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ZS61II Star Shape Issue

Astrophotography Beginner Collimation Imaging Optics Refractor
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#1 MDL1

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Posted 26 January 2022 - 11:10 AM

Need help diagnosing this issue and recommendations on a solution.  My WO ZS61 is not producing tight round stars.  The scope was bought new 1 year ago when I started astrophotography.  As a beginner, I didn't notice how misshapen the stars are on the ZS61 until I upgraded to a GT81.  I've looked through the history of the images I've taken with the ZS61 using DSLR, ASI533 and ASI 183MC Pro and the defraction pattern is present in all of them.  Do I have pinched optics, collimation issues, both?  Is there a DIY solution or do/can I send back to WO to fix?  The image is a tight crop of one sub that has a minimal stretch. 

 

 

ZS61 single image minor stretch.jpg

 

 

Thanks for your thoughts and clear skies.

 

 


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#2 crowley213

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Posted 26 January 2022 - 12:21 PM

As I am also just a beginner I may most likely lack the knowledge and experience to present you a solution.

 

But as I am also using the WO ZS61II here some thoughts from my side:

- Which flattener do you use? Is the flattener correctly adjusted?

- Could it be a focus issue? I can observe in my telescope that depending on how far I extend the focusser and from which direction I try to achieve focus there a slight variations.

- Is it possible that the form of the stars is related to a not completely stable mount / setup, means small vibrations when capturing the images? What mount and tripod do you use?

- Is this pattern also visible when exposing less, means when no "bloating" is visible?

 

Again, also just a beginner and just some ideas from my side.

 

Herbert



#3 MDL1

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Posted 26 January 2022 - 04:15 PM

Herbert--all great questions...

 

The mount is an EQ6R PRO on a pier and I use an ASIAIR with multi-star tracking...pretty much always less than 1 arcsecond combined between RA and DEC.  The flattener is WO flattener for the scope--the Flat 61.  The back-focus seems right since I can easily achieve focus with the integral Bahtinov and I've recently added an EAF.  I've looked at various exposure lengths and gain--60 seconds, 120 seconds and 180 seconds.  The above picture is a heavily cropped single 120 sec exposure with minimal stretch at gain 50 using 183MC pro, but I still see the same pattern on shorter exposures and different gains with the 183, 533 and DSLR. It is really pronounced on mid to large stars--the above star is a pinpoint in the uncropped image.



#4 MDL1

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Posted 26 January 2022 - 04:46 PM

This is another image with zero stretch and less crop

 

61 full size unstretched image copy.jpg

 

Maybe I'm expecting too much?



#5 crowley213

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Posted 26 January 2022 - 04:57 PM

Okay, considering your way more sophisticated mount and setup than I have I cannot imagine that they are the root cause…!

 

Just one additional question: your flattener, is it the adjustable flattener, means the so called “New Adjustable Flat61 for Z61”?

 

Herbert



#6 MDL1

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Posted 26 January 2022 - 05:20 PM

Herbet--it is the new adjustable flattener for the Z61.  Mike



#7 Holtdoa

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Posted 26 January 2022 - 11:38 PM

I'm no expert, but recently I got interested in what measurements there were for "seeing". In the typical rambling bouncing around the www and YouTube the discussion was dealing in how many arc seconds the current "seeing" would permit with a given instrument. This moved into arc seconds per pixel for a given camera and what the results would be for over and under sampling. That discussion described how stars would appear a little blocky or pixelated. My first look at your pic struck me as a little blocky. Maybe your camera and your first scope don't "match". The article I was reading spoke about adjusting bining to get the "pixel size" closer to the scopes sweet spot. I'm probably all wet, but maybe the folks explain how dumb this post is cam help wit your issue 😀.

#8 crowley213

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Posted 27 January 2022 - 12:57 AM

Herbet--it is the new adjustable flattener for the Z61.  Mike

Hmm, then I am out of ideas what could be the root cause for this problem, sorry!

 

Herbert




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