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A Tri-Bahtinov mask for SCT collimation and focusing

astrophotography imaging SCT
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#401 fortranguy

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Posted 27 July 2020 - 04:59 PM

Wouldn't it get soggy/soft in the dew overnight ?

Steve.


I added a waterproof coating. The material is very similar to the eco-friendly containers used for food takeaway.


Edited by fortranguy, 27 July 2020 - 06:54 PM.


#402 fortranguy

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Posted 27 July 2020 - 05:04 PM

I tried three types of masks made of

1. Clear Lexan (clear polycarbonate plastic) 1/8" - CNC cut
2. Opaque Lexan 1/8" - CNC cut
3. 3D printed with black ABS in 5mm thickness

All design parameters are the same like Bhatinov factor, outer/inner margin etc. So amount of open/clear area the same.
Among these, 2 and 3 are about the same. There might be but they're in the same ballpark.
On the other hand, #1 has much brighter and longer spikes in totally different league.

But I can't use #1 for any use because patterns are not Tri-B patterns anymore. There are many more patterns overlapped on each other.
My guess is, 1) when I ordered this mask to CNC guy, Satoru's pattern generator has slight symmetry issue. Due to bright nature of this material, any imperfection would be apparent. and 2) Lexan is not coated for light transmission. So there are surfaces that light reflects and make offset patterns.
I couldn't confirm which one contributes more though as I dropped the experiment.
Meanwhile, William Optics is selling the clear B-mask with claim 'much brighter', you can see it's not just a marketing phrase. It's indeed a (or several) notch brighter.


I tried optical grade clear acrylic 2mm thick. The spikes are awesome but it becomes very fragile.

If you are using a RC or a newtonian, you are quite safe, but if you are using a SCT or a MCT, the risk of damaging the front lenses is quite high.


Edited by fortranguy, 27 July 2020 - 06:42 PM.


#403 Zebul

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Posted 01 August 2020 - 06:25 AM

Thanks Jinux & Cytan for your comments on my questions.

I understand much better the information given on the construction of masks. The rounded ends probably lessen the parasitic diffrations. Indeed a line segment produces its own diffraction.

 

To complete my previous citation, I'm not sure that 3D printing is the best way to produce a DIY mask. Personaly I prefer laser cutting. It may possible to use thinner and rigid raw material.

An other point about my diffraction understand ('far from perfect'). This is not so important to have equal 'black and white' slits. The most important is to place at the good intervals obstructions, even with very thin opaque lines. The objectif is to generate well harmonic diffraction franges. This is why in Bahtinov mask generator equation, one of most important key factor is the focal distance.

If you have not the good factor you will loose spikes contrast.

You have in other forum some experiences about mask DIY building... (eg: https://stargazerslo...sk-with-wires/)



#404 Zebul

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Posted 02 August 2020 - 03:34 AM

I tried three types of masks made of

 

1. Clear Lexan (clear polycarbonate plastic) 1/8" - CNC cut

2. Opaque Lexan 1/8" - CNC cut

3. 3D printed with black ABS in 5mm thickness

 

All design parameters are the same like Bhatinov factor, outer/inner margin etc. So amount of open/clear area the same.

Among these, 2 and 3 are about the same. There might be but they're in the same ballpark.

On the other hand, #1 has much brighter and longer spikes in totally different league.

 

But I can't use #1 for any use because patterns are not Tri-B patterns anymore. There are many more patterns overlapped on each other.

My guess is, 1) when I ordered this mask to CNC guy, Satoru's pattern generator has slight symmetry issue. Due to bright nature of this material, any imperfection would be apparent. and 2) Lexan is not coated for light transmission. So there are surfaces that light reflects and make offset patterns.

I couldn't confirm which one contributes more though as I dropped the experiment.

Meanwhile, William Optics is selling the clear B-mask with claim 'much brighter', you can see it's not just a marketing phrase. It's indeed a (or several) notch brighter.

Hi Jinux,

 

Even if we are not in full Tri Bahtinov problem. The mask realization is very important to improuve spike contaste, in particular because we divide the light source, by at least 3, to generate diffraction fringes...

 

It is important to fully understand how diffraction work. Each edge is source of a diffraction fringe and the thicker the mask, the more this phenomenon is accentuated.

To my opinion, with cristal mask finaly cutting is not the solution, because in the previous situation, you add generated mirror and opaque surfaces (in vertical dimension). You add many parasits diffractions sources.

If you pay attention of the W Optics mask, this one is just finely engraved and that's the big difference.

And this is why I also suggest, for 3D printing, to build mask with the beveled opennings creating much more of a cut and sharp edge. With this type of construction, the diffraction will be much more contrasted. You tend to generate less diffraction origin.



#405 Zebul

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Posted 06 August 2020 - 05:43 AM

Hi All,

 

I re use my Tri Bahtinov mask to check my collimation, in visual mode. It look like prety good in the 3 directions, the same with concentrics circles in & out focus.

 

I work with Arcturus in the center of my C8 edge, using a Hyperion 5mm. I am a little over the usual capacity of magnification of this OAT

I have a question about Tri Bahtinov mask work.

If the Tri Bahtinov mask is correctly focus, it looks like natural, I will be at the perfect stars 'focus' ?

As I remove the mask, my surprise, stars are out focus ! This is not the first time I note this trouble...

 

Did some one of you, have experience the same trouble or in oposit have an other feed back ?

Maybe you have explanation about this problem ?

To be honest, it surprise me.

 

BR. Zebul



#406 cytan299

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Posted 06 August 2020 - 05:52 AM

Hi All,

 

I re use my Tri Bahtinov mask to check my collimation, in visual mode. It look like prety good in the 3 directions, the same with concentrics circles in & out focus.

 

I work with Arcturus in the center of my C8 edge, using a Hyperion 5mm. I am a little over the usual capacity of magnification of this OAT

I have a question about Tri Bahtinov mask work.

If the Tri Bahtinov mask is correctly focus, it looks like natural, I will be at the perfect stars 'focus' ?

As I remove the mask, my surprise, stars are out focus ! This is not the first time I note this trouble...

 

Did some one of you, have experience the same trouble or in oposit have an other feed back ?

Maybe you have explanation about this problem ?

To be honest, it surprise me.

 

BR. Zebul

This is a very surprising observation. Do you have an ordinary Bahtinov to cross check?  I’d expect the Tribahtinov and the Bahtinov to give the same answers. With regards to your question, no, I’ve never seen your observation. It is very strange.

 

cytan



#407 archer1960

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Posted 06 August 2020 - 10:44 AM

This is a very surprising observation. Do you have an ordinary Bahtinov to cross check?  I’d expect the Tribahtinov and the Bahtinov to give the same answers. With regards to your question, no, I’ve never seen your observation. It is very strange.

 

cytan

I have seen this in the past with a standard bahtinov on my AT-10RC, but I didn't follow up with more detailed testing. Once I found SGP's autofocus routine, I don't use the mask any more.



#408 Ladyhawke

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Posted 27 October 2020 - 04:19 PM

Would you guys be kind enough to explain how to download the TriBahtinov Grabber program? I downloaded the .Zip file from GitHub but there's no executable file. Thanks!



#409 Bingoin

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Posted 27 October 2020 - 04:50 PM

I'm actualy not at home to look for it, but I remember, that it's written in Python. So you need a Python-Interpreter installed on your computer to run it. The Cairo-Lib is also required.

 

 

Greetings,

Juergen



#410 ChiTownXring

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Posted 17 November 2020 - 01:34 AM

Looks like I need a lot of help!!

I am 2 months into the hobby and from the beginning I knew my focus was off and I guess this image doesn't lie.. 

Procyon 45" in the sky Canon T3i ISO 1600 30 Sec Exp Celestron 8SE with Tri-Bahtinov mask.. Please be kind as I have tried to take in a lot of information the last couple of months and I love it but still am green as a frog. I used the tri-bahtinov mask calculator and got the mask that I am using but don't know if its the right size.. I tried to focus as best as I can using live view in APT and snapped this picture. Can someone please help me as so far I am loving the hobby.

 



#411 cytan299

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Posted 17 November 2020 - 08:11 AM

 

Looks like I need a lot of help!!

I am 2 months into the hobby and from the beginning I knew my focus was off and I guess this image doesn't lie.. 

Procyon 45" in the sky Canon T3i ISO 1600 30 Sec Exp Celestron 8SE with Tri-Bahtinov mask.. Please be kind as I have tried to take in a lot of information the last couple of months and I love it but still am green as a frog. I used the tri-bahtinov mask calculator and got the mask that I am using but don't know if its the right size.. I tried to focus as best as I can using live view in APT and snapped this picture. Can someone please help me as so far I am loving the hobby.

 
 

 

 

Your focus is way off. First focus the telescope so that only *one* set of spikes give you the pattern that I highlighted in yellow and red:

 

track.jpg

 

Then if the other set of spikes don't resemble the set that I highlighted then you have a collimation problem.

 

Follow the instructions here to get your scope collimated:

 

https://github.com/c...rain-Telescopes

 

And since you're a newbie, here's my advice: take your time when you collimate your scope. The collimation screws should be turned a little bit at a time, 1/16 to 1/8 turn at a time, no more than that each time. When you turn the collimation screw, the star will move. It is important to re-centre (use the bullseye in APT) and refocus with the reference set of spikes after each correction. 

 

And if you have trepidations doing about collimation, don't. It's actually very easy once you see what happens when you turn a collimation screw a little bit and do the first correction.

 

Hope the above helps.

 

 

cytan


Edited by cytan299, 17 November 2020 - 08:43 AM.

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#412 Psittacula

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Posted 02 January 2021 - 11:51 PM

Hi, folks.

If you have access to a cutting machine like the silhouette CAMEO which I use, it is relatively easy to make your own tri-bahtinov mask as shown in this movie. Of course, the same goes for the bahtinov mask. I use thin plastic sheets or card board as material smile.gif

TBM_CuttingMachine.jpg

 

Satoru


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#413 archer1960

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Posted 03 January 2021 - 04:31 PM

Anybody out there willing to cut or 3D-print a tri-bahtinov mask for my AT-10RC? I can send the dimensions, but I don't have access to a precision cutter or 3D printer, and don't have the patience to cut it out by hand. I have a friend with a 3D printer, but it only goes up to 8", which isn't big enough for this. Price negotiable...



#414 omomomo

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Posted 03 January 2021 - 10:56 PM

You can split the model into smaller pieces for printing, and then glue it back together.
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#415 omomomo

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Posted 05 January 2021 - 03:56 PM

Here is an example, obviously the tri-mask will be trickier to split up, but should be possible.

 

https://www.thingive...om/thing:266400



#416 Psittacula

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Posted 17 January 2021 - 09:53 PM

If I understand the 'rounded corners' correctly, it means the end of slit is rounded rather than sharp edged like attached drawings. 

I guess it will reduce higher order diffraction and less spurious spikes but my 5mm thickness edged 3d-printed one works similar order of magnitude as ~3mm thickness rounded CNC'ed one.

Hello.
I am working on the implementation of the 'rounded corners' option.
So far, I have completed the implementation for the sensitivity-enhanced version of the pattern and for the regular bahtinov mask pattern. I'm also working on the implementation for the original tri-bahtinov mask. (This pattern is a little more complicated, so I'm having a hard time with it.wink.gif )

 

https://satakagi.git..._symmetric.html

 

TBM.png

 

By the way, since the well known original bahtinov mask generator site astrojargon.net has disappeared, I have also prepared webApps for it to ensure redundancy. These are static githubpages, so they are almost costless and can probably be operated long term.

You can find it at https://satakagi.git...s/Bahtinov.html

Satoru


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#417 cytan299

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Posted 18 January 2021 - 01:07 PM

Hello.
I am working on the implementation of the 'rounded corners' option.
So far, I have completed the implementation for the sensitivity-enhanced version of the pattern and for the regular bahtinov mask pattern. I'm also working on the implementation for the original tri-bahtinov mask. (This pattern is a little more complicated, so I'm having a hard time with it.wink.gif )

 

https://satakagi.git..._symmetric.html

 

attachicon.gifTBM.png

 

By the way, since the well known original bahtinov mask generator site astrojargon.net has disappeared, I have also prepared webApps for it to ensure redundancy. These are static githubpages, so they are almost costless and can probably be operated long term.

You can find it at https://satakagi.git...s/Bahtinov.html

Satoru

Good job! You're a much better programmer than I am

 

cytan



#418 Psittacula

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Posted 19 January 2021 - 08:08 AM

Good job! You're a much better programmer than I am

 

cytan

Thank you.

I believe the activity you and bahtinov have started, which could be called a kind of open source hardware, is very valuable.

I'm just contributing to improve it.

 

Satoru



#419 cytan299

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Posted 19 January 2021 - 11:32 AM

Thank you.

I believe the activity you and bahtinov have started, which could be called a kind of open source hardware, is very valuable.

I'm just contributing to improve it.

 

Satoru

A suggestion, instead of rounding everything, perhaps just round off the areas I indicated below. I tried to do rounding in my python program and failed miserably frown.gif

 

track.jpg

 

It's a lot easier because it is easy to round the indicated edge of the slots when they are parallel to the x-axis. Then you just do a rotation transform. Unfortunately the other edge and the intersections are a lot more difficult. Hence my failure.

 

cytan



#420 Jinux

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Posted 19 January 2021 - 01:31 PM

Satoru,

 

Great job working on these! I found that original B-mask generator also already have rounding working!

But also found that outW parameter doesn't apply when generating mask. You may want to check.

 

-Jinux



#421 Jinux

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Posted 19 January 2021 - 02:50 PM

And I found that rounded B-mask generator doesn't go well with Fusion 360 on 'some' slits. I don't know what makes some slits are left unselected for extrusion.

I magnified the drawings to max but couldn't find disconnect between segments but still Fusion 360 refuses to select some. 

Strange thing is, some mirrored slits are fine. I couldn't figure out why yet.

Non-rounded masks are fine.

 

-Jinux



#422 MarMax

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Posted 21 January 2021 - 07:21 PM

I've created a mask using Satour's generator for my CPC 1100 and then just drew it up in Autocad since I could not get Inkscape to save as a DXF file. It's the first generation mask below. My question before I go any further is if it looks correct. Thanks in advance and this thread has been a great read and learning experience!

 

I'm going to have this printed and mounted on 3/16" gator board at FedEx Office and then cut the openings with a razor knife. I know 3/16" is a bit thick but hopefully as a 1st try this should work.

 

gallery_332504_13715_107421.jpg



#423 cytan299

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Posted 21 January 2021 - 08:32 PM

I've created a mask using Satour's generator for my CPC 1100 and then just drew it up in Autocad since I could not get Inkscape to save as a DXF file. It's the first generation mask below. My question before I go any further is if it looks correct. Thanks in advance and this thread has been a great read and learning experience!

 

I'm going to have this printed and mounted on 3/16" gator board at FedEx Office and then cut the openings with a razor knife. I know 3/16" is a bit thick but hopefully as a 1st try this should work.

 

 

The pattern looks fine to me. But I cannot confirm the actual ID and OD because I don't have a CPC 1100.

 

cytan



#424 MarMax

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Posted 21 January 2021 - 09:57 PM

The pattern looks fine to me. But I cannot confirm the actual ID and OD because I don't have a CPC 1100.

 

cytan

Thanks cytan, it's good to know you think it looks OK. It was hard for me to figure out which lines to delete (trim) to create the cutouts. It took a couple tries to get something that looked right, plus there are no C11 samples I've seen. The good thing is you only need to work on 1/3 of it since it's just a copy and paste at 120 degrees to make the rest.



#425 Psittacula

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Posted 22 January 2021 - 01:42 PM

Hi,

I've also added a rounded corners option to the original tri-bahtinov mask generator webApps.

https://satakagi.git...i-Bahtinov.html

It's a bit ugly, but probably not too inconvenient. It also works when there are boundary stems.

TBM_rounding.png

 

Satoru


Edited by Psittacula, 22 January 2021 - 01:59 PM.



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