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Comparing Tri-Bahtinov mask and Airy disk for SCT collimation

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#1 cytan299

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Posted 30 May 2016 - 10:13 PM

Hi guys,

   I'm still testing my Tri-Bahtinov mask and here's a comparison between the Tri-Bahtinov mask and Airy disk diffraction patterns before and after using my LX200. I took these pictures pointing at Polaris on 28 May 2016.

 

Here's the as found Airy disk and Tri-Batinov patterns. My LX200 is badly collimated :)

 

asfound_small.png

 

And here it is after correction

 

corrected_small.png

 

Anyway, I think the Tri-Bahtinov mask seems to work :)

 

I will be writing a python script to generate arbitrary sized Tri-Bahtinov masks and that will be uploaded to my github site https://github.com/c...299/tribahtinov

  

 

cytan

 



#2 Cotts

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Posted 31 May 2016 - 07:33 AM

Picky point.   The Airy disc is only visible with the telescope in perfect focus.   You are using the 'out of focus diffraction pattern' which lacks a shorter, more convenient name...  Some  use the term Fresnel pattern....I'm not sure this is strictly correct, either.

 

Can you post a photograph of the Tri-Bahtinov mask?  Curious to see it.

 

Dave



#3 cytan299

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Posted 31 May 2016 - 08:06 AM

Picky point.   The Airy disc is only visible with the telescope in perfect focus.   You are using the 'out of focus diffraction pattern' which lacks a shorter, more convenient name...  Some  use the term Fresnel pattern....I'm not sure this is strictly correct, either.

 

Can you post a photograph of the Tri-Bahtinov mask?  Curious to see it.

 

Dave

 

Here's a picture of the Tri-Bahtinov mask:

 

IMG_0086_small.jpg

 

If you use a LX200 8", you can download the printable mask for you to make one at:

 

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

 

I am in the process of writing a python script to generate arbitrary masks to fit different scopes. Hopefully, it will be done soon.

 

 



#4 cytan299

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Posted 31 May 2016 - 08:11 AM

Just to summarize my intention of "inventing" the Tri-Bahtinov mask are:

  • I use the Bahtinov mask to focus anyway, so using a Tri-Bahtinov not only allows me to focus, it also shows whether my scope is collimated at the same time
  • I found from using this mask that when I find that the spikes are non-symmetric, the non-symmetric spikes indicate which screw to tighten or loosen.
  • I have bad judgement :) as to whether the donut is centered or not and IMO there is less arbitrariness when I use the Tri-Bahtinov focus pattern.

Anyway, I hope this will be useful

 

cytan


Edited by cytan299, 31 May 2016 - 08:11 AM.


#5 TheRock

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Posted 10 June 2016 - 10:17 AM

Just to summarize my intention of "inventing" the Tri-Bahtinov mask are:

  • I use the Bahtinov mask to focus anyway, so using a Tri-Bahtinov not only allows me to focus, it also shows whether my scope is collimated at the same time
  • I found from using this mask that when I find that the spikes are non-symmetric, the non-symmetric spikes indicate which screw to tighten or loosen.
  • I have bad judgement :) as to whether the donut is centered or not and IMO there is less arbitrariness when I use the Tri-Bahtinov focus pattern.

Anyway, I hope this will be useful

 

cytan

 

Not sure why this has not received more interest... an ingenious way to get rid of two problems with one simple tool/method. Have you validated the collimation and/or focus with other techniques?



#6 rgsalinger

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Posted 10 June 2016 - 12:22 PM

There's a commercially available similar system that does something just a tad better because it comes with software the measures the error in either collimation or focus. Here's a link http://www.goldastro...etails_plus.php - it's not free but it does work. They make masks in all sizes pretty much.  

Rgrds-Ross



#7 cytan299

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Posted 10 June 2016 - 02:49 PM

 

Just to summarize my intention of "inventing" the Tri-Bahtinov mask are:

  • I use the Bahtinov mask to focus anyway, so using a Tri-Bahtinov not only allows me to focus, it also shows whether my scope is collimated at the same time
  • I found from using this mask that when I find that the spikes are non-symmetric, the non-symmetric spikes indicate which screw to tighten or loosen.
  • I have bad judgement :) as to whether the donut is centered or not and IMO there is less arbitrariness when I use the Tri-Bahtinov focus pattern.

Anyway, I hope this will be useful

 

cytan

 

Not sure why this has not received more interest... an ingenious way to get rid of two problems with one simple tool/method. Have you validated the collimation and/or focus with other techniques?

 

 

My validation is with the Airy disk method. Focusing works fine too. That's what I've been using for the past 2 imaging sessions. The advantage is that I can immediately see whether my scope requires adjustment in collimation or not.

 

Anyhow, I am in the process of writing a python script to generate arbitrary sized masks for different telescope sizes. It should be ready in a few weeks, if not sooner. 

 

cytan



#8 cytan299

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Posted 10 June 2016 - 02:51 PM

There's a commercially available similar system that does something just a tad better because it comes with software the measures the error in either collimation or focus. Here's a link http://www.goldastro...etails_plus.php - it's not free but it does work. They make masks in all sizes pretty much.  

Rgrds-Ross

 

It was pointed out to me in my original announcement in this forum by someone. It's always good to have alternatives. I'm sure some enterprising programmer among us can write a program to do the alignment exactly. But for now, I'm writing a program to generate arbitrary sized masks.

 

IMO, especially for masks, it should be "free".

 

cytan




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