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A Tri-Bahtinov mask for SCT collimation and focusing

astrophotography imaging SCT
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#1 cytan299

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Posted 07 May 2016 - 08:52 PM

Hi,

  I made a Tri-Bahtinov mask that I think would be useful for both SCT collimation and focusing. My mask is shown here:

IMG_0086.jpg

 

The diffraction pattern that I see when I first point at Polaris is shown below:

as_found.png

 

It is clear that 2 orientations are in focus while the third (indicated with red arrows) is not.

After collimation (still not quite perfect), I get all three orientations in focus

collimated.png

 

I built this mask because my judgement with the Airy disk method is not as objective as I would like it. I think by using my favorite tool, a Bahtinov mask, modified for collimation would make collimation much less subjective.

 

My full design and writeup can be found here:

https://github.com/c...299/tribahtinov

 

I hope this is useful for the community.

 

cytan


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#2 noisejammer

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Posted 07 May 2016 - 09:39 PM

An Interesting idea, thank you for posting it.

Can you estimate its sensitivity to miscollimation?

 

Have you considered (or better tested) what happens when the mask isn't perfectly flat on the corrector?

 

Could you use it to test how square a CCD is?



#3 cytan299

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Posted 07 May 2016 - 09:59 PM

An Interesting idea, thank you for posting it.

Can you estimate its sensitivity to miscollimation?

 

Have you considered (or better tested) what happens when the mask isn't perfectly flat on the corrector?

 

Could you use it to test how square a CCD is?

 

All very good questions :) I'm actually not sure how to answer them. Is there a standard measure for miscollimation? I can always come up with one, but it'll probably not be something that people here use.

 

Are there any references for me to read up on to answer your questions? The answers to your questions might actually be interesting for me to write up for Sky and Telescope :)

 

FYI, this is the first time I've used it after I made it. The weather here is atrocious right now -- raining all the time. It was only clear last night, and only just enough before the clouds rolled in for my test.

 

cytan



#4 UAwildcat

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Posted 07 May 2016 - 11:52 PM

Have you tried comparing this method to the Duncan mask method? It would be interesting to hear how they compare. Specifically, it would be great to have an all-in-one tool like this for both collimation and focusing. Duncan mask is great for collimation, which Bahtinov is great for focusing. On the other hand, if this only combination tool does a fair job at both objectives, I don't think it would be as useful.



#5 Steve OK

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Posted 08 May 2016 - 06:35 AM

Very intersting idea.  It has me wondering about using a standard B-mask, placed in three successive orintations.  Perhaps with the axis of symmetry of the mask aligned with each of the collimating screws in succession?  Would that show an out-of-collimation condition, and could you determine which collimation screws need tweaking from that?

 

Steve



#6 austin.grant

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Posted 08 May 2016 - 06:44 AM

Gold Focus already offers a similar product aimed at being used for colimation and focus. He has lots of documentation on the website, so maybe check it out and see how yours compares.

Edited by austin.grant, 08 May 2016 - 06:44 AM.

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#7 cytan299

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Posted 08 May 2016 - 06:55 AM

Have you tried comparing this method to the Duncan mask method? It would be interesting to hear how they compare. Specifically, it would be great to have an all-in-one tool like this for both collimation and focusing. Duncan mask is great for collimation, which Bahtinov is great for focusing. On the other hand, if this only combination tool does a fair job at both objectives, I don't think it would be as useful.

 

I had tried both the Duncan mask and the Hartmann mask for collimation first:

IMG_0096.jpg

 

I did not find the Duncan mask to be that accurate for collimation because I had a hard time judging whether I got the "Y" because the "Y" was (a) rather dim (b) rather small even at 400x. I think it got me close because I had used the Duncan mask last and the "as found" with my Tri-Bahtinov had 2 directions in focus while the third was not as shown in my previous post.

 

I think the tri-Bahtinov works as well as the normal Bahtinov for focusing but with the advantage of showing whether you are collimated or not. This is the result I had from Maskulator:

 

theory.png

 

My hypothesis is that with the normal Bahtinov, you only get perfect focus in one orientation while in other orientations you might not be if you are not in collimation. One test that you can do (I'll do it myself, but looking at the weather forecast, it's either going to be cloudy or raining for the next 10 days here!), is to focus in one Bahtinov orientation and then rotate it by 90 deg to see if your focus is still perfect. My guess is that you won't be unless you are collimated.

 

Any other suggestions for tests or evaluations are welcome.

 

cytan



#8 cytan299

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Posted 08 May 2016 - 07:02 AM

Very intersting idea.  It has me wondering about using a standard B-mask, placed in three successive orintations.  Perhaps with the axis of symmetry of the mask aligned with each of the collimating screws in succession?  Would that show an out-of-collimation condition, and could you determine which collimation screws need tweaking from that?

 

Steve

 

Yes, I have! I started out with just having three standard Bahtinov masks placed in successive orientations. The result is not so good. This is the mask with 3 Bahtinov masks placed in successive orientations:

lx200.jpg

 

And this is the diffraction pattern calculated with Maskulator:

 

bahtinov_lx200.png

 

There are too many spikes to be useful.

 

cytan


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#9 cytan299

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Posted 08 May 2016 - 07:07 AM

Gold Focus already offers a similar product aimed at being used for colimation and focus. He has lots of documentation on the website, so maybe check it out and see how yours compares.

 

Interesting ... but I see that Gold Focus costs real money :) They definitely have a nice suite of tests. I'm not sure I'm in the position to compare their product with mine.

 

However, the Tri-Bahtinov mask is *free* (as in free beer :) ) and open source, so people can easily try it out to see if it works for them. 

 

I'd really like some feedback and tests that I can easily do to show that it works. 

 

The next thing I'd like to do is to take Airy diffraction pics with collimation a little bit off and not off to see if the Tri-Bahtinov is as good.

 

cytan


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#10 James Cunningham

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Posted 08 May 2016 - 07:09 AM

Are you going to make it commercially available?
Jim

#11 cytan299

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Posted 08 May 2016 - 07:13 AM

Are you going to make it commercially available?
Jim

 

It's open source. See

 

https://github.com/c...299/tribahtinov

 

It has the source svg file for sending the mask to Ponoko for laser cutting. Unfortunately, I only have the 8" LX200 mask. The svg file can be edited and rescaled for other sizes with for example, Inkscape . I can certainly write a python script to generate other sizes on the web but I don't have a host for it.

 

cytan


Edited by cytan299, 08 May 2016 - 07:18 AM.


#12 Steve OK

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Posted 08 May 2016 - 09:12 AM

 

Very intersting idea.  It has me wondering about using a standard B-mask, placed in three successive orintations.  Perhaps with the axis of symmetry of the mask aligned with each of the collimating screws in succession?  Would that show an out-of-collimation condition, and could you determine which collimation screws need tweaking from that?

 

Steve

 

Yes, I have! I started out with just having three standard Bahtinov masks placed in successive orientations. The result is not so good. This is the mask with 3 Bahtinov masks placed in successive orientations:

attachicon.giflx200.jpg

 

And this is the diffraction pattern calculated with Maskulator:

 

attachicon.gifbahtinov_lx200.png

 

There are too many spikes to be useful.

 

cytan

 

I was thinking more of using a single mask rotated 120° for each trial, rather than three used simultaneously.  I'll have to give it a try.  Then I may be breaking out the X-acto knife to cut out a Cytan-Bahtinov mask!  Sounds fancy!

 

Steve



#13 cytan299

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Posted 08 May 2016 - 11:59 AM

 

I was thinking more of using a single mask rotated 120° for each trial, rather than three used simultaneously.  I'll have to give it a try.  Then I may be breaking out the X-acto knife to cut out a Cytan-Bahtinov mask!  Sounds fancy!

 

 

Steve

 

 

Hi Steve,

   Please do perform the trial with the 120 degree rotation with the Bahtinov mask and post here. I can't do this for at least the next 10 days! Weather is atrocious right now, it's either rain or cloudy. My hypothesis is that focus can only be achieved at one orientation of the Bahtinov mask if there is bad collimation. It'll be interesting to see if this hypothesis is true.

 

   And if you need any other graphics format for the Tri-Bahtinov mask (Cytan-Bahtinov does have a nice ring to it :) ) in order for you to make one, please email me or post here. I will try to help you if I can. Personally, I think the Bahtinov mask is a must have and if this works, the Tri-Bahtinov is even more useful.

 

cytan



#14 Wford

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Posted 16 February 2017 - 08:07 AM

thanks for this tool.I tried to send the svg file to a laser cutter.They now request a vector image.Can anybody advise how do I resolve this, my understanding as that the svg file was a vector image



#15 cytan299

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Posted 16 February 2017 - 12:31 PM

Hi Wayne,

   If your scope is similar to an 8" LX200, I would recommend that you use the file in ponoko/tribahtinov.svg and modifying it to your needs. This is definitely a vector file because I had used to at Ponoko for laser cutting.

 

   Unfortunately, my python program does not generate a vector file at this time :(

 

cytan



#16 Wford

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Posted 17 February 2017 - 12:48 AM

Thank you for the reply I will look into that



#17 Psittacula

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Posted 28 May 2017 - 04:31 AM

Hi,

I ported Cytan's Tri-Bahtinov mask drawing generator.py to webApps. It runs on the web browsers and generates mask drawings by entering some parameters into the form. Of course, you can get the source of masks in SVG format that supports Inkscape. The URL is below.

 

http://svg2.mbsrv.ne...i-Bahtinov.html

 

Satoru


Edited by Psittacula, 28 May 2017 - 05:33 AM.

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#18 cytan299

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Posted 30 May 2017 - 09:54 PM

Hi,

I ported Cytan's Tri-Bahtinov mask drawing generator.py to webApps. It runs on the web browsers and generates mask drawings by entering some parameters into the form. Of course, you can get the source of masks in SVG format that supports Inkscape. The URL is below.

 

http://svg2.mbsrv.ne...i-Bahtinov.html

 

Satoru

Hi Satoru,

   Great job! Hopefully, this will make the tri-bahtinov mask more popular :)

 

cytan


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#19 Psittacula

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Posted 02 June 2017 - 08:45 PM

Thanks!

Last night I first tried the effect of Tri-Bahtinov mask using live view of CMOS USB camera. I realized that SCT's collimation can be done considerably easily and objectively.

 

By the way, when I tried to make a mask for larger C11 with relatively thin paper, I was concerned that the mechanical strength of the mask would be weakened.
So I added options to webApps to insert a stem at the boundary of the upper right and lower right slots of the mask. ("Build boundary stem" check box) Does this have a serious effect on mask performance?

Satoru
 



#20 cytan299

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Posted 03 June 2017 - 07:32 AM

Hi Satoru
  I can run the mask through Maskulator to see it's effects later today. I spent last night imaging so I'm a still in bed right now :) My gut feeling is that the extra support stem will make one of the spikes dimmer.

 

 

cytan

 

P.S. I will add a link to your site on my GitHub repository today as well.

 

Thanks!

Last night I first tried the effect of Tri-Bahtinov mask using live view of CMOS USB camera. I realized that SCT's collimation can be done considerably easily and objectively.

 

By the way, when I tried to make a mask for larger C11 with relatively thin paper, I was concerned that the mechanical strength of the mask would be weakened.
So I added options to webApps to insert a stem at the boundary of the upper right and lower right slots of the mask. ("Build boundary stem" check box) Does this have a serious effect on mask performance?

Satoru
 


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#21 Poochpa

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Posted 03 June 2017 - 05:20 PM

I generated a Tri-Bahtinov mask for my 9.25" EdgeHD using Satoru's mask drawing generator with the following parameters: 260mm for the outer diameter, 90mm for the inner diameter and FL of 2321mm. I then saved it as a svg file and took a look at it in Inkscape. Does it look correct? Where should I get it cut and what material and thickness to ask for? Thanks.

Mike

(File attached as pdf because I can't attach svg to a CN post)
Attached File  TribahtinovMask.pdf   9.77KB   295 downloads



#22 cytan299

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Posted 03 June 2017 - 06:57 PM

I generated a Tri-Bahtinov mask for my 9.25" EdgeHD using Satoru's mask drawing generator with the following parameters: 260mm for the outer diameter, 90mm for the inner diameter and FL of 2321mm. I then saved it as a svg file and took a look at it in Inkscape. Does it look correct? Where should I get it cut and what material and thickness to ask for? Thanks.

Mike

(File attached as pdf because I can't attach svg to a CN post)
attachicon.gifTribahtinovMask.pdf

Hi Poochpa,

   Just cut out the slots that I have colored in red.

 

bahtinov.png

 

There are extra lines at the transition between the two angled slots in the file generated by Satoru because it's based on my python script. This is where my programming limits are shown. 

 

   The tribahtinov can be made by printing out the mask to form a template and then pasting it onto cardboard that has some rigidity. You then cut the slots out with the template pasted onto it. I actually laser cut mine, but the method I described works very well too.

 

cytan



#23 cytan299

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Posted 04 June 2017 - 01:24 PM

Thanks!

Last night I first tried the effect of Tri-Bahtinov mask using live view of CMOS USB camera. I realized that SCT's collimation can be done considerably easily and objectively.

 

By the way, when I tried to make a mask for larger C11 with relatively thin paper, I was concerned that the mechanical strength of the mask would be weakened.
So I added options to webApps to insert a stem at the boundary of the upper right and lower right slots of the mask. ("Build boundary stem" check box) Does this have a serious effect on mask performance?

Satoru
 

Hi Satoru,

   I ran through the with/without the boundary stem mask using the default mask parameters that is on your website. I don't see too much of a difference between the two results:

 

combined.jpg

 

So the addition of the boundary stem to strengthen the mask should be ok.

 

cytan

 

P.S. I've also updated my github repository and wiki with links to your webapp.


Edited by cytan299, 04 June 2017 - 01:39 PM.

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#24 Psittacula

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Posted 05 June 2017 - 08:43 AM

 

Hi Satoru,

 

   I ran through the with/without the boundary stem mask using the default mask parameters that is on your website. I don't see too much of a difference between the two results:

 

attachicon.gifcombined.jpg

 

So the addition of the boundary stem to strengthen the mask should be ok.

 

cytan

 

P.S. I've also updated my github repository and wiki with links to your webapp.

 

Hi cytan,

 

Thank you for your investigation. Then leave the support stem option on webApps. And I will add a link to this article.



#25 Poochpa

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Posted 05 June 2017 - 02:16 PM

Cytan and Satoru,

If I were to have a shop laser cut a mask, what material and thickness would you recommend? Thanks.

Mike




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