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Atmospheric Dispersion or Focuser/ Sensor Tilt? Yellow/Blue fringing. Any ideas?

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#1 aatdalton

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Posted 25 August 2019 - 03:59 PM

I've recently been trying to get the kinks out of my imaging rig and I've come across an interesting issues. I get pretty bad yellow/ blue fringing on my stars right now and I'm wondering if it's an issue with the atmosphere or with my imaging train. The misalignment is almost exactly Up-Down or North-South in DEC, which is also the direction my DSLR would hang out of the focuser tube.

 

My imaging train is: 

  • SW 130PDS
  • SW 2-speed focuser - 2 thumb screws
  • GSO 2" Coma Corrector 
  • 20mm of threaded spacers
  • T-Ring to EOS Adapter
  • Canon 80D unmodified

These images are from very near the center of the image and from the extreme bottom left. I also included a lunar image with the saturation boosted to exaggerate the effect. I believe the orientation of the scope and camera would have been flipped between the stars and moon shots. Any ideas? Coma corrector spacing? Sag? Atmosphere?

 

Thanks for looking!

 

Center Left.jpg

 

LowerRight.jpg

 

Moon TIFSmyoon251789-2.jpg



#2 rockstarbill

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Posted 25 August 2019 - 04:09 PM

Likely CA caused by the coma corrector.
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#3 TOMDEY

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Posted 25 August 2019 - 05:35 PM

It's interesting that is splays pure lateral vector (vs radial) --- some sort of misalignment of a refractive element or group. Not likely the atmosphere.    Tom



#4 freestar8n

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Posted 25 August 2019 - 10:13 PM

To me it looks like dispersion if it is truly aligned up and down with vertical. I would try a shot near the zenith.

What was the altitude above horizon for those pics?

Frank

#5 AtmosFearIC

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Posted 26 August 2019 - 02:16 AM

I get this on anything below 40ยบ with my Mewlon but not when pointing higher so I know that it is dispersion in that case.

If it happens irrespective of altitude (happens at zenith) then it is most likely lateral CA introduced by the coma corrector. This is something I noticed on a Sigma 24mm F/2.8 I used to own.




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