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A Tri-Bahtinov mask for SCT collimation and focusing

astrophotography imaging SCT
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#451 Kaydubbed

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Posted 01 April 2021 - 01:52 PM

Thanks. I'll check it out. Any advantage to the TriB vs a Duncan mask besides the bonus of being able to focus at the same time? 



#452 Broscheanu

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Posted 01 April 2021 - 02:31 PM

I'll buy one for my Edge 11 SCT if your friend wants to print one!

 

I just don't have the resources [or energy] to hunt down someone local with a 3D printer and go through that trial and error if they aren't familiar with these masks. 

Tri-Bahtinov masks are commercially available at Farpointastro.com and they have one for your C11. I ordered mine there


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#453 WB91

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Posted 01 April 2021 - 03:30 PM

Does anyone have an windows exe of the TriBahtinov Collimator software they can share?  I cannot seem to find one available online other than source code.     Any help would greatly be appreciated as I am doing some collimation testing tonight.  Thanks!!

 

Bill



#454 MarMax

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Posted 01 April 2021 - 05:59 PM

Thanks. I'll check it out. Any advantage to the TriB vs a Duncan mask besides the bonus of being able to focus at the same time? 

I like both the Duncan and the Tri-B but I think the Tri-B is a bit easier to interpret and may allow for a more precise collimation. I can't say for sure because I collimated with a Duncan and it's spot-on with the Tri-B so have not needed to adjust collimation yet.

 

It's also rare that I have the seeing conditions to really do it right so I'll just have to wait.



#455 rohanpai

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Posted 07 April 2021 - 06:42 PM

I generated a Tri-B mask using the pattern generator link for my Skywatcher 200PDS and printed it 1:1 scale. The diameter of the printed version is smaller than the OTA.

 

I gave the following inputs -

FL: 1000mm

Outer dia: 217mm

Internal dia: 34mm

 

in W:8mm

out W: 8.5mm

 

What am I missing? I also tried saving the html and opening in Inkspace. Same result! 



#456 Psittacula

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Posted 08 April 2021 - 08:34 AM

This is the result I just tried. The browser I used is chrome on windows10.

tmbgn.png

 

And, this is the result measured with Inkscape's ruler tool, and the dimensions seem to be as set.

tbmis.png

 

It might be possible that the Draw Tri-Bahtinov Mask button was not pressed after setting the parameters.


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#457 rohanpai

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Posted 08 April 2021 - 09:41 PM

Thanks for checking. I will give this a try again now.



#458 Zebul

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Posted 25 April 2021 - 03:57 PM

Hi, is there anyone, following this forum, able to provide a theoretical explanation of focus error calculation with bahtinov mask. I have read in the Russian forum the Pavel Bahtinov message which explains the theory but I cannot reproduce the calculation. I succed to calculate the distance between central line and x lines generat by bahtinov mask diffraction. I would like to compare 'my 'how' calcul' and the result with bahtinov grabber... I will appreciate calcul 'demonstration'.

Datas I have are, D=280mm, Focal 2800mm, pixel 4.16µm and I have 36.7µm distance between the ref line and x diffraction. I use tribahtiniov mask second generation, if it is usefull.

Thanks for any answer



#459 cytan299

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Posted 25 April 2021 - 04:38 PM

Hi, is there anyone, following this forum, able to provide a theoretical explanation of focus error calculation with bahtinov mask. I have read in the Russian forum the Pavel Bahtinov message which explains the theory but I cannot reproduce the calculation. I succed to calculate the distance between central line and x lines generat by bahtinov mask diffraction. I would like to compare 'my 'how' calcul' and the result with bahtinov grabber... I will appreciate calcul 'demonstration'.

Datas I have are, D=280mm, Focal 2800mm, pixel 4.16µm and I have 36.7µm distance between the ref line and x diffraction. I use tribahtiniov mask second generation, if it is usefull.

Thanks for any answer

Not me :) I just use it.

 

cytan

 

P.S. It's one of those things that it'll be interesting to derive, but I have a lot of other calculations to do and this is not that high on my priority list.



#460 Edmond S.

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Posted 01 May 2021 - 12:41 AM

Not sure if someone has posted here before ( as I come very late to this thread ). Anyway, I just used the TriB mask to collimate my 8RC.

 

Basically, I have used the DSI method for quite sometime. So to for the TriB mask, I use it to fix the on-axis star to make all 6 spikes even (adjusting/tuning the primary mirror). Then run the iteration between the 2nd to balance the stars at the corners, and the TriB mask for on axis coma. 

 

It really is better at telling me how far am I from fixing the on axis coma than before.  Without it, I have no way to tell if I am on it or not.

 

The fixing the off axis via the 2nd is somewhat getting easier as focus stars (say a globular cluster) will surely tell you what direction of astig is pointing at and can fix accordingly. For fixing the secondary, the TriB mask may help but I haven't investigated yet. 


Edited by Edmond S., 01 May 2021 - 09:13 AM.

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#461 nateman_doo

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Posted 12 May 2021 - 01:04 PM

Not sure if someone has posted here before ( as I come very late to this thread ). Anyway, I just used the TriB mask to collimate my 8RC.

 

Basically, I have used the DSI method for quite sometime. So to for the TriB mask, I use it to fix the on-axis star to make all 6 spikes even (adjusting/tuning the primary mirror). Then run the iteration between the 2nd to balance the stars at the corners, and the TriB mask for on axis coma. 

 

It really is better at telling me how far am I from fixing the on axis coma than before.  Without it, I have no way to tell if I am on it or not.

 

The fixing the off axis via the 2nd is somewhat getting easier as focus stars (say a globular cluster) will surely tell you what direction of astig is pointing at and can fix accordingly. For fixing the secondary, the TriB mask may help but I haven't investigated yet. 

This is currently on my to-do list tonight.  Perhaps do what you said by centering a star, and moving the primary around until all 6 spikes are even, then going back to the DSI to balance with the secondary?  



#462 fetoma

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Posted 12 May 2021 - 05:46 PM

Does anyone have an windows exe of the TriBahtinov Collimator software they can share?  I cannot seem to find one available online other than source code.     Any help would greatly be appreciated as I am doing some collimation testing tonight.  Thanks!!

 

Bill

Is this it Bill→ http://www.njnoordhoek.com/?p=376



#463 nateman_doo

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Posted 12 May 2021 - 09:10 PM

It actually worked really well.

 

I centered Arcturus, and got (after 20 minutes of adjusting on the primary) even spikes.  Then took the mask off and tight/loosen the secondary screws to get a balanced image.  Then put the TriB back on, and got the spikes even, took it off and adjusted the secondary again.  Took perhaps close to an hour ( cause I wanted to take the time and do it right) and It is MUCH better.

 

Here is the one that I used:

4jbStCs.jpg

 

cqGKXn3.jpg

 

Cut on my CNC out of clear lexan, then bead blasted for an opaque finish.  Worked like a champ.  


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#464 meegja

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Posted 13 May 2021 - 02:57 AM

He does mean this: https://www.cloudyni...4#entry10044031

I too am looking for that but it seems to be difficult to get that right.



#465 nateman_doo

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Posted 13 May 2021 - 01:26 PM

Not sure if someone has posted here before ( as I come very late to this thread ). Anyway, I just used the TriB mask to collimate my 8RC.

 

Basically, I have used the DSI method for quite sometime. So to for the TriB mask, I use it to fix the on-axis star to make all 6 spikes even (adjusting/tuning the primary mirror). Then run the iteration between the 2nd to balance the stars at the corners, and the TriB mask for on axis coma. 

 

It really is better at telling me how far am I from fixing the on axis coma than before.  Without it, I have no way to tell if I am on it or not.

 

The fixing the off axis via the 2nd is somewhat getting easier as focus stars (say a globular cluster) will surely tell you what direction of astig is pointing at and can fix accordingly. For fixing the secondary, the TriB mask may help but I haven't investigated yet. 

Got 2 very different but arguably decent collimation images from Sharp cap.

Attached Thumbnails

  • Collimation.JPG


#466 nateman_doo

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Posted 13 May 2021 - 01:29 PM

and this:

Collimation.PNG



#467 meegja

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Posted 13 May 2021 - 03:09 PM

Got 2 very different but arguably decent collimation images from Sharp cap.

The collimation feature still is experimental. Plus it's mainly there for Newton telescopes. Robin is almost finished with version 4 but also in there it's still in experimental phase.



#468 Borodog

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Posted 13 May 2021 - 09:27 PM

I have a question about using a regular Bahtinov mask (non-tri that is) for collimation. It seems like I should be able to put the mask on, say aligned with my lower two collimation knobs, focus on an on-axis star, and then rotate my mask 90° degrees, and if I am well collimated, I should still be in focus. It seems I should be able to rotate the mask to any angle and still be in focus. But if my collimation is out, I will see focus shift as I rotate the mask.

 

Am I thinking straight here? Or am I missing something that the tri mask accomplishes other than convenience?



#469 Jinux

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Posted 14 May 2021 - 11:26 AM

I have a question about using a regular Bahtinov mask (non-tri that is) for collimation. It seems like I should be able to put the mask on, say aligned with my lower two collimation knobs, focus on an on-axis star, and then rotate my mask 90° degrees, and if I am well collimated, I should still be in focus. It seems I should be able to rotate the mask to any angle and still be in focus. But if my collimation is out, I will see focus shift as I rotate the mask.

 

Am I thinking straight here? Or am I missing something that the tri mask accomplishes other than convenience?

I think your reasoning is correct from the perspective of 'tri' mask. The only difference is that 'tri' mask can detect other type of collimation error as well, which I can't articulate as I'm ignorant of optics. 

One of the 'beta' tester of my collimation program told me as below. Note his mention about 'Magenta' line below.

 

 

 

I was never able to get a perfect collimation where the results showed in focus for all 4 lines and a low focus error .(the Magenta middle line was the problem - no matter what I adjusted, it always gave me the same result).

The best I got was 2 lines in focus with 3 lines showing a focus error of about a pixel.

I then did a normal collimation without the mask, then put the mask back on and re tested. This gave me all lines within critical focus, with errors of 1 pixel or less. - including the troublesome Magenta line. Thus it confirmed good collimation.

I also found that rotating the mask even a centimeter or so gave differing results.

Normal B-mask with 120deg rotation would yield the same result if done one by one properly, but won't give you all 4 axis (3 masks and collection of middle one) reports at the same time.

Thing is, normal 3 screw collimation will tilt mirror in a way that the other two also get affected as noted by John Hayes on this mask approach earlier, so it's iterative process and keep rotating mask would be quite of hassle to run. 

My process for this is to run 'auto focus' (or a focus to one group of pattern focused), then leaving the best focused group (reference) alone and adjust the other two to get all three focused. If reference group is deviated during adjustment, I put it back using focus not collimation knob. Imagine this work process with keep rotating masks, on top of already confusing which knob correspond to which group of lines and which direction of turn makes middle line to which directions... it would easily become nightmare.

 

Note.

For the people who is waiting for the analysis SW, I ran a couple of beta testing with volunteers including myself field testing, the SW is 'quite' difficult to use on the field while it worked very well under indoor environment. There are multiple of issues and one of them is seeing. It's turbulent to shake lines and SW is faltering to track them. If I use Hough Line Transform to detect lines instead of original Bhatinov grabber's cryptic one, I guess it would improve quite a lot, but I'm lazy bum and don't know when would it happen.  

I 'almost' declared it's useless on the field and adjust collimation by eye, which actually works quite well. So, I stopped development of the SW. 

So, please don't waste your time looking for the SW which is not available anywhere on the net as it's not as useful as I hoped. Sorry.

But Tri-B mask certainly works VERY well. I don't know why this is not as widely used as B-mask. 

 

-Jinux


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#470 Borodog

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Posted 14 May 2021 - 04:42 PM

Thank you, Jinux, for the clear explanation.

 

Also I still haven't had a chance to use the camera adapter you sent me yet. ;O)



#471 Psittacula

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Posted 15 May 2021 - 11:11 AM

Hi,

There seems to be a difference in the ability to detect misalignment or out of collimation between the result of simply rotating the regular Bahtinov Mask by 120 degrees and the Tri-Bahtinov Mask.

Both the original Tri-Bahtinov Mask pattern by cytan and the modified pattern are designed to check the focus on an optical system that is divided into three non-coaxial parts as shown in the figure below. (The blue part, the red part, and the green part)

On the other hand, if you simply rotate the normal Bahtinov Mask by 120 degrees, it is still coaxial.

 

TBM.png


Satoru


Edited by Psittacula, 15 May 2021 - 11:16 AM.


#472 Nowthatsorignal

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Posted 15 May 2021 - 02:41 PM

Hey,

I have a Meade 14in lx200gps and I am trying to make one of these and I am having trouble getting a good SVG file that does not clip the edge of the circle.

2021-05-15 (6).png

This is what my output looks like. I looked into the reason for this and to me it appears that the coding assumes that no one would ever use this for greater than a 13.8888in diameter telescope. 

 

I looked into the HTML and I was able to find the values for the svg viewbox and increase them. It appears that the scaling of the drawing adjusted to 16.666in with this change

2021-05-15 (8).png

 

Is there anything else I need to worry about?

I have skimmed this thread and read at least 10 percent so far so it is possible that this question already came up so forgive me if it has.

 

Also if any one has any input on the parameters I am using please let me know if there are better settings to use. From what I have read so far it sounds like there is not a substantial amount of difference between the the results for most of the settings


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#473 Psittacula

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Posted 15 May 2021 - 10:05 PM

Hey,

I have a Meade 14in lx200gps and I am trying to make one of these and I am having trouble getting a good SVG file that does not clip the edge of the circle.

 

Hi,
Thank you very much for reporting the issues. I've just improved it.

 

Satoru



#474 Nowthatsorignal

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Posted 16 May 2021 - 09:30 AM

Thats awesome! Thank you, I wasn't expecting it to get fixed so quickly.
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#475 Peter_S

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Posted 24 August 2021 - 06:45 AM

Hi,
...I ordered 2 acrylic TriBahtinov masks (laser cut) for my C8 EHD, one in black and one in transparent / translucent.
I´d like to add an engraving on the translucent one like on those masks from Williams Optics.
I am not able to write a program for that. Is there anyone who can write an "engraving generator"? that would be great...

cs Peter

 

TriBahtinov_transparent.jpeg


Edited by Peter_S, 24 August 2021 - 07:34 AM.



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