but now I simply look at that reflection directly from the front of the tube, looking down to the main mirror. Thanks to the multiple reflections, the donut shadow is easy to see.
Much easier and with same effectiveness.
Ciao
Lorenzo
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