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A Tri-Bahtinov mask for SCT collimation and focusing

astrophotography imaging SCT
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#576 Poochpa

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Posted 06 February 2023 - 09:51 AM

Thanks Siouxsie. I previously used a tri-b mask (I'm one of the early posters in this thread) when I had a larger SCT and I had the mask made with pretty tight tolerances for the outer and inner circumferences, so it didn't have the slop in fit that this one does. I'll just try it and see how it works and  compare it to a traditional collimation.

Mike



#577 MarMax

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Posted 06 February 2023 - 11:51 AM

I just received a tri-b mask I purchased for an 8” EdgeHD. The mask sits off center on the corrector plate because the inner diameter of the mask is larger than the secondary's retaining ring.  Will this affect the accuracy of the mask? Thanks. 
Mike

If there is a way to center it with some spacers, maybe felt dots on the inner (secondary) circle, that would be good. Logically it just seems like centering is best if you can do it easily.



#578 Psychlist1972

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Posted 10 March 2023 - 02:42 AM

Ben,

 

I rotated your center image and converted to gray, adjusted contrast/brightness so that my program can detect the lines properly and did analysis. (You can only trust the direction and amount, not the numbers as I don't have necessary infos like your 1:1 image,focal length nor sensor pixel size.) 

 

It shows Red side is tilted to right and Blue side is off center to left. Please refer this with rotated image and your original to find corresponding patterns of yours.

You can actually visually see middle spikes are not in the exact middle in your corner images. This is 'part' of the problem of your corner stars and you can improve it a bit. But I think one of the main reason you're not satisfied on corner stars are from clippings by FR. 

I attached corner crop captured by full frame DSLR when I tried F/4 FR/FF stacking. (You can find full image from the link I put on previous post) You can see blue circled star is reasonably round but red circled ones are not round with flat side facing to outer edges. If your corner stars has clipping enough to missing spikes, I guess your star shape will be similar to this at the corners. Interesting thing is, stock EDGE optics and their plain 0.7x FR shows that much of clipping. I may have to try 0.63x FR/FF tonight to see how much clipping is happening on my conventional SCT.

 

-Jinux

 

ps. My program has too many flaws and precarious to distribute to anybody. Working on it with one volunteer as alpha tester but we hit immediate road block from ASI ASCOM driver is not responding to my expectation where I don't have any ASI cameras. Dang.

Did you ever end up finding testers? If not, I have a bunch of ZWO cameras here, and would be primarily using the mask with the 6200mm Pro, once I print out the mask. Scope is an SCA260.

 

If you're no longer working on it, did you put the source anywhere like Github? Your app looked like it had real promise. Maybe, with some help, it could be turned into a NINA plugin and use all the camera driver integration already present in that?

 

Pete



#579 tahtgeo

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Posted 22 March 2023 - 11:16 AM

Hello, dear CN-ers. 
I would really appreciate some help in using this tool. I've read here that some people got a success creating this mask for refractors, but was not able to do so on my own.
I've read a description of parameters, but still am a bit confused.

Could anyone help me to pick parameters for the specific telescope model, please? 
Would really appreciate that.

The scope is Askar FRA-400 (72mm aperture, 400mm focal length).


 



#580 Fred76

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Posted 13 May 2023 - 12:43 PM

In fact it can be prooved mathematically that for an angle of +/-20° of the mask's angled slits, and an accuracy of 5%, which is what the original design of Pavel Bartinov was, the "Bartinov ratio" is:

 

     F/S = 26.6 * N / m

 

where:

  • N if the aperture (N=F/D) of the telescope,
  • m the order to maximize (usually m=1)
  • F is the focal length of the telescope
  • S is the step of the mask's slits (expressed in the same length-unit than F)
  • D is the telescope reference optical diameter (expressed in the same length-unit than F)

 

For example, classic telescopes (m=1 is OK):

  • f/4, F/S = 26.6*4 = 106 (e.g. N1000/250)
  • f/5, F/S = 26.6*5 = 133 (e.g. N1000/200)
  • f/6.9, F/S = 26.6*6.9 = 183 (e.g. WO ZS AP 81/559)
  • f/11, F/S = 26.6*11 = 293 (e.g. Celestron C14)

 

This is why the recommended range for F/S is usually set between 100 and 300, you now know why !

 

It will then give you the ratio for short focal lenses, but you should better use m=3 otherwise the diffraction grating will be too dense: 

  • f/1.4, F/S = 26.6*1.4/3 = 12.4 (e.g. Sigma 50mm:1.4 Art)
  • f/1.8, F/S = 26.6*1.8/3 = 16.0 (e.g. Sigma 14mm:1.8 Art)
  • f/2.0, F/S = 26.6*2.0/3 = 17.7 (e.g. Samyang 135mm:2)
  • f/2.8, F/S = 26.6*2.8/3 = 24.8 (e.g. Sigma 200mm:2.8 Sports)

In practice, the Bartinov mask diffraction spikes are almost unreadable with lenses of focal length shorter than abt. 25 mm.

 

Fred


Edited by Fred76, 13 May 2023 - 01:00 PM.

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#581 Fred76

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Posted 14 May 2023 - 01:22 PM

I made a typo above, the Bahtinov factor is not 26.6 but 25.7. It doesn't change a lot but... here is the corrected text.

 

In fact it can be prooved mathematically that for an angle of +/-20° of the mask's angled slits, and an accuracy of 5%, which is what the original design of Pavel Bartinov was, the "Bartinov ratio" is:

 

F/S = 25.7 * N / m

 

where:

  • N if the aperture (N=F/D) of the telescope,
  • m the order to maximize (usually m=1)
  • F is the focal length of the telescope
  • S is the step of the mask's slits (expressed in the same length-unit than F)
  • D is the telescope reference optical diameter (expressed in the same length-unit than F)

For example, classic telescopes (m=1 is OK):

 

  • f/4, F/S = 25.7*4 = 103 (e.g. N1000/250)
  • f/5, F/S = 25.7*5 = 129 (e.g. N1000/200)
  • f/6.9, F/S = 25.7*6.9 = 177 (e.g. WO ZS AP 81/559)
  • f/11, F/S = 25.7*11 = 283 (e.g. Celestron C14)

This is why the recommended range for F/S is usually set between 100 and 300, you now know why !

Fred



#582 iandh2010

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Posted 23 May 2023 - 04:49 AM

Hi Guys,

I have only just found this thread after watching a YouTube by Cuiv The Lazy Geek https://www.youtube....h?v=eJ9_14Jvenk . I am having problems critically focusing my 8" LX200 Classic SCT and thought that being able to collimate it as well should cure most of my problems. I use a Bahtinov to focus my 11" NexStar which is very easy and precise,  so I now want to make a Tri-Bahtinov mask for my 8" LX200 Classic.

 

After reading quite a bit of this thread I used the tri-Bahtinov design tool at https://satakagi.git..._symmetric.html  which appeared to be already set-up with the correct details for my scope.

 

Several people have talked about printing it out and pasting onto card, as a trial, but there does not appear to be an option for printing it?

 

Then I want to add a central tube to the mask as in Cuiv's video as that seems to be an ideal solution as it allows the mask to sit flat on the corrector retaining ring whilst making it simple to remove without getting a screwdriver close to my corrector plate in the dark!

 

Can anyone help and advise me on how to print the tri-Bahtinov design out onto paper and then also get it into TinkerCad at the correct size.

 

Many thanks

Ian

 

 



#583 rajones19

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Posted 23 May 2023 - 09:20 PM

I was looking for a tri-bahtinov mask for my Nexstar 6SE, and wound up buying one from Farpoint for about $20. If you're up for the project, I totally get that - but if you just want it done, check here. I used mine only for collimation, but did a fantastic job. No financial interest.



#584 sctbrd

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Posted 23 May 2023 - 11:20 PM

I did a screen shot of the image and adjusted the size to get it printable. It took a few tries to get it how I wanted it.

 

----

Have you tried to import the svg file to Tinkercad? https://www.tinkerca...files-tinkercad



#585 iandh2010

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Posted 24 May 2023 - 04:00 AM

Hi rajones and sctbrd,

 

Thanks for your advice. 

 

I found that I could open the svg file in Google Chrome web browser and print from that. However, I had overlooked that A4 paper is not large enough for and 8" Bahtinov mask......doh!

 

I have attempted to import it into Tinkercad but it does not appear to display at the correct size. It does look good though. So all I will now need to do is learn how to drive Tinkercad and then get it 3D printed.

 

Thanks

Ian



#586 Acapulco Rolf

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Posted 24 May 2023 - 04:15 AM

Hi rajones and sctbrd,

 

Thanks for your advice. 

 

I found that I could open the svg file in Google Chrome web browser and print from that. However, I had overlooked that A4 paper is not large enough for and 8" Bahtinov mask......doh!

 

I have attempted to import it into Tinkercad but it does not appear to display at the correct size. It does look good though. So all I will now need to do is learn how to drive Tinkercad and then get it 3D printed.

 

Thanks

Ian

 

ICYMI, see here: https://www.cloudyni...-for-3d-prints/



#587 Jinux

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Posted 05 June 2023 - 04:33 PM

Hi rajones and sctbrd,

 

Thanks for your advice. 

 

I found that I could open the svg file in Google Chrome web browser and print from that. However, I had overlooked that A4 paper is not large enough for and 8" Bahtinov mask......doh!

 

I have attempted to import it into Tinkercad but it does not appear to display at the correct size. It does look good though. So all I will now need to do is learn how to drive Tinkercad and then get it 3D printed.

 

Thanks

Ian

You may want to read "fine print" on the generator page by Satoru.

 

----------

SVG Source
Download using upper buton or copy following source text and save as
[maskName].svg using text editor, and open it by inkscape.

If you want a DXF file, you can export the DXF with Inkscape's "Save As". Provided, to maintain scale,
you should change the base unit to "mm" or "in" in the options dialog that appears.

----------

Import this DXF into Fusion 360, then you'll get exact size in CAD.



#588 iandh2010

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Posted 05 June 2023 - 05:21 PM

Hi Jinux,

Many thanks for that. I don't have Inkscape or Fusion 360 but I did learn how to drive TinkerCAD to do everything that I needed togenerate a.stl file.

 

I then gave the output .stl file from TinkerCAD, after resizing it to  the correct outer diameter and adding a central tube, to a friend who has a 3D printer just large enough for my 8" mask.

 

He then had to "Slice" the file for his printer which appears to have gone wrong so what came out was a flat tri-Bahtinov only 1mm thick and without the central tube/handle.

 

He has now "sliced" the same .stl file again and he says that it has over doubled the estimated print time so hopefully 2mm thick. He is going to see whether there any updates /bug fixes for his slicing software.

 

In the meantime, I am going to test the 1mm thick, flat mask, before going for a re-print.

 

I did use the APT Collimation Aid on my 8" LX200 a couple of nights ago while pointing at Vega. It looked absolutely spot on collimation. So it will be interestig to see what results I get from the tri-Bahtinov mask, as I think that I am seeinng coma abberation on stars close to M57 and fairly central in my FOV. 

 

Regards

Ian



#589 cpl43uk

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Posted 28 June 2023 - 06:07 PM

Could i ask a late question to the group. Is it better to max out the resolution when collimating? I have 8”SCT with focal reducer but i also go native with a 2.5 Telemate for lunar imaging.  Should i use the full resolution of my scope or does it make no real difference to the tri-B?

 

thanks


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#590 iandh2010

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Posted 29 June 2023 - 02:53 AM

Could i ask a late question to the group. Is it better to max out the resolution when collimating? I have 8”SCT with focal reducer but i also go native with a 2.5 Telemate for lunar imaging.  Should i use the full resolution of my scope or does it make no real difference to the tri-B?

 

thanks

Hi cpl43uk,

I have now had my tri-Bahtinov mask 3D printed by a friend but have not yet had an  opportunity to use it!

 

However, from my understanding, the tri-B design depends on the aperture and focal length of your telescope. 

 

My 8" SCT is an LX200 Classic F10 which has a native FL of 2000mm. If your NexStar 8" is similar, in native mode it will have a focal length of 2000mm but when using your powermate,  it will increase the FL to 5000mm. Your focal reducer will have the opposite effect and reduce  the FL by whatever factor the reducer has on it, usually F6.3. This will therefore result in an effective focal legth of 1260mm on your NexStar..

 

In fact, when using APT to plate solve on my Meade LX200 Classic, once it suceeds in plate solving, it tells me that my scope with camera has a FL of 2140mm, presumably due to the back focus required due to my Strarlight Xpress AO unit which puts the camera some 120mm behind the focal back of the OTA. So when I desinged my T-B I specified an FL of 2140mm.

 

However, I have been informed that FL does not make a huge difference to  the T-B design. Unfortunately I have not tested mine yet.

 

I am no expert, so it would be good to get some other feedback from guys who undertsand the physics of what goes on!

 

Regards

Ian


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#591 Jinux

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Posted 03 July 2023 - 11:52 AM

Could i ask a late question to the group. Is it better to max out the resolution when collimating? I have 8”SCT with focal reducer but i also go native with a 2.5 Telemate for lunar imaging.  Should i use the full resolution of my scope or does it make no real difference to the tri-B?

 

thanks

It would make sense to me that first step is to collimate main scope with least amount of accessory first - with SCT, native F/10 + visual back + EP, then add other optical trains and handle their tilt etc, though it would be a huge pain to go back and forth between EP and front collimation screws especially when you don't know which knob will move which pattern's middle spike to which direction. 

If your main usage of the scope is imaging, collimating whole image train may work, in the end the goal is to reduce aberration at the image and having camera feedback right next to you while adjusting collimation screw is so much easier. But if your main optics is not collimated and there is camera tilt, you might not be able to fix both using secondary adjust knob alone. So the basic method mentioned above is the usual steps. Also, depending on the source of tilt in imaging train, rotation of it can throw off the previous collimation as well. So setup repeatability would be an issue, or you have to adjust it on every imaging sessions.

My 2 cents of experiences.



#592 iandh2010

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Posted 04 July 2023 - 09:26 AM

If your main usage of the scope is imaging, collimating whole image train may work, in the end the goal is to reduce aberration at the image and having camera feedback right next to you while adjusting collimation screw is so much easier. But if your main optics is not collimated and there is camera tilt, you might not be able to fix both using secondary adjust knob alone. So the basic method mentioned above is the usual steps. Also, depending on the source of tilt in imaging train, rotation of it can throw off the previous collimation as well. So setup repeatability would be an issue, or you have to adjust it on every imaging sessions.

My 2 cents of experiences.

Hi Jinux,

Which basic method were you referring to, please.

 

I know that on my 8" LX200 Classic the whole image train shows a very good defocussed star image that I have checked using the APT Collimation Aid but I still see what appears to be coma distortion when I zoom in very close on my images. I have a 2" nose piece on my Starlight Xpress AO unit which I fully insert into the Baader ClickLock that is screwed directly onto the visual back. My camera is then screwed onto the back of AO unit. However, even when the ClickLock is done up tight onto the AO unit nosepiece I can still detect  a very small amount of flexure, which must result in my camera chip being slightly out of collimation......

 

I totally agree that having a camera to view the collimation and focus in more or less real time is very helpful. I guess  that this will be even more important when trying to collimate and focus with a Tri-B mask!

 

Regards

Ian



#593 archer1960

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Posted 04 July 2023 - 11:50 AM

Hi Jinux,

Which basic method were you referring to, please.

 

I know that on my 8" LX200 Classic the whole image train shows a very good defocussed star image that I have checked using the APT Collimation Aid but I still see what appears to be coma distortion when I zoom in very close on my images. I have a 2" nose piece on my Starlight Xpress AO unit which I fully insert into the Baader ClickLock that is screwed directly onto the visual back. My camera is then screwed onto the back of AO unit. However, even when the ClickLock is done up tight onto the AO unit nosepiece I can still detect  a very small amount of flexure, which must result in my camera chip being slightly out of collimation......

 

I totally agree that having a camera to view the collimation and focus in more or less real time is very helpful. I guess  that this will be even more important when trying to collimate and focus with a Tri-B mask!

 

Regards

Ian

That small amount of flexure is where a camera tilt adapter comes in.



#594 iandh2010

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Posted 04 July 2023 - 02:15 PM

That small amount of flexure is where a camera tilt adapter comes in.

Aha! So get the OTA collimated with the focal back, using an eyepiece and then bolt a tilt adapter to my ASI294MC and then collimate the camera/imaging train using the tilt plate. Right?

 

If so, it sounds like it is probably something one only wants to do once in a while, and not every night!

 

Regards

Ian


Edited by iandh2010, 04 July 2023 - 02:15 PM.

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#595 13rannon

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Posted 21 January 2024 - 05:49 PM

Hello!

 

If I use the mask creator for my edge11 hyperstar, and make it slightly bigger to go over the dew shield is it ok to leave the hole in the middle of the mask, or would I need to fill that in?

 

Thanks!



#596 Zuul

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Posted 30 January 2024 - 10:42 AM

Hello!

 

If I use the mask creator for my edge11 hyperstar, and make it slightly bigger to go over the dew shield is it ok to leave the hole in the middle of the mask, or would I need to fill that in?

 

Thanks!

Take this with a grain of salt, as I have no practical experience with that set-up and limited theoretical training...

 

In practice, I suspect it would make no observable difference.  From the standpoint of better mechanical structure for the mask, and reducing spurious reflections, it should be beneficial to fill it in.


Edited by Zuul, 30 January 2024 - 10:43 AM.



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