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A Tri-Bahtinov mask for SCT collimation and focusing

astrophotography imaging SCT
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#101 Mert

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Posted 31 May 2018 - 03:20 PM

Wow, this is a very interestng thread!!

I just generated a mask for my Maksutov and will try to print it

soon.

Hopefully I get some clear night ( lately only clouds and rain ) to

test things out.

Wonderful invention!!

A bit of a struggle to use Inkscape, install the extension svg2openscad

and then from openscad to STL.

Will post the result once all made and tested!

A big thanks to you Satoru!! :bow:


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#102 Mert

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Posted 01 June 2018 - 02:24 PM

Only a question on the generator, why do the stems are soo distant

in between then while the bahtinov mask I generated for the same scope

are very much nearer ( do I say this correctly? )

 

Maybe I will put some stems equidistant in between them to make it

look more like the standard bahtinov mask that works great???

 

Any feedback is apreciated!!

Regards,

-Mert

 

Attached Thumbnails

  • Tribahtinov1.png
  • bahtinov.jpg


#103 cytan299

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Posted 01 June 2018 - 02:42 PM

Hi Mert,

   What's the parameters of your scope? Focal length, diameter, etc. I'll see what's going on ...

 

cytan



#104 Mert

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Posted 02 June 2018 - 03:00 AM

Thank you for your effort, much apreciated!!

 

The Maksutov is a Skywatcher 6" but with moonlight crayford

on the back so due to backfocus the FL is now aprox. 1900 mm.

 

Diameter is 150mm and FL 1900 ( native focal length is 1800 mm )

 

Thank you again!!

Regards,

-Mert



#105 Mert

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Posted 02 June 2018 - 03:35 AM

Just tried to generate again and it looks much more like the original

bahtinov mask, the distances.

Thanks for your work!!

-Mert



#106 1CM69

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Posted 07 June 2018 - 06:45 AM

This is a fascinating thread and correct collimation is something that I have been severely struggling with since without knowing differently I removed my corrector to clean my mirrors without first marking it's position.

 

I think that I've been lucky and dodged a bullet as my collimation does not seem to be obviously too far out.

 

I have used various other methods to collimate such as defocused stars & using Al's Collimation Aid to try to make sure that all rings are concentric but it seems almost impossible to get the star to be still long enough for very fine tuning.

 

Recently I have made a Duncan Mask to try, this was a fail. I had to raise the gain such much that the image was dancing around the screen, also I was getting a fainter ghost image of the 'Y' with a slight rotational offset to the original 'Y'

 

Though hours of trawling the web I stumbled on the thread & have used the generator to create a Tri-Bahtinov mask for my CPC9.25.

 

I left all the settings on the generator as default, only entering my FL of 2350mm, Outer Diameter of mask as 263mm & the diameter of my Fastar boss as 90mm.

 

Here is the resulting plot:

KirkEvans_3mm_ACRYLIC.jpg

 

Here is the image I made from the plot in order to try it in Maskulator:

CelestronCPC9.25.jpg

 

When the generator created the above plot there were 3 very, very small triangle cutouts, one by each of the Mount Holes, I deleted these in Inkscape - hope this does not effect the masks use.

 

I have emailed my svg to a Laser Cutter for a quote, just waiting to hear back with a price.

 

While on the subject, when I travel below focus on my scope and then back up to a focus point I can see a very distinct & central Poisson point, as it should be but moving up away from a focus point this Poisson point gradually fades only to give way to a brighter spot that is much more off center but my overall secondary shadow does not move.

It alomost like there are 2 focus points very close to the center.

I have attempted to correct this but tweaking the collimation screws moves the secondary about but this odd spot stays in the same place relative to the secondary shadow.

 

Hoping this Tri-Bahtinov will help in some way for me to check my collimation to such a degree that I can rule out any errors there before figuring out what this other off center point could be.

 

Thanks to cytan299 for starting this thread & everyone else who has helped progress it so far.

 

I''l post back my findings.

 

Regards..,

 

Kirk



#107 cytan299

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Posted 07 June 2018 - 07:26 AM

Hi Kirk,

   Please post whether it works for you or not. More empirical evidence is always helpful. If it doesn't work, it'll be good to know as well.

 

cytan



#108 1CM69

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Posted 07 June 2018 - 08:54 AM

Hi Kirk,

   Please post whether it works for you or not. More empirical evidence is always helpful. If it doesn't work, it'll be good to know as well.

 

cytan

Yes, I definitely will.

 

I got a quote back from the Laser Cutter I found: 16.75GBP for the cutting & 7.95GBP postage, so I have ordered it in 3mm Black Acrylic.

The cutting service I used for anyone in the UK is: https://www.fredaldo...es/laser-studio



#109 1CM69

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Posted 07 June 2018 - 12:13 PM

Just heard from my cutter, the mask has been shipped already & will be with me tomorrow.

 

Looking forward to seeing it although I won't be able to try it out until early next week weather permitting.



#110 1CM69

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Posted 08 June 2018 - 06:37 AM

Very happy this morning when the mask arrived.

 

IMG_1114.JPG

 

but then like an idiot within 5 minutes I broke it when trying to see if it would sit closer to the corrector by pushing it under the two retaining pins for my scopes lens cap

 

IMG_1115.JPG

 

attempting to superglue the mask back together before contacting my supplier to cut another one, doh bawling.gif

 


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#111 James Cunningham

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Posted 08 June 2018 - 06:55 AM

Can you order two of them?  I will pay you for it including shipping.  You can reply to me off posting at vikelj@comcast.net.  Or if the moderator will not allow e-mail addresses, at vikelj(at)Comcast.net.  Thanks

Jim



#112 cytan299

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Posted 08 June 2018 - 07:08 AM

Very happy this morning when the mask arrived.

 

attachicon.gif IMG_1114.JPG

 

but then like an idiot within 5 minutes I broke it when trying to see if it would sit closer to the corrector by pushing it under the two retaining pins for my scopes lens cap

 

attachicon.gif IMG_1115.JPG

 

attempting to superglue the mask back together before contacting my supplier to cut another one, doh bawling.gif

Hi Kirk,

   Big oops! But supergluing will work. I've had broken Bahtinov masks before and superglue fixes all ills smile.gif

 

cytan


Edited by cytan299, 08 June 2018 - 07:08 AM.


#113 1CM69

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Posted 08 June 2018 - 07:25 AM

Can you order two of them?  I will pay you for it including shipping.  You can reply to me off posting at vikelj@comcast.net.  Or if the moderator will not allow e-mail addresses, at vikelj(at)Comcast.net.  Thanks

Jim

Hi Jim, prob best if you find a supplier near you, I am in the UK so shipping would be crazy money.

 

Hi Kirk,

   Big oops! But supergluing will work. I've had broken Bahtinov masks before and superglue fixes all ills smile.gif

 

cytan

Tried it cytan, twice. It will not stick.

Have made a mod to the design now by placing cutouts where the lens cap pins are & I think I'll get this one cut in 5mm instead of 3mm.

Good thing is that I used both parts to check for proper fitment & yes it's a nice snug interference fit between the outer edge of the mask & the inner edge of my OTA as well as the same where the centre cutout of the mask meets the Fastar/secondary boss. 



#114 James Cunningham

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Posted 08 June 2018 - 07:29 AM

I have a problem on what to send to a mask cutter.  Can you send to me what you sent to yours.  Thanks.

Jim



#115 1CM69

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Posted 08 June 2018 - 07:32 AM

My v2.0 with Lens cap retaining pin cutouts.

 

KirkEvans_5mm_ACRYLIC.jpg

 

 



#116 1CM69

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Posted 08 June 2018 - 07:35 AM

I have a problem on what to send to a mask cutter.  Can you send to me what you sent to yours.  Thanks.

Jim

Did you try using the mask generator?



#117 James Cunningham

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Posted 08 June 2018 - 07:37 AM

I could not get it to generate the right sized mask.

Jim



#118 1CM69

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Posted 08 June 2018 - 07:51 AM

I could not get it to generate the right sized mask.

Jim

OK, I see that you also have a 9.25" CPC, is this what you are requiring the mask for?

 

Does your CPC have the Fastar boss exactly as shown in the image here: https://www.rotherva...humbscrews.html ?

 

Does you CPC have two locating/securing pins for the lens cap?



#119 James Cunningham

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Posted 08 June 2018 - 07:52 AM

Yes.



#120 1CM69

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Posted 08 June 2018 - 07:59 AM

Yes.

YES to all 3 questions?

 

If so then here is the .svg file that I have that was generated for my scope: https://www.dropbox....with_FASTAR.svg

 

Send this .svg file to your local cutter after discussing with them which material you wish it to be cut from.

 

Regards..,



#121 darkstar3d

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Posted 08 June 2018 - 08:31 AM

YES to all 3 questions?

Not poking at anyone but I experience this issue almost daily in my IT business. :)


Sent from my iPhone using Tapatalk
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#122 James Cunningham

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Posted 08 June 2018 - 09:12 AM

Can you just send the file that I can send out.



#123 1CM69

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Posted 08 June 2018 - 09:21 AM

Can you just send the file that I can send out.

I added a dropbox link to the post, download it from there.

 

A please wouldn't go amiss wink.gif


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#124 James Cunningham

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Posted 08 June 2018 - 09:23 AM

I do not subscribe to Dropbox from my IPad.

Jim



#125 StarmanDan

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Posted 08 June 2018 - 09:47 AM

Using the mask generator, and I keep getting an error from Pokono saying the stroke is too thick and to change them to .01mm.  Will this impact the design?




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