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How can I tell if I have Chromatic aberration in my APO?

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#1 Midnight Skies

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Posted 20 September 2020 - 01:24 AM

I apologize for this stupid question; I am not very knowledgable... I have an ES ED102-FCD100 scope (Apo triplet), I love it.  But tonight I was looking at the planets (Jupiter, Saturn, Mars) and I could swear I was consistently seeing colour that shouldn't be there - blue in the upper left of the planet, and red in the lower right.  

 

The colours appeared to be there regardless of planet or eyepiece.  So now I'm worried.  Does my scope have a problem?  Or could there be some other explanation?  Or could I just be imagining it?

 

I cannot ask anyone else to look and check (covid isolation) nor go to anyone else, nor do I have another Apo to compare against.  I also can't take an image of it (tried, failed).  Is there any way I can check or test I can easily do?

 

Please tell me it's something stupid and I'm just asking a dumb question.  Thanks in advance to any responses.

 



#2 Jon Isaacs

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Posted 20 September 2020 - 01:41 AM

I apologize for this stupid question; I am not very knowledgable... I have an ES ED102-FCD100 scope (Apo triplet), I love it.  But tonight I was looking at the planets (Jupiter, Saturn, Mars) and I could swear I was consistently seeing colour that shouldn't be there - blue in the upper left of the planet, and red in the lower right.  

 

The colours appeared to be there regardless of planet or eyepiece.  So now I'm worried.  Does my scope have a problem?  Or could there be some other explanation?  Or could I just be imagining it?

 

I cannot ask anyone else to look and check (covid isolation) nor go to anyone else, nor do I have another Apo to compare against.  I also can't take an image of it (tried, failed).  Is there any way I can check or test I can easily do?

 

Please tell me it's something stupid and I'm just asking a dumb question.  Thanks in advance to any responses.

 

Hi:

 

Your question is not a dumb question nor is it something stupid.  You saw what you saw and it would be visible in a Newtonian or other telescope with perfect color correction. 

 

What you are seeing is chromatic aberration but it is not your objective that is causing it, it is caused by the atmosphere. It is called atmospheric dispersion and is the result of the planets being relatively close to the horizon. 

 

They make ADC, atmospheric dispersion correctors.  

 

Jon


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#3 Astrojensen

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Posted 20 September 2020 - 02:11 AM

Yes, that's atmospheric dispersion. You can get rid of it with an atmospheric dispersion corrector, as Jon points out, but you can also use the lateral color in your eyepieces to do the same (unless your eyepieces are highly corrected for it, which some advanced eyepieces are), by moving the planet off-center on the eyepiece, usually towards the lower half. It works in my orthoscopics, plössls and ES 82's. 

 

 

Clear skies!
Thomas, Denmark


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#4 havasman

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Posted 20 September 2020 - 03:01 AM

And the very high level technology of your scope's glass selection and coatings should give you confidence in the integrity of the images you enjoy, especially now that you know what you were seeing. Sometimes it is rare and always it is important to have justifiable confidence in your gear.

 

Concern is natural. Your Q is a very good one and you got a good true answer right away. And Thomas' suggestion is new to me and something I will try.


Edited by havasman, 20 September 2020 - 03:02 AM.

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#5 Midnight Skies

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Posted 20 September 2020 - 10:00 AM

Oh my gosh, thank you all so very much for the explanation!  Wow.  I can breathe again lol. 

 

I try to study a lot (it's been cloudy so long) and read Cloudy Nights regularly but hadn't learned about atmospheric dispersion and its effects until last night.  It was a beautiful night and I did have lovely views, excepting the scare of seeing colour.  I feel relieved.  I do love my 102, it's just the right size and it's so sharp to look both through and at.  

 

Thank you again, wow, what a relief!


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#6 Wildetelescope

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Posted 21 September 2020 - 07:00 PM

Hi:

 

Your question is not a dumb question nor is it something stupid.  You saw what you saw and it would be visible in a Newtonian or other telescope with perfect color correction. 

 

What you are seeing is chromatic aberration but it is not your objective that is causing it, it is caused by the atmosphere. It is called atmospheric dispersion and is the result of the planets being relatively close to the horizon. 

 

They make ADC, atmospheric dispersion correctors.  

 

Jon

Absolutely!   No worries!  You have a good scope.

 

JMD


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#7 Wildetelescope

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Posted 21 September 2020 - 07:05 PM

Oh my gosh, thank you all so very much for the explanation!  Wow.  I can breathe again lol. 

 

I try to study a lot (it's been cloudy so long) and read Cloudy Nights regularly but hadn't learned about atmospheric dispersion and its effects until last night.  It was a beautiful night and I did have lovely views, excepting the scare of seeing colour.  I feel relieved.  I do love my 102, it's just the right size and it's so sharp to look both through and at.  

 

Thank you again, wow, what a relief!

No worries! This happens to the best of us.   Every once and a while I look through one of my refractors at low power and panic that all the stars have tails and my heart stops.  Until I remember that I have strong astigmatism in both eyes and I need to put my durn glasses on!:-).  Enjoy your very nice scope!

 

Cheers!

 

JMD


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#8 gnowellsct

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Posted 21 September 2020 - 10:50 PM

The point of a good triplet apo is to get the most accurate rendition possible of the colors related to atmospheric dispersion. You don't want to mess that effect up with false color from an achromat. A well corrected Apo will give you redder reds and bluer blues on those low planet fringes than some random achromat. Be proud of your atmospheric dispersion and your apo making it possible for you to see what the good Lord put there for all of us to see.
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#9 civis

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Posted 29 September 2020 - 08:46 PM

I apologize for this stupid question; I am not very knowledgable... I have an ES ED102-FCD100 scope (Apo triplet), I love it.  But tonight I was looking at the planets (Jupiter, Saturn, Mars) and I could swear I was consistently seeing colour that shouldn't be there - blue in the upper left of the planet, and red in the lower right.  

 

The colours appeared to be there regardless of planet or eyepiece.  So now I'm worried.  Does my scope have a problem?  Or could there be some other explanation?  Or could I just be imagining it?

 

Do a star test on a bright one, like Vega, your diffraction patterns in and out should be close to symmetrical, also notice any color fringing (CA) when in focus.

https://www.youtube....h?v=fXQSaIL4WpI
 




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