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what decides the color of the Chromatic Aberration?

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#1 iseegeorgesstar

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Posted 23 September 2022 - 05:19 PM

This might not be subject to one factor. But basically I was wondering what if anything determines the color band of the CA seen in a scope? Does it have to do with how the lens might be figured such as it's figured more into the green wave length than the red? Or due to atmospheric conditions? Etc. 

 

For example, not always perhaps but I feel like I tend to see red CA most often on the limb of a full moon in my doublet. I do see other colors as well such as turquoise/yellow (or perhaps the latter is in whitelight with a hershel wedge). But I was wondering does seeing the red fringe mean my scope isn't figured into the red wavelength? Apparently most scopes aren't figured into the IR end since our eyes see green the best (?)

 

Is the CA dependent on the type of object perhaps and whether it reflects light or is a light source?


Edited by iseegeorgesstar, 23 September 2022 - 05:22 PM.


#2 Ihtegla Sar

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Posted 23 September 2022 - 05:28 PM

How high above the horizon was the moon at the time?

Red on the moon limb could be atmospheric dispersion rather than chromatic abberation. Atmospheric dispersion is the atmosphere bending light like a prism. Its worse the lower an object is to the horizon but still visible above 60 degrees at very high magnifications. On a bright star it looks like blue on top and red on the bottom.

Tourquoise and yellow sound like chromatic abberation.

Edited by Ihtegla Sar, 23 September 2022 - 05:29 PM.

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#3 iseegeorgesstar

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Posted 23 September 2022 - 06:13 PM

This wasn't recently so it's hard to tell. I recall one viewing session where it was about 3/5th's above the horizon towards zenith. However I also remember catching the moon on the horizon so I really don't know.

 

This was more full view of the moon not super zoomed in. And to be clear, it's not a line of CA. Just this fringe that phases in and out of existence is the best way I can describe it. 

 

Besides wanting to know more. I was also asking because I read an interesting comment that high end refractors aren't always optimized towards the red end thus for halpha viewing an achromat might be better suited in that regard, in addition to cost and not needing the extra glass because you're only working with one wavelength, et cetera.



#4 KBHornblower

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Posted 23 September 2022 - 07:08 PM

For H alpha viewing, I would just put the filter in place and then find the best focus for that wavelength.  The color correction of the objective would be immaterial.

 

A typical achromat doublet is designed to bring the Fraunhofer C and F wavelengths (H alpha and H beta respectively) to the same focus.  Those are red and cyan tints.  The yellow and green in the middle come to a slightly shorter focus.  When we focus to the sharpest overall look, the extreme red and blue ends of the spectrum form a magenta fringe around a star.

 

I don't know what the lesser amount of false color in an apochromatic triplet would look like.


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#5 PKDfan

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Posted 23 September 2022 - 07:25 PM

Hi iseegeorgesstar!

The way i process optical rankings.

Every objective will perform slightly different depending upon the formula and glass quality with the Focal ratio as the prime driver of performance limits. Never will you get EXACTLY the same performance i.e. 99.32 Strehl or 99.33 etc.


An object under 45degrees usually displays some AD BUT its highly dependent upon stability of the air column.


The correction is wholly dependent upon the human implimentation of the formula-

Glass quality, polish and lens cell construction.

A triplet for AP
A doublet for visual
The usual paradigm.


The slower the F ratio the less steep the curves to cut. Equals more consistent quality.

IME when done superb a slow F scope is the BEST. BAR NONE.


A flourite/FPL-53 lens will usually have better performance at the long end of the spectrum rather than the shorter with the third element of a triplet there usually to helping out the blue corrections past it towards the violet.




Clear highest quality glass striae/stress free skies
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#6 iseegeorgesstar

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Posted 24 September 2022 - 11:39 AM

Thank you for both of the replies. Things make a little more sense now. 

 

I sometimes wonder if I can't see a tint using certain EPs but at that point I think it's more the EP and not the lens objective. Thank you.



#7 RichA

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Posted 24 September 2022 - 05:59 PM

This might not be subject to one factor. But basically I was wondering what if anything determines the color band of the CA seen in a scope? Does it have to do with how the lens might be figured such as it's figured more into the green wave length than the red? Or due to atmospheric conditions? Etc. 

 

For example, not always perhaps but I feel like I tend to see red CA most often on the limb of a full moon in my doublet. I do see other colors as well such as turquoise/yellow (or perhaps the latter is in whitelight with a hershel wedge). But I was wondering does seeing the red fringe mean my scope isn't figured into the red wavelength? Apparently most scopes aren't figured into the IR end since our eyes see green the best (?)

 

Is the CA dependent on the type of object perhaps and whether it reflects light or is a light source?

The lens.  How it is corrected.  I've seen some weird CA colours (green, cyan, pink) on scopes and some lenses used as scopes which were not designed as scopes.  Mostly, you see purplish-blue in-focus and blue or red on either side of focus, for conventionally-corrected lenses.  Defocussed blurs (around bright objects) are composed mostly of blue.  The emission of the light source matters a bit but since most targets and light sources are polychromatic, they exhibit the same CA colours.  Fascinating subject.




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