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A Tri-Bahtinov mask for SCT collimation and focusing

astrophotography imaging SCT
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#326 Psittacula

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Posted 11 March 2020 - 09:21 AM

Oops, I have configured the redirection settings and it should be accessible.

Satoru


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#327 archer1960

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Posted 12 March 2020 - 01:29 PM

Hmmm, neither http://svg2.mbsrv.ne...i-Bahtinov.html, nor the _symmetric one is working for me. Both Firefox and IE say they can't find the server.



#328 cytan299

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Posted 12 March 2020 - 06:24 PM

Hmmm, neither http://svg2.mbsrv.ne...i-Bahtinov.html, nor the _symmetric one is working for me. Both Firefox and IE say they can't find the server.

I just tried the link on my mac with chrome and it found it. Maybe a cache flush is required?

 

cytan



#329 Psittacula

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Posted 12 March 2020 - 07:26 PM

Both Firefox and IE say they can't find the server.

It looks like an error that can't find the server IP address or resolve domain name.
If other people have the same problem, the mask generator pages may need to be moved to another domain, IP address.

 

The URLs of the generators are as follows.

 

Satoru


Edited by Psittacula, 12 March 2020 - 08:33 PM.


#330 wvreeven

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Posted 13 March 2020 - 04:09 PM

Thanks Satoru. Both URLs work for me.

 

 

Wouter



#331 wvreeven

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Posted 13 March 2020 - 05:44 PM

Cut my mask just now. All I need now is a clear night!

 

IMG_0375.jpg

 

 

Wouter


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#332 Jinux

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Posted 14 March 2020 - 12:20 PM

Cut my mask just now. All I need now is a clear night!

 

 

Wouter

I recommend to make a sort of artificial star and try it indoor if your scope can focus.

I put it as 'sort of' because this diffraction based method doesn't require good quality (say small star that can show airy disc size star with diffraction patterns around)  artificial star.

Any pinhole with bright LED flashlight works. I know this because I created real artificial star based on link below and tested.

 

https://sites.google...ar(pointsource)

 

I used 0.2mm pinhole with 4mm eyepiece and it indeed shows very good star image. But it didn't help that much on this mask. Star is faint. This real artificial star can be used for double confirming collimation with airy disk though.

When I removed eyepiece, the star gets whole lot brighter and so are diffraction patterns, while starts get bigger too but in the end we just need pattern to align not the star itself.

 

The best part of indoor artificial method is that now you have seeing free environment. If you try this out under the star, middle spike 'shakes' without focus/collimation change. So it's hard to know whether you're hitting the middle or not but you're free from it in indoor.

Another very important benefit is, now you have full hours under the star not on collimation any single second.

 

-Jinux


Edited by Jinux, 14 March 2020 - 12:21 PM.


#333 wvreeven

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Posted 14 March 2020 - 10:31 PM

Thanks for the recommendation Jinux. I'll give it a thought though I am not sure if I can focus on an artificial star that's too close with this 8" F/5.4 RC.

 

Tonight I collimated my telescope with a cheshire and then pointed at an m=2 star. This is the pattern I saw:

 

Screen Shot 2020-03-14 at 23.53.21.jpg

 

Fortunately it wasn't a very clear night so I could spend some time on collimating my RC without losing imaging time. My problem now is that I haven't got a clue on how to proceed. Should I move the secondary mirror or primary? Or both? How do I know which one to move in what direction? It is quite challenging to be honest.

 

The collimation isn't bad but it can be better. I just don't know how. Any thought would be very much appreciated.

 

 

Clear skies, Wouter



#334 Jinux

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Posted 14 March 2020 - 11:30 PM

Hi,

 

I'm getting better and better to deal with C# image handling and coding. Also COVID-19 lock down made me stay at home whole day long, so made some progress!

Currently only missing feature that I planned but not implemented is "Continuous Analysis" with real time image. I think it's doable but probably will hit with processing speed limit. It's already very slow.

Soon I may need a couple of volunteers to be a 'alpha tester' of this program before release to public. Hopefully the one who already had experience with tri-Bahtinov masks and provide me a feedback how well it works and any feedback on interface improvement. Of course, putting aside whether I'm capable of reflecting those valuable feedback via C# coding. lol.gif Believe it or not, this is my first C# program ever written. My only previous experience of Windows programming is VB6.0 in 15 years ago and I'm NOT a programmer.

If you're interested to be 'alpha tester', please PM me.

 

Clear Skies and stay healthy!

Jinux

 

 

 

 

Attached Thumbnails

  • ASCOM_full_analysis.png

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#335 Jinux

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Posted 14 March 2020 - 11:49 PM

Thanks for the recommendation Jinux. I'll give it a thought though I am not sure if I can focus on an artificial star that's too close with this 8" F/5.4 RC.

 

Tonight I collimated my telescope with a cheshire and then pointed at an m=2 star. This is the pattern I saw:

 

Fortunately it wasn't a very clear night so I could spend some time on collimating my RC without losing imaging time. My problem now is that I haven't got a clue on how to proceed. Should I move the secondary mirror or primary? Or both? How do I know which one to move in what direction? It is quite challenging to be honest.

 

The collimation isn't bad but it can be better. I just don't know how. Any thought would be very much appreciated.

 

 

Clear skies, Wouter

Wouter,

 

Goal is to align all middle spikes right in the middle of each group. You can move either mirror but I would definitely take secondary.

Don't worry about which screw is which direction of which pattern. Once you start move one screw, you'll see one of middle spike moves. Find which one is which by trial and error. I recommend you to make a V2 pattern. That is easier to align three directions of spikes to three adjusting screws on secondary, IMHO.

I tried to analyze your pattern using my program but red axis lines are quite fuzzy and program fails to detect proper lines. I may try some adjustment to analyze it properly. The other two directions looks better. Blue axis is quite on and Yellow is a bit off. Since I don't have your scope's focal/diameter and pixel size, you can't trust analysis numbers though.

 

-Jinux 

Attached Thumbnails

  • analysis.png

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#336 wvreeven

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Posted 15 March 2020 - 09:45 AM

Thanks for the help Jinux. I appreciate it a lot!!!

 

As I wrote, I collimated the RC with a cheshire. Based on what I noticed when turning the collimation screws, is that turning the secondary screws leads to a big offset of the center circle wrt the overall image. I refer to the center ring in this image

 

0FB574A0-7D08-424E-AFC3-1FD82D2B9EE7.jpeg

 

Which is why I am inclined to play with the primary. But moving the primary means moving the secondary as well and that's where I get lost. Please note that the cheshire doesn't stay parallel when I tighten the focuser screw which explains the off-center image and also is why I cannot fully collimate the RC with the cheshire only...

 

And FYI I use a 203 mm F=1084 RC with an ASI1600MM-C Pro camera which has pixels of 3.8 micron.

 

Finally, I'd love to beta test your application but I am on a Mac. Not sure if your app will run on Wine but I am willing to give it a try. Please PM me.

 

 

Clear skies, Wouter


Edited by wvreeven, 15 March 2020 - 09:47 AM.


#337 cytan299

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Posted 15 March 2020 - 01:40 PM

Hey Wouter,

   From my experience with the TriBahtinov, I can tell from your post/photo of the spike pattern that your collimation is not good. However, before messing around with the primary or secondary mirrors, I really hope that someone will give you good advice as to what to do with an RC. (I can't because I don't own one). I fear that if you screw up, it will be very difficult to get the scope back into collimation -- which is not good at this time, but can easily become terrible if done wrong --  whether you use the Tribahtinov mask or not. Personally, I don't believe Jinux's program can help you at this stage. You *have* to know which mirror to move *without* the program. Using Jinux's program will eventually get you to perfection, but IMO you're not there yet.

 

As usual YMMV

 

cytan


Edited by cytan299, 15 March 2020 - 01:41 PM.


#338 HxPI

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Posted 01 April 2020 - 07:48 PM

This is an interesting tool. Looking forward to more info!

 

Thanks for sharing.

 

Ciao,

Mel



#339 JoseBorrero

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Posted 04 May 2020 - 11:09 AM

Hello,

 

Anywhere I can download Tribahtinov Collimator  v0.01?

 

Thanks in advance

 

JB



#340 Manitu

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Posted 19 May 2020 - 10:47 AM

Yes I also would like to try this Collimator tool :)

But there is no link available for download yet. I hope it will be available soon. Any target date available from the originator?

Tom



#341 meegja

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Posted 02 July 2020 - 09:22 AM

Keeping my eyes on this too.
New CN user but I did discover this thread here last week. And tomorrow my custom made Tri-Bahtinov arrives here :)

Having great difficulties with finetuning collimation my SCT so maybe this is also a bit of help.



#342 Zebul

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Posted 05 July 2020 - 06:40 AM

Realy happy to see that someone tends to succeed to manage to adapt Bahtinov soft to Tri Bahtinov mask. I follow this discussion from the beginning, a good story.

 

I'm impatient to discover the fist result of your developent, Jinux. Many thanks for your great job and sharing.waytogo.gif

 

I tried, 1 year ago, to do the job by myself, starting from Tri Bahtinov Grabber, an evolution of Bahtinov Grabber soft.

I only know a little Python, finaly it was too hard for me... There is too many parameters to manage and to succeed in this work (analysis picture or video to indentify spikes, make calculations and give visual results...). The advantage, which I see, is that Python can be multi OS and be open to many of us to tune and implement it.

 

A point of view of my experience using Tri Bahtinov. This mask is not as good as I expect on my C8 Edge to realize collimation.

Tri Bahtinov mask tends to limit the ability so by the fact that it diaphragms the opening of the telescope. There is less semsibility at the end.

I try to do a well collimation with only a simple Bahtinov mask rotating 2 times 120° to control my C8 collimation. It is a not such a bad solution but I need to do screenshot to compare results evolution...

 

To control and possibly refine the colimation, I finish my development work by checking the concentricity of the circles in and out focus.

For that I developed under Python a simple control of the circles concentricity. It does not work so badly and gives me the divergence of the centers in pixels and direction. Like this, I can fine tune directly on my system.

 

I will be happy to do a new evaluation of this new soft for sure. We all need to have a good focus and any simple process to do this job will be great, naturaly.bow.gif


Edited by Zebul, 05 July 2020 - 06:47 AM.


#343 Ben Diss

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Posted 05 July 2020 - 07:33 AM

@Zebul I too had little luck using a tri mask at first, but two things I did helped. First, I printed a mask very thin, only 1mm.  Second, I reduced my exposure greatly so that there is a little space between the central circle and the spikes. This allowed me to see it well enough to collimate.



#344 gatocosmico

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Posted 06 July 2020 - 10:08 PM

Hi guys,

 

I'm new in this hobby and I been looking for this (SCT collimation) for a while. I had no idea about that this topic has been in this forum for years. Anyways, my question is how do I run the "Tri-Bathinov grabber" software?

 

I follow the links and I go straight to the Github website. I found a lot of folders and CS. files but I can't find an exe. or run. file to start the program. I already downloaded the master folder but I can't find a way to make it work on my Win10 computer.

 

I read that this program is written on C# but I have no idea about programming and anything else, I think is way beyond of my understanding.

 

If somebody could explain this for me that would be really, really appreciated.

 

Thanks



#345 wpullen

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Posted 07 July 2020 - 04:28 PM

I apologize for and acknowledge my ignorance, but I'm confused by what seems to be the amateur astronomer's definition of collimation.  By definition (according to my optics background) collimation simply refers to parallel rays, or planar wavefronts, but throughout this thread as well as many other amateur astronomer resources the word seems to simply be used as a synonym for misalignment.

 

With this in mind, while this tri-Bahtinov mask idea is quite clever and beneficial, it does not provide a measure of collimation.  Collimation/miscollimation is an axial-only misalignment between optics in an afocal telescope or of a collimation optics after an intermediate focus and because of that the path-length change is an azimuthally symmetric error (spherical aberration).

 

The tri-Bahtinov mask then provides a measure of non-azimuthally symmetric errors, namely coma, astigmatism (you can't detect trefoil with this unless you iterated a 60 degrees rotation, but then you can't correct for trefoil really either so there's no harm there).  Coma and astigmatism arise from errors in tilt and/or decenter of elements in the optical train.

 

Again, this is still very beneficial and pretty neat extension to the Bahtinov mask that allows an easy way to align out three of the five primary aberrations!


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#346 meegja

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Posted 08 July 2020 - 03:07 AM

Again, this is still very beneficial and pretty neat extension to the Bahtinov mask that allows an easy way to align out three of the five primary aberrations!

True ... at least a few parts, I do not have enough knowledge about other parts :)

But, as I understand it, this is not that important for SCT's: the collimation (or misalignment) can only be tuned with 3 screws of the secondary mirror. If there is for example astigmatism, nothing can be done about that anyway since the primary mirror is fixed.


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#347 cytan299

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Posted 08 July 2020 - 07:36 AM

I apologize for and acknowledge my ignorance, but I'm confused by what seems to be the amateur astronomer's definition of collimation.  By definition (according to my optics background) collimation simply refers to parallel rays, or planar wavefronts, but throughout this thread as well as many other amateur astronomer resources the word seems to simply be used as a synonym for misalignment.

 

With this in mind, while this tri-Bahtinov mask idea is quite clever and beneficial, it does not provide a measure of collimation.  Collimation/miscollimation is an axial-only misalignment between optics in an afocal telescope or of a collimation optics after an intermediate focus and because of that the path-length change is an azimuthally symmetric error (spherical aberration).

 

The tri-Bahtinov mask then provides a measure of non-azimuthally symmetric errors, namely coma, astigmatism (you can't detect trefoil with this unless you iterated a 60 degrees rotation, but then you can't correct for trefoil really either so there's no harm there).  Coma and astigmatism arise from errors in tilt and/or decenter of elements in the optical train.

 

Again, this is still very beneficial and pretty neat extension to the Bahtinov mask that allows an easy way to align out three of the five primary aberrations!

Hi wpullen,

   I'm the originator of the Tri-Bahtinov. IMO, the Tri-Bahtinov only provides two functions: enabling the correction of coma that arises from misalignment of the "tilt and/or decenter of elements in the optical train" (that's the collimation part) and to ensure that all the rays have the same path length to the image at the center of the eyepiece (that's the focusing part). It does not correct for astigmatism or trefoil or any other optical errors.

 

  For SCTs, we can only use the 3 screws on the secondary to correct for the tilt, and this is the best we can do for collimation. And I'm using "collimation" definition used in the universe of us amateurs.

 

As usual YMMV

 

cytan


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#348 wpullen

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Posted 10 July 2020 - 12:16 AM

Hi wpullen,

   I'm the originator of the Tri-Bahtinov. IMO, the Tri-Bahtinov only provides two functions: enabling the correction of coma that arises from misalignment of the "tilt and/or decenter of elements in the optical train" (that's the collimation part) and to ensure that all the rays have the same path length to the image at the center of the eyepiece (that's the focusing part). It does not correct for astigmatism or trefoil or any other optical errors.

 

  For SCTs, we can only use the 3 screws on the secondary to correct for the tilt, and this is the best we can do for collimation. And I'm using "collimation" definition used in the universe of us amateurs.

 

As usual YMMV

 

cytan

Okay, it's clear to me now that "collimation" her just means misalignment, still don't understand why/how that became the case, but so be it.

 

Yes, there's nothing you can do about trefoil since that is not an alignment error but a mounting error.  Also you won't measure it with a 120 deg symmetry anyway.  I have no experience with SCT's, but yes, if you can only adjust the tilt of one mirror then you can't correct both astig and coma.  Astigmatism comes from attempting to correct for coma with the wrong motion, for example trying to tilt coma out when the coma was due to a decenter error.  I guess my point was that you can see both coma and astigmatism with this type of mask whether or not you can correct them both.

 

The big reason I brought this up, btw, is because I am polishing up a paper for publication about using the Bahtinov mask to measure collimation (in the parallel rays sense) so I've been looking around to see what else might have been looked at with the B-mask and collimation (again, the parallel rays sense).



#349 meegja

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Posted 10 July 2020 - 02:49 AM

Okay, it's clear to me now that "collimation" her just means misalignment, still don't understand why/how that became the case, but so be it.

 

 

That might be a tiny typo on my part also I see now :)

Collimation means aligning the optics of the telescope. If the telescope shows stars that are out of alignment (can show several effects), the telescope is not properly collimated.

Or as Celestron describes it: "Collimation is the proper alignment of the optical elements–lenses and mirrors in your telescope. Poor collimation will result in optical aberrations and distorted images. The optical axis of the objective (main) lens must be aligned with the optical axis of the eyepiece.".



#350 Ben Diss

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Posted 10 July 2020 - 05:46 AM

The big reason I brought this up, btw, is because I am polishing up a paper for publication about using the Bahtinov mask to measure collimation (in the parallel rays sense) so I've been looking around to see what else might have been looked at with the B-mask and collimation (again, the parallel rays sense).

Have you looked at Carey masks? I did, and I'm hooked. Much easier to interpret than Bahtinov.




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