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Issue with CDK 12,5" star shapes - please help

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#1 AstroLF

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Posted 25 July 2021 - 08:07 AM

Hello all,

 

this is my 1st post here even though i have been reading this forum for a while. This is a great forum with a lot of helpful people.

 

I have a CDK 12,5" with a secondary that seems to be spot-on, following the instructions from the manual.

 

However, when looking at the star shares, in particular in the field, they are "blurred" and not clearly defined - see attachment. What can i do to get this corrected?

 

There some more files here, with defocused stars and the full .fits.

 

https://1drv.ms/u/s!...LAVssw?e=b2nUAV

 

Any help is highly appreciated!

 

Lucas

 

Attached Thumbnails

  • 60sec_L_1200.jpg

Edited by AstroLF, 25 July 2021 - 08:59 AM.


#2 akulapanam

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Posted 25 July 2021 - 10:35 AM

Yeah the field doesn’t look flat. Are you sure your backfocus is correct? Could also be a mirror spacing issue.

#3 rockstarbill

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Posted 25 July 2021 - 12:47 PM

That flaring problem is due to mirror spacing being incorrect. The CDK scopes should come with a ronchi eyepiece and adapters that you can use to correct the spacing. Instructions are in the manual.

#4 AstroLF

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Posted 06 August 2021 - 03:51 PM

Sorry for the late answer but i wanted to check spacing. Its accurate - when i look through the ronchi adapter, i see a white donut and no lines. The lines appear when i pull out the eyepiece.

 

So assume i can exclude spacing as the cause of the issue ...



#5 photoracer18

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Posted 09 August 2021 - 03:46 PM

Yeah but that error is definitely an optical spacing issue. It shows as the opposite of coma in a way, like the center of the field is a high spot as in out of focus. See how the inner edge of the stars is fuzzy all around the center of the field.



#6 carolinaskies

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Posted 09 August 2021 - 05:37 PM

If you take a close look at your images you'll see the stars are all drawing towards the center of the frame.
This means you've got a coma issue which means your secondary spacing is just a bit out. Did you have the Ronchi seated fully and furthest point in it could go?  If you had even 1mm gap this would cause an issue as the Ronchi test is to precisely set the spacing and if the adapter or the ronchi grate aren't properly seated you won't get the proper spacing.  

It amazes me that no CDK maker or vendor has a good set of examples of CDK spacing images.  It would be good to have a field like you shot showing too far, too close, and correct spacing.  




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