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A Tri-Bahtinov mask for SCT collimation and focusing

astrophotography imaging SCT
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#526 CosmicArk

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Posted 19 February 2022 - 10:09 PM

I printed a Tri Bahtinov mask yesterday afternoon for my Edge C8 HD
Printed it in two halves to get it on my print bed.

I set the the z height for the mask to be 0.4mm
Each half printed in about 56 minutes

Did some brief tests with it last night

Cloud cover meant I only got about an hour of time under the stars last night

Hoping for clear skies to continue with this...

Thanks to Satoru, Cytan, Kirk, Jinux and all those from whom I've learnt new things in this forum thread

Some images from last night's session
 

 

 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

Could you, or someone else share how to install the Tri Grabber program? It seems theres more work than just running an .exe file. 



#527 Raginar

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Posted 20 February 2022 - 12:33 PM

https://github.com/F...ave/AstroJargon

No it’s that easy. Just fill j. The specs, download the svg. Upload to tinker cad for setting the thickness. Download the stl and print.

#528 CosmicArk

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Posted 20 February 2022 - 03:13 PM

https://github.com/F...ave/AstroJargon

No it’s that easy. Just fill j. The specs, download the svg. Upload to tinker cad for setting the thickness. Download the stl and print.

I understand how to create mask, I was referring the software that analyzes the image



#529 jetty

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Posted 28 February 2022 - 04:26 AM

Thanks to everyone for their hard work on this.

 

After 20 years of having inaccurately collimated (the secondary holder isn't concentric to the corrector plate, I never get seeing good enough for an airy disk and aligning

visually with rings is just too subjective), for the first time tonight I got good collimated results that were repeatable, could be checked and weren't subjective.

 

Created a Tri-Bahtinov (modified), made it 3D printable and added the cover for my LX200 10" f/10.

 

Took about 10 mins to get a perfect collimation and about 4 iterations over the bobs knobs, then I picked various positions over the sky to make sure

it held and it did.

 

Duncan didn't work or produce the lines, but Tri-Bahtinov is the bomb.


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#530 Raginar

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Posted 28 February 2022 - 11:05 AM

I understand how to create mask, I was referring the software that analyzes the image

It's visual... Or with a camera to see it.  There isn't a software tool to tell you its correct.



#531 ChristopherLP

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Posted 22 March 2022 - 01:05 PM

I have a tri-mask and I'm finding that the diffraction spikes and airy disk don't show the same result.

I used an artificial star to perform the collimation.

After using the tri-mask, I could see that the airy disk alone, or the airy disk with the mask showed different things.

Why is this happening?  Does this indicate that the optic train is out of alignment?

After collimating and sticking with the diffraction spikes, taking the scope outside confirmed that the diffraction spikes were not giving proper alignment.

I had the scope out and I could see that it was out of collimation.  I could see the coma indicative of poor collimation.

I have a '76 and '83 C8.  Both scopes resulted in the same result.  Diffraction spikes didn't match the airy disk  = diffraction spikes led to poor collimation.

Any help would be appreciated.  I've tried to read through this thread, but it is very, very long and I haven't found my answer yet.

Thank you,

Chris

DSC 1526
Album: Collimation-C8
3 images
0 comments


Edited by ChristopherLP, 22 March 2022 - 01:11 PM.


#532 cytan299

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Posted 22 March 2022 - 02:41 PM

I have a tri-mask and I'm finding that the diffraction spikes and airy disk don't show the same result.

I used an artificial star to perform the collimation.

After using the tri-mask, I could see that the airy disk alone, or the airy disk with the mask showed different things.

Why is this happening?  Does this indicate that the optic train is out of alignment?

After collimating and sticking with the diffraction spikes, taking the scope outside confirmed that the diffraction spikes were not giving proper alignment.

I had the scope out and I could see that it was out of collimation.  I could see the coma indicative of poor collimation.

I have a '76 and '83 C8.  Both scopes resulted in the same result.  Diffraction spikes didn't match the airy disk  = diffraction spikes led to poor collimation.

Any help would be appreciated.  I've tried to read through this thread, but it is very, very long and I haven't found my answer yet.

Thank you,

Chris

This is interesting ... there is one possibility, the Tri-Bahtinov mask collimation is done at *focus* while your "airy" disk (it's actually the shadow) is done out of focus. You might actually want to move in and out of focus to see whether the diffraction rings are concentric with the Poisson dot. If the diffraction ring is symmetric about the Poisson dot, you are collimated regardless as to what the shadow actually tells you.

 

See this video:

 

https://youtu.be/Ie9j0Z9MNC8

 

to see what I mean.

 

cytan



#533 ChristopherLP

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Posted 22 March 2022 - 03:26 PM

This is interesting ... there is one possibility, the Tri-Bahtinov mask collimation is done at *focus* while your "airy" disk (it's actually the shadow) is done out of focus. You might actually want to move in and out of focus to see whether the diffraction rings are concentric with the Poisson dot. If the diffraction ring is symmetric about the Poisson dot, you are collimated regardless as to what the shadow actually tells you.

 

See this video:

 

https://youtu.be/Ie9j0Z9MNC8

 

to see what I mean.

 

cytan

Hi Cytan,

Thanks for the reply and video link.

I'm still confused because before getting the mask, I would collimate using the airy disk/concentric rings/Poisson dot.  This method has served me well, resulting in tack sharp stars.  I recently started doing some imaging and wanted to get my collimation as good as I could.  I have a friend who printed me the tri-mask. 

When I'm collimating using the airy disk and the shadow, aren't I attempting to get the shadow perfectly centered, resulting in the concentric circles?

 

If my concentric circles are centered around the Poisson dot, but my shadow appears off center, is this indicative of something being out of alignment in the optics train?  Like the secondary being off center or the focusing tube being off center/sagging? 

 

The discrepancy between good diffraction spikes and the actual seeing results puzzles me.  It seems the diffraction spikes are lying.

I'll need to look again and pay attention to the concentric rings and Poisson dot. 

 

Thanks again,

Chris

 



#534 cytan299

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Posted 22 March 2022 - 06:39 PM

Hi Cytan,

Thanks for the reply and video link.

I'm still confused because before getting the mask, I would collimate using the airy disk/concentric rings/Poisson dot.  This method has served me well, resulting in tack sharp stars.  I recently started doing some imaging and wanted to get my collimation as good as I could.  I have a friend who printed me the tri-mask. 

When I'm collimating using the airy disk and the shadow, aren't I attempting to get the shadow perfectly centered, resulting in the concentric circles?

 

If my concentric circles are centered around the Poisson dot, but my shadow appears off center, is this indicative of something being out of alignment in the optics train?  Like the secondary being off center or the focusing tube being off center/sagging? 

 

The discrepancy between good diffraction spikes and the actual seeing results puzzles me.  It seems the diffraction spikes are lying.

I'll need to look again and pay attention to the concentric rings and Poisson dot. 

 

Thanks again,

Chris

Hi Chris

  I assume that the star that you used is at the center of the FOV. Can I assume that the set of 3 photos are of the same star? If yes, then I don’t have a good explanation for what you’re seeing.

 

If the photos are not of the same star, then the middle photo with the mask not in focus shows some asymmetry in the 4 o’clock position already. And yet in focus, the spikes look pretty symmetric. My guess is that there is a tilt in your image train.

 

cytan



#535 ChristopherLP

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Posted 22 March 2022 - 11:25 PM

Hi Chris

  I assume that the star that you used is at the center of the FOV. Can I assume that the set of 3 photos are of the same star? If yes, then I don’t have a good explanation for what you’re seeing.

 

If the photos are not of the same star, then the middle photo with the mask not in focus shows some asymmetry in the 4 o’clock position already. And yet in focus, the spikes look pretty symmetric. My guess is that there is a tilt in your image train.

 

cytan

Hi Cytan,

 

Yes, I re-centered the star throughout the collimation process and the pictures are all from the same artificial star, same time.

The pictures represent the discrepancy between the spikes and the the airy disk off of the collimation done with the diffraction spikes.

It makes sense that something is out of alignment in some way.  I'll investigate further.

If I find out what is causing it, I'll let you know.

Thanks again.

Chris



#536 cytan299

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Posted 23 March 2022 - 05:39 AM

Hi Cytan,

 

Yes, I re-centered the star throughout the collimation process and the pictures are all from the same artificial star, same time.

The pictures represent the discrepancy between the spikes and the the airy disk off of the collimation done with the diffraction spikes.

It makes sense that something is out of alignment in some way.  I'll investigate further.

If I find out what is causing it, I'll let you know.

Thanks again.

Chris

If you’re using an artificial star, is the OTA horizontal? And are you moving the mirror for focusing? I can imagine that if it is a moving mirror and the mirror is essentially vertical when the a OTA is horizontal that the mirror is flopping differently as you move in and out of focus.

 

cytan


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#537 ChristopherLP

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Posted 23 March 2022 - 11:16 AM

If you’re using an artificial star, is the OTA horizontal? And are you moving the mirror for focusing? I can imagine that if it is a moving mirror and the mirror is essentially vertical when the a OTA is horizontal that the mirror is flopping differently as you move in and out of focus.

 

cytan

Yes, the scope is essentially horizontal.  When I used the regular mask to attain my initial focus, before moving to the Tri-mask, I was ending my focusing in the counter-clockwise direction.  After your messages yesterday, I cast my mind back to that collimation session and was walking through everything in my mind. 

I did think about the fact that the mirror wasn't stably weighted downward as it would be looking at a star close to the zenith.  I also wondered if the extension tubes required to gain focus with the DSLR wasn't sagging in some way.  So, yes, a couple things that could be confusing the situation.  

I will try to do some experimentation, eliminating these two factors, and see if the result is the same.  It might take a while.  I live in the WA state, so finding a night with clear skies is the main obstacle.  I'll think about some ways to bypass these obstacles.  I'm thinking that if I can rig up the artificial star on the high part of the wall of my 2nd floor staircase, I would be able to get the scope to around a 50 degree angle.  Not sure how to deal with the sag of the camera gear, but I could try to move the camera rig in a different direction, for example, moving the extension tube thumb screws to different sides, etc.  Some of the focuser tube extensions only have two thumb screws to secure things.  This may be part of the problem and rotating the thumb screw orientation may show a different result.

I'll sleuth is out and see what I come up with. 

 

Thanks again for thinking about my issues and helping with these ideas.

Chris
 


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#538 Zebul

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Posted 27 March 2022 - 04:25 AM

Hi Christopher,

I have experience the same trouble about collimation and focusing.quality on my C8 Edge.

=> https://www.cloudyni...7#entry10398311

 

I have done some test between tribahtinov (2 ring design) and bahtinov classic mask => my surprise the result is not exactly the same. There is a focusing delta...

   My process :

     - check the shadow circle concentric => start pre-colimation position (myself software picture check)

     - tribahtinov focusing to adjust colimation =>  result shadow circle is no more concentric (what!???)

     - with bahtinov mask, I turn 120° to check and readjust colimation (=> fine tuninig after tribahtinov) => bahtinov mask give me a better focus. Shadow circle is more concentric but not at the best (I don't understand situation...)

 

One of my personnal analyze is, tribahtinov mask probably tend to diaphram the aperture (divide by 3 the quantity of lite...). I am not expert but probably this have an optical impact.

May be the tribahtinov mask desing have an impact too. I would like to check with a new design in one ring, maybe better. I will try to produce a mask that have the same surface for all diffraction area.

=> https://www.cloudyni...0#entry11348592

 

No more for me...

I expect one day in this forum or elsewhere, a specialist analyze how tribahtinov work and give is result to much better understand such particular situation...smile.gif


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#539 cytan299

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Posted 27 March 2022 - 08:22 AM

Hi Christopher,

I have experience the same trouble about collimation and focusing.quality on my C8 Edge.

=> https://www.cloudyni...7#entry10398311

 

I have done some test between tribahtinov (2 ring design) and bahtinov classic mask => my surprise the result is not exactly the same. There is a focusing delta...

   My process :

     - check the shadow circle concentric => start pre-colimation position (myself software picture check)

     - tribahtinov focusing to adjust colimation =>  result shadow circle is no more concentric (what!???)

     - with bahtinov mask, I turn 120° to check and readjust colimation (=> fine tuninig after tribahtinov) => bahtinov mask give me a better focus. Shadow circle is more concentric but not at the best (I don't understand situation...)

 

One of my personnal analyze is, tribahtinov mask probably tend to diaphram the aperture (divide by 3 the quantity of lite...). I am not expert but probably this have an optical impact.

May be the tribahtinov mask desing have an impact too. I would like to check with a new design in one ring, maybe better. I will try to produce a mask that have the same surface for all diffraction area.

=> https://www.cloudyni...0#entry11348592

 

No more for me...

I expect one day in this forum or elsewhere, a specialist analyze how tribahtinov work and give is result to much better understand such particular situation...smile.gif

Hi Zebul

 

  The Tribatinov’s idea is trivial: it just checks that the path length taken by the light from a distant star, i.e. focus, is the same in all three sectors. That’s all. If the focus is the same in all three sectors, then collimation should be perfect. 
 

  That’s why your observation that a normal Bahtinov focus at the three 120 deg rotated positions is different from the Tribahtinov is so mysterious.

 

  As for the shadow circle, it’s better to check whether the first ring is concentric around the Poisson point like what I had posted in the video above.

 

  Finally, do you lock your mirror and use an external zero image shift focuser or do you move your mirror to do focusing? There is a possibility that it is mirror flop that’s the source of the problem if the moving mirror focusing is used.

 

cytan


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#540 MarMax

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Posted 02 April 2022 - 12:56 PM

I've been using a TriB mask for quite some time now, all as a result of this thread. I would also like to thank buckeyestargazer, a vendor that has provided custom TriB masks for my C11. Even though I've been using a TriB mask for more than a year, I've never really had what I'd call an easy success with collimation and focusing.

 

I've been doing EAA for about six months now so perhaps it has brought me to a better position to best utilize a TriB mask. Regardless, I would like the thank the OP again for this fabulous thread and share my most recent experience with a TriB mask.


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#541 Zebul

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Posted 03 April 2022 - 03:51 AM

Hi Zebul

 

  The Tribatinov’s idea is trivial: it just checks that the path length taken by the light from a distant star, i.e. focus, is the same in all three sectors. That’s all. If the focus is the same in all three sectors, then collimation should be perfect. 
 

  That’s why your observation that a normal Bahtinov focus at the three 120 deg rotated positions is different from the Tribahtinov is so mysterious.

 

  As for the shadow circle, it’s better to check whether the first ring is concentric around the Poisson point like what I had posted in the video above.

 

  Finally, do you lock your mirror and use an external zero image shift focuser or do you move your mirror to do focusing? There is a possibility that it is mirror flop that’s the source of the problem if the moving mirror focusing is used.

 

cytan

Hi Cytan,

 

I agree on all answer point you give.My OAT is a C8 edge, that have a low shift.

To try to check the well position of my secondary miror, I develop a Python concentric circles calculator (inside & outside). It looks like there is no trouble with this point, both side with miror lock.

For sure I use the sérial focusing miror, probably not the best.

I'm not an expert and I need to experience again to understand why I have in the past such result. And I consider Tribathinov as an help not a full solution...

 

My only today deception is not to find a Grabber updated for Tribathinov.

 

Thanks laugh.gif



#542 Procyon

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Posted 05 April 2022 - 09:20 AM

The cover for the original has been prepared.smile.gif

 attachicon.gifTBMCss.png

 

Satoru

Hi all, I bought a Tri-Bahtinov collimating mask from Farpoint but have no mask cover?, nor do I have a 3d printing machine or even know how to 3d print. I will be collimating without a camera. Do I need this mask cover? Can someone sell me one if needed? I'll pay for the shipping cost and your time and everything. Thanks.

 

I bought this one for my 11" SCT:

 

Screenshot 2022-04-05 102124.jpg


Edited by Procyon, 05 April 2022 - 09:20 AM.


#543 cytan299

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Posted 05 April 2022 - 01:54 PM

Hi all, I bought a Tri-Bahtinov collimating mask from Farpoint but have no mask cover?, nor do I have a 3d printing machine or even know how to 3d print. I will be collimating without a camera. Do I need this mask cover? Can someone sell me one if needed? I'll pay for the shipping cost and your time and everything. Thanks.

 

I bought this one for my 11" SCT:

 

attachicon.gifScreenshot 2022-04-05 102124.jpg

The cover can be easily made out of paper/cardboard. Once you are familiar with the spikes, IMO, you don't even need the cover. You just need your fingers to shadow out the spikes to know which slots the spikes originate and which screw to turn.

 

As usual YMMV

 

cytan


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#544 Harshad

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Posted 08 April 2022 - 10:25 AM

Hi all,

I have created a web based mask generator, that can directly emit STL files (for 3D printing).

The tool is available at this link.

Here's a post announcing it on CN.


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#545 TareqPhoto

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Posted 11 April 2022 - 06:31 AM

Hi all,

I ordered the mask from Agena for my coming yet to arrive 10" RC, i hope it can do the good job.

 

I want to know how people can focus on stars using narrowbanding filters, as sometimes it is difficult to see stars to focus on unless you choose very very bright star.

 

I hope someone or some can make more of this kind of tri bahtinov mask for other scopes, i am thinking about something for my Meade 8" F/5 Newt and GSO 6" 6" F/4 Newt.



#546 cytan299

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Posted 11 April 2022 - 07:02 AM

Hi all,

I ordered the mask from Agena for my coming yet to arrive 10" RC, i hope it can do the good job.

 

I want to know how people can focus on stars using narrowbanding filters, as sometimes it is difficult to see stars to focus on unless you choose very very bright star.

 

I hope someone or some can make more of this kind of tri bahtinov mask for other scopes, i am thinking about something for my Meade 8" F/5 Newt and GSO 6" 6" F/4 Newt.

Ummm, personally I wouldn’t use the Tribahtinov for narrow band filters for focusing. It’ll be challenging even with a Bahtinov because it’ll be very dim.

 

Since you’re using NINA, why don’t you just use their autofocus routine? Just expose for a longer time.

 

cytan



#547 TareqPhoto

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Posted 11 April 2022 - 08:41 AM

Ummm, personally I wouldn’t use the Tribahtinov for narrow band filters for focusing. It’ll be challenging even with a Bahtinov because it’ll be very dim.

 

Since you’re using NINA, why don’t you just use their autofocus routine? Just expose for a longer time.

 

cytan

I didn't know that NINA has that feature or can do this, mostly i don't use NINA and instead i use SGPro, but it is time for me to move to NINA really seriously and see all what i can do there.

 

Well, i was thinking about buying something like WO Bahtinov mask that is like transparent, so the focus is easier or it can be not very dim, until i can afford an auto focuser or motor then i can do things semi automated or fully automated.



#548 fewayne

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Posted 29 April 2022 - 11:12 PM

If you can be reasonably consistent on  how much you turn the knob, you can be the motor in the autofocus routine. It will yield a stream of figure-of-merit values (for Ekos, Half-Flux Radius or HFR) numbers that are pretty linear to focus drawtube movement. So you move the knob, see the new number, move the knob, etc.

 

It's not that easy, of course, but it's almost that easy.



#549 calypsob

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Posted 18 July 2022 - 08:39 PM

Hi, I tried collimation using tri-bahtinov mask with newtonian reflector. The tri-bahtinov mask seems useful for the final fine-tuning phase. The method is roughly as follows.

 

First of all, do an initial collimation with another existing collimation tool.

For example, if it is a laser collimator, adjust the secondary mirror so that the optical axis passes through the center of the primary mirror. Next, roughly adjust the primary mirror so that the laser light reflected from the primary mirror returns to the center of the collimator.

 

Next, remove that collimation tool and set up tri-bahtinov mask. At this time, match the orientation of the marks of the mask with the adjustment screws of the primary mirror. It is not those of the secondary mirror.

Then introduce a test star in the center of the field of view and focus. Be sure to evaluate the collimation with the test star always in the center of the field of view. This is the same for SCT.

 

The collimation using the tri-bahtinov mask after that is done only by adjusting the primary mirror. Because tri-bahtinov mask seems difficult to evaluate whether the optical axis passes through the center of the primary mirror.

 

The relationship between the adjustment screws of the primary mirror and the diffraction image looks the same as that of the SCT, so adjust it just like SCT.

 

Incidentally, newtonian has a spider so we can put a mask on it.

 

This is the diffraction image of tri-bahtinov mask with newtonian.

attachicon.gif29663910_941219066036434_1696944629_n.jpg

 

Satoru

Satoru, would it be possible to do this with a newtonian that uses 4 secondary screws instead of 3 ? 



#550 calypsob

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Posted 18 July 2022 - 08:54 PM

Hi Steve,

  You are absolutely correct. It's just more convenient to have all 3 Bahtinov masks etched so that all three directions can be checked at the same time and have consistency.

 

  Recall that I made the TriBahtinov so that the focus should be the same in all three orientations which implies good collimation. So by rotating the normal Bahtinov, it is aso possible to do collimation as well.

 

  At this time, Kirk's program looks at one set of spikes at a time. Perhaps an upgrade will be able to see all three spikes. But let's try this version out first to see whether the upgrade is even necessary.

 

 Obviously, with Kirk's program, you can always use the normal Bahtinov and rotate and check as well.

 

cytan

how would you orient the mask to do this? 




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