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Strange Red Star Fringing

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#1 AstroPics

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Posted 07 December 2018 - 10:52 AM

Anyone have any idea what could be causing this?

 

I am seeing red fringes around my stars. This is using my AT65EDQ which I have never had an issue with before. However, I just recently tightened the retaining ring on the front lens element and corrector after noticing it was much too loose (a previous imaging session had elongation in my stars). That resolved the elongation but now I see these red fringes...

 

Sample1.jpg

 

Sample2.jpg



#2 Jerry Lodriguss

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Posted 07 December 2018 - 11:31 AM

How high was the field you were shooting?  

 

This looks like prismatic atmospheric diffraction. That's in the sky, not in your scope.

 

Normally it's blue on the top and red on the bottom, but I don't know how your scope was oriented. The lower in the sky, the worse it is.

 

Jerry



#3 Brett Waller

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Posted 07 December 2018 - 11:52 AM

If the alignment of fringing is consistent all across the image, and if the orientation of the fringing is along the vertical direction (not north) from your location, then the problem is most likely atmospheric dispersion.  What was the altitude of your object at the time the image was taken? Atmospheric dispersion becomes markedly worse as you decrease in altitude.

 

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#4 AstroPics

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Posted 07 December 2018 - 01:17 PM

How high was the field you were shooting?  

 

This looks like prismatic atmospheric diffraction. That's in the sky, not in your scope.

 

Normally it's blue on the top and red on the bottom, but I don't know how your scope was oriented. The lower in the sky, the worse it is.

 

Jerry

 

If the alignment of fringing is consistent all across the image, and if the orientation of the fringing is along the vertical direction (not north) from your location, then the problem is most likely atmospheric dispersion.  What was the altitude of your object at the time the image was taken? Atmospheric dispersion becomes markedly worse as you decrease in altitude.

 

Brett

Brett/Jerry,

 

The object is M74 which I started shooting right at dusk when the object was at an altitude of 52 degrees. It was a pretty long run for about 5 hours total so it passed the meridian at an altitude of 72 degrees and when the imaging was done, it was back down to an altitude of 52 degrees. 

 

It was a combination of RGB and Lum frames with the Lum frames taken at the beginning and end of imaging while G and B were taken close to the meridian and R taken slightly off from the meridian.

 

The orientation of the red is predominantly to the North to North-East in the image though although the fringing is consistent across all the stars in the image.

 

Does this still sound like atmospheric dispersion? If so, is there much that can be done to minimize it?

 

Kevin



#5 Der_Pit

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Posted 07 December 2018 - 03:40 PM

Given the altitudes you mention, rather not.  And as you had recently worked on the lens, I'd try to get a cheshire and/or Ronchi eyepiece and check the collimation.  Might be you tightened it too much - optics don't like physical stress.



#6 AstroPics

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Posted 07 December 2018 - 04:53 PM

Given the altitudes you mention, rather not.  And as you had recently worked on the lens, I'd try to get a cheshire and/or Ronchi eyepiece and check the collimation.  Might be you tightened it too much - optics don't like physical stress.

So I checked the collimation of the refractor using a Hubble Optics Artificial Star and the collimation looks perfect... So I don't think that is it either. 

 

Any other thoughts about what could be causing the effect?

 

The effect has shown up on both my ASI1600 and ASI183 so that would exclude the cameras. The scope seems to check out as well. The only other intervening items in the optical train include an OAG and the filter wheel.

 

If it isn't atmospheric diffraction, are there other areas where diffraction could occur? It does seem like there is some color diffraction in the system.



#7 Jerry Lodriguss

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Posted 07 December 2018 - 05:45 PM

So I checked the collimation of the refractor using a Hubble Optics Artificial Star and the collimation looks perfect... So I don't think that is it either. 

 

Any other thoughts about what could be causing the effect?

 

The effect has shown up on both my ASI1600 and ASI183 so that would exclude the cameras. The scope seems to check out as well. The only other intervening items in the optical train include an OAG and the filter wheel.

 

If it isn't atmospheric diffraction, are there other areas where diffraction could occur? It does seem like there is some color diffraction in the system.

Collimation of refractors is generally not a problem,  but that doesn't mean you might not have some kind of tilted element, or who knows what in the optics.

 

PAD would have been so simple. That's not something you can really fix when you shoot, but you can align the channels in processing. You can do that in the meantime while searching for a scope setup problem.

 

Jerry



#8 AstroPics

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Posted 07 December 2018 - 06:19 PM

Collimation of refractors is generally not a problem,  but that doesn't mean you might not have some kind of tilted element, or who knows what in the optics.

 

PAD would have been so simple. That's not something you can really fix when you shoot, but you can align the channels in processing. You can do that in the meantime while searching for a scope setup problem.

 

Jerry

When I had the elongation in the stars, I went through everything to eliminate tilt but I can always double check that.

 

A thought just occurred to me though... I've seen the effect the last few imaging sessions. Once with the ASI183 and twice with the ASI1600. Both nights, I had dew collecting on the primary lens. I've been waiting for my dew heaters to come in.

 

Anyway, this wouldn't by chance cause a diffraction effect?




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