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weird CA

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#1 oldtimer

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Posted 26 June 2019 - 09:22 PM

I picked up a 90mm F-6.7 achro. I knew it was going to have abundant CA at higher power but the ca is weird . I spent some time aligning the optics with a Cheshire . With Jupiter at 100X the CA was shifted to just one side of the planet. What is this telling me?



#2 Jim Waters

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Posted 26 June 2019 - 09:27 PM

What's the scope?   Likely it still needs aligning.  Does the lens cell allow for aligning one or two lens?

 

EDIT - Is it a Meade?

 

https://www.cloudyni...nd-the-focuser/


Edited by Jim Waters, 26 June 2019 - 09:43 PM.


#3 oldtimer

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Posted 26 June 2019 - 09:44 PM

Its a Gskyer. The lenses are in a metal cell but it is not a push/pull alignment cell. I have played with the focuser and am confident it is square with the tube. I guess I am going to see if the cell can be removed from the tube. That may be a problem as I don't think it is screwed in. 



#4 Jim Waters

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Posted 26 June 2019 - 09:50 PM

Are the lens plastic, coated...?



#5 Richard O'Neill

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Posted 26 June 2019 - 10:44 PM

Atmospheric refraction?


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#6 petert913

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Posted 26 June 2019 - 10:51 PM

"Its a Gskyer".....that could be the problem grin.gif



#7 mistateo

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Posted 26 June 2019 - 11:05 PM

You may be confusing chromatic aberration with atmospheric dispersion?  With atmospheric dispersion, there is a blue cast on top and red on bottom (or vise versa depending on if using a diagonal or not).  This effect gets much more prevalent the lower a planet is in the sky, as you are looking through much more atmosphere near the horizon than pointing upwards.  Given that the planets are low in the northern hemisphere this year and you are in IL, I think this is what you are seeing. Try pointing at a really bright star near zenith with a good amount of magnification.  Or same thing with the moon at it's highest point in the sky.  That should show you the chromatic aberration of your scope.


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#8 SeattleScott

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Posted 26 June 2019 - 11:20 PM

Yes I was thinking atmospheric dispersion based on the description. And the position of the planets this year. There is a simple solution for atmospheric dispersion. Move to Hawaii.
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#9 oldtimer

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Posted 27 June 2019 - 12:02 PM

Very interesting. I must experiment.



#10 Alan French

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Posted 27 June 2019 - 01:15 PM

Rotate the tube and see if the chromatic oddity stays put or rotates.

 

Clear skies, Alan


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#11 oldtimer

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Posted 27 June 2019 - 06:51 PM

It stays put.  Logic seems (?) to suggest the optics are not the problem. I need some bright overhead stars to do more testing.


Edited by oldtimer, 27 June 2019 - 06:53 PM.


#12 oldtimer

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Posted 28 June 2019 - 01:03 PM

OK more description on my weird CA. When looking at Jupiter I am NOT seeing purple,red or green fringing. What I was seeing is a partial second dimmer white/yellowJupiter protruding from only one direction. Is this what some here have been calling atmospheric refraction?

 

Secondly a previously poster to this thread said that my problem was that the scope was a Gskyer. I ask this poster if he has had a bad experience with this brand.



#13 Redbetter

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Posted 28 June 2019 - 04:48 PM

I don't understand how what you are describing could stay put when rotating the OTA.  While this indicates it is not the objective that has a problem, your other observations say it isn't atmospheric dispersion. 

 

Are you rotating the diagonal when you do this?  Or leaving it in the same (original) orientation with respect to the tube?  If you are rotating the diagonal for the same viewing angle as before (resulting in a different angle with respect to the OTA) then you have just isolated the problem:  the diagonal.  It could be that the diagonal is cocked or that it has some sort of defect or it could be slop in the way it sits in the focuser that results in in being skewed.

 

If you are leaving the diagonal in the original orientation when rotating the tube (so that the diagonal is now pointed in a different position relative to the horizon than it was before) then the problem would likely be some sort of sag in the assembly...although with the angle of the diagonal relative to the horizon changing it might be expected to so some sort of left/right flip...not sure on this latter but it complicates things.

 

Are you testing with a diagonal that you already know to be good and, most importantly, well aligned?  In my experience cheap 1.25" diagonals are almost never properly aligned or they have some sort of other major defect.  What type of diagonal?  Because whether this proves to be the diagonal or somewhere else it sounds like misalignment of the backend/focuser/diagonal/eyepiece tilt.  I have seen this with simple eyepiece tilt in a straight through ~f/5--and the actual tilt of the eyepiece was not obvious, but the impact on the image was.  A short refactor collimation tube can help identifying misalignment.

 

Have you done a star test?  It would be a lot simpler to start with a bright star placed higher in the sky and see whether it produces a uniform airy disk and first diffraction ring, or if it displays coma, astigmatism, etc.  If one side of the diffraction ring is bright and the other dim or non-existing then there is most likely some sort of misalignment that is causing coma and lateral color.  That could be in the objective, focuser, diagonal, etc.



#14 Alan French

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Posted 28 June 2019 - 06:14 PM

OK more description on my weird CA. When looking at Jupiter I am NOT seeing purple,red or green fringing. What I was seeing is a partial second dimmer white/yellowJupiter protruding from only one direction. Is this what some here have been calling atmospheric refraction?

 

 

From what I can find, it looks like this telescope has an "48-degree" erecting prism. If you are rotating the tube but leaving this is the same position, I suspect you may have some sort of ghost reflection from the erecting system.

 

See if it rotates with the erecting system. 

 

Clear skies, Alan



#15 KerryR

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Posted 28 June 2019 - 06:22 PM

From what I can find, it looks like this telescope has an "48-degree" erecting prism. If you are rotating the tube but leaving this is the same position, I suspect you may have some sort of ghost reflection from the erecting system.

 

See if it rotates with the erecting system. 

 

Clear skies, Alan

Yup. My money's here, if the scope is being used with said prism. Low end erecting prisms are notorious for doing wacky things to astro targets.

Test without the diagonal, straight through, assuming you can reach focus.



#16 SeattleScott

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Posted 28 June 2019 - 08:49 PM

Sounds like ghosting to me! Cheap optics tend to cause this. Could potentially be the diagonal or the eyepiece.

Scott

#17 oldtimer

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Posted 29 June 2019 - 09:23 AM

Finally a clear night tonight. I'm hoping 'ghosting' is the issue because that can be cured with a better diagonal. I'll know after tonight and post results. 



#18 oldtimer

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Posted 15 August 2019 - 01:40 PM

Mystery Solved

 

This is an airspaced lens. Close inspection revealed that one of the three spacers between the lenses had slipped way out of its 120 degree position to the other two spacers. This meant that the "3 points determine the plane' was not happening and the 2 lenses were not square to each other. Removing the lenses and placing the 3d spacer in the appropriate position solved the 'ghosting' problem.

 

Gary (oldtimer)


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