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A Tri-Bahtinov mask for SCT collimation and focusing

astrophotography imaging SCT
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#276 mrpugh55

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Posted 08 November 2019 - 05:33 AM

Thank you Cytan



#277 mrpugh55

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Posted 08 November 2019 - 07:26 AM

Hi Cytan

 

so I am using my mask for the first time tonight, but I am getting dotted diffraction spikes, not solid lines.  Not normal or is it?

 

Scope is a 36cm RASA f2.2 ,FL 790mm.  Perhaps the slots are too wide?

 

cheers

Martin


Edited by mrpugh55, 08 November 2019 - 07:52 AM.


#278 cytan299

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Posted 08 November 2019 - 01:56 PM

Hi Cytan

 

so I am using my mask for the first time tonight, but I am getting dotted diffraction spikes, not solid lines.  Not normal or is it?

 

Scope is a 36cm RASA f2.2 ,FL 790mm.  Perhaps the slots are too wide?

 

cheers

Martin

Perhaps. If you have photos, then I can have a look. I'd point at a bright star like Vega as a first test to make sure that the diffraction spikes look ok.

 

cytan


Edited by cytan299, 08 November 2019 - 01:58 PM.


#279 mrpugh55

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Posted 10 November 2019 - 05:21 AM

My bad....sorry.  I had a Ha filter in there.  Diffraction spikes looking as they should now.

 

First attempt tonight!

cheers

Martin



#280 mrpugh55

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Posted 10 November 2019 - 06:28 AM

Hi there.  Would be most grateful for some directions here.

I am trying to collimate an f2.2 36cm RASA and have just acquired and fitted a tri-B mask.

I focussed on axis, and then moved that same star to each corner and took an image:  Below.

TOPRIGHT
TOPLEFT
BOTTOMLEFT
BOTTOMRIGHT

I started to adjust each set of collimation screws just to experience the change in the diffraction pattern and to try and identify what screws I should be adjusting.  I started with the top left as that appeared to be the worst.

So, am I supposed to be adjusting the collimation screws until all 3 sets of diffraction spikes appear like the star on axis?  Because after making multiple changes on all 3 sets the only thing I saw move was the central spike on the only set of diffraction spikes that were there.  Then I rotated the mask a small amount and all 3 sets of diffraction spikes appeared.  I did not expect that, but what does that mean?

 

A second question is I was using the Tri-B Grabber and it stated my mask was 37degs when the star was at the top left, then 60degs on axis - is this angle relevant or important to the overall process?

 

many thanks for the help in advance.

cheers

Martin



#281 cytan299

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Posted 10 November 2019 - 07:10 AM

Hi Martin,

   I'm sorry, the only time I've seen such a bad set of diffraction spikes is with my ETX125 which wasn't even remotely close to collimation. I believe that your Tribahtinov images also indicate that your scope is *way* off collimation because the spikes are so far off symmetry frown.gif The TriBahtinov will not work at this stage, I'm afraid. 

 

   If it were a normal SCT, I'd suggest doing this first:  

 

http://www.mira.org/...ures/collim.htm

 

This method really gets you close and then the TriBahtinov can be used for final tuning.

 

However, posters here who have RASAs at f2 will have to chime in because I have never collimated, or touched this type of SCT before.

 

cytan


Edited by cytan299, 10 November 2019 - 07:18 AM.


#282 mrpugh55

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Posted 10 November 2019 - 07:43 AM

Thanks Cytan.  I have work to do obviously.

 

But why did 3 sets of diffraction spikes appear with just a slight rotation of the mask?

 

thanks

Martin



#283 cytan299

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Posted 10 November 2019 - 07:53 AM

Thanks Cytan.  I have work to do obviously.

 

But why did 3 sets of diffraction spikes appear with just a slight rotation of the mask?

 

thanks

Martin

 

Hi Martin

  IMO, the scope is too far off collimation to make any comment or conclusion about your observation. Let’s revisit it after you get the classic Tribahtinov spike pattern first.

 

cytan



#284 mrpugh55

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Posted 10 November 2019 - 08:14 AM

ok....but to be clear, with the star at the centre and focused, the 3 sets of spikes were very good.

 

cheers

Martin



#285 cytan299

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Posted 10 November 2019 - 08:42 AM

ok....but to be clear, with the star at the centre and focused, the 3 sets of spikes were very good.

 

cheers

Martin

I’m confused obviously. If the 3 set of spikes look at the center and focused (like for my ETX125)

 

https://www.cloudyni...goodbad-is-it/#

 

then I’d say that you are collimated.

 

If that’s the case, then the results at the corners is beyond my experience. Some other people who use RASA at f2 will have to comment.

 

cytan


Edited by cytan299, 10 November 2019 - 08:44 AM.


#286 lambermo

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Posted 17 November 2019 - 08:45 AM

I finally was able to collimate my SCT thanks to the TriBahtinov mask. Many thanks for it !
'Before' on the left, 'after' result on the right.

20190531-collimation-with-tribahtinov.jpg

I laser cut the mask locally at http://www.seeyoulaser.nl/tarieven/ for 14 Euro. Good price.
 
I never tried the TriBahtinov grabber software as I'm not on WIndows (I use Linux instead). I wonder what needs to be done to make it portable so that Mac and Linux users could use it as well.
 
-- Hans


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#287 cytan299

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Posted 17 November 2019 - 12:47 PM

Hi Hans

 

  I’m glad that the mask worked for you.

 

  The grabber is immature software. You’re not missing too much. Unfortunately, I’m not a programmer and don’t have the resources to make a better fitting program for it. Besides, I’m a Mac user smile.gif

 

cytan


Edited by cytan299, 17 November 2019 - 12:47 PM.


#288 archer1960

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Posted 17 November 2019 - 01:05 PM

I finally was able to collimate my SCT thanks to the TriBahtinov mask. Many thanks for it !
'Before' on the left, 'after' result on the right.

attachicon.gif 20190531-collimation-with-tribahtinov.jpg

I laser cut the mask locally at http://www.seeyoulaser.nl/tarieven/ for 14 Euro. Good price.
 
I never tried the TriBahtinov grabber software as I'm not on WIndows (I use Linux instead). I wonder what needs to be done to make it portable so that Mac and Linux users could use it as well.
 
-- Hans

Write it in Java would be one way to make it cross-platform.



#289 archer1960

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Posted 17 November 2019 - 01:06 PM

Hi Hans

 

  I’m glad that the mask worked for you.

 

  The grabber is immature software. You’re not missing too much. Unfortunately, I’m not a programmer and don’t have the resources to make a better fitting program for it. Besides, I’m a Mac user smile.gif

 

cytan

What is it written in right now?



#290 cytan299

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Posted 17 November 2019 - 04:24 PM

What is it written in right now?

Here's the code:

 

https://github.com/1...ahtinov_Grabber

 

written by K. Evans.

 

It's written in C#.

 

cytan



#291 Freakshow

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Posted 08 December 2019 - 07:37 PM

Has anyone considered a Tri-Carey mask? Carey mask seems easier to use than Bahtinov.

#292 t-ara-fan

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Posted 10 December 2019 - 04:37 PM

 I need a tri-bahtinov mask for my 36cm RASA. 

I am going to try collimation of my  EdgeHD8 with CCDInspector.  Quite a versatile program, I have used it for tweaking tilt and curvature.   Bonus is it will work with all my scope configurations.

 

The temp was -20°C the other night, so I didn't try collimating with CCDInspector then. Hopefully this weekend.
 



#293 pagpatrice

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Posted 24 December 2019 - 02:05 AM

Hello,
do you have a link for "tri-bahtinov grabber" for windows?

 

sorry i just found it


Edited by pagpatrice, 24 December 2019 - 02:10 AM.


#294 Psittacula

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Posted 03 January 2020 - 11:56 AM

Hi,
I will post the information for reference.

  1. The usefulness of Tri-Bahtinov mask for RASA has been demonstrated. He uses a modified version.
  2. Some vendors are selling the products.


#295 Psittacula

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Posted 03 January 2020 - 12:12 PM

Hi there.  Would be most grateful for some directions here.

I am trying to collimate an f2.2 36cm RASA and have just acquired and fitted a tri-B mask.

I focussed on axis, and then moved that same star to each corner and took an image:  Below.

 

I started to adjust each set of collimation screws just to experience the change in the diffraction pattern and to try and identify what screws I should be adjusting.  I started with the top left as that appeared to be the worst.

So, am I supposed to be adjusting the collimation screws until all 3 sets of diffraction spikes appear like the star on axis?

I’m confused obviously. If the 3 set of spikes look at the center and focused (like for my ETX125)

 

https://www.cloudyni...goodbad-is-it/#

 

then I’d say that you are collimated.

 

If that’s the case, then the results at the corners is beyond my experience. Some other people who use RASA at f2 will have to comment.

 

cytan

I think we need to use the star at the center of the field for collimation.
Stars in the corners may be affected by vignetting.


Edited by Psittacula, 03 January 2020 - 09:18 PM.

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#296 CvBadengoth

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Posted 10 January 2020 - 06:14 AM

....transparent tri-baht arrived today for my RASA8. Thank you Cytan & Satoru Takagi for providing this tool to the community, you are heroes!

 

As its transparent I hope to have very intense spikes.

 

I would be happy if the mask also works from the distant position on top of the dew-shield, for focussing only. I made a mask holder for it (last image).

Any experiences so far that it needs to be close to the corrector plate?

 

Lightroom3406-9.jpg
Lightroom3406-11.jpg
Lightroom3406-13.jpg
Lightroom3406-16.jpg


Edited by CvBadengoth, 10 January 2020 - 06:45 AM.

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#297 cytan299

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Posted 10 January 2020 - 08:32 AM

Hi CvBadengoth,

   The transparent mask looks great! Please post pics and comments of the results if you can.

 

  I've never used one that is on the dew shield on my SCT for collimation because it will make it difficult to get to the collimation screws. 

 

cytan


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#298 CvBadengoth

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Posted 10 January 2020 - 09:47 AM

Hi CvBadengoth,

   The transparent mask looks great! Please post pics and comments of the results if you can.

 

  I've never used one that is on the dew shield on my SCT for collimation because it will make it difficult to get to the collimation screws. 

 

cytan

....I only meant focussing when on the dew shield, do you think that might work? I read somewhere that it needs to be close to the corrector plate, but I dont understand why....



#299 cytan299

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Posted 10 January 2020 - 12:23 PM

....I only meant focussing when on the dew shield, do you think that might work? I read somewhere that it needs to be close to the corrector plate, but I dont understand why....

Although the TriBahtinov can work as a focusing mask, I actually use a normal Bahtinov mask because the diffraction spikes are brighter with a normal one. I place my Bahtinov mask over my dew shield for focusing.

 

I'd imagine that having the TriBahtinov over a dew shield for focusing will work. But the proof is in the pudding -- so you'll have to tell us whether it works or not smile.gif

 

cytan


Edited by cytan299, 10 January 2020 - 12:23 PM.

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#300 RaulTheRat

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Posted 13 February 2020 - 08:25 PM

Can anyone please provide me with the parameters to put into http://svg2.mbsrv.ne...symmetric4.html to generate an SVG for an edgehd 1100. I have ordered one (I don't have it yet to measure) and would like to get a mask laser cut before it arrives, so if anyone can tell me the outer and inner diameters to use that would be useful.

Also, is there any reason I would not use the 2nd version with improved sensitivity of the mask generator?


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