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Eyepieces and Chromatic Aberration

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#1 penguinx64

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Posted 18 April 2019 - 01:50 PM

I never really noticed Chromatic Aberration or CA, until I got a refractor scope.  Then, I noticed some eyepieces also cause CA.  A good example is my Meade 4000 8-24mm zoom eyepiece.  I had problems focusing zoomed in to high magnification and the problem turned out to be CA.  I even had the same problem using the zoom in my reflector scope.  I though something was wrong with the eyepiece.  The real problem was CA.  I started using a Baader Contrast Booster Filter and now  the CA is practically gone.  I looked at the Moon tonight with the zoom eyepiece and the Contrast Booster filter.  Zoomed in all the way, I hardly noticed any CA at all.  I see CA using a cheap SVBony 23mm Aspheric eyepiece without the filter and with inexpensive Barlows without the filter too.  Now I keep the Contrast Booster filter attached to my refractor's diagonal all the time.  I'm curious to try this filter with these eyepieces in my reflector scope sometime.  Anybody else notice eyepiece CA before?


Edited by penguinx64, 18 April 2019 - 01:53 PM.


#2 db2005

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Posted 18 April 2019 - 02:22 PM

Yup, I've seen plenty of eyepieces cause false color, in some cases even to a very high degree. False color caused by the eyepieces is extremely common, especially in inexpensive, simpler eyepieces, but also some complex eyepieces are prone to showing false color. However, In high-quality eyepieces the CA and lateral color are typically confined to the edge of the field of view and usually barely detectable elsewhere. I have found my ~15 year old Series 4000 eyepieces to add a distinct yellowish hue to the views and usually adding yellow color fringes, but otherwise I found them to be quite good performers for the asking price at the time when I purchased them. By contrast, my Pentax XWs barely show any false color except at the very edge.

 

The focal ratio of the telescope also significantly affects eyepiece performance: many simple and complex eyepiece designs alike that may perform excellently at, say, f/9 frequently perform poorly in fast scopes, for instance an f/5 short tube refractor or a fast reflector. A large part of the relatively asking price of premium quality eyepieces vs inexpensive eyepieces is justified by their excellent performance and outstanding correction of optical aberrations even when used with fast telescopes. This is one reason why owners of quality APOs usually also find it worthwhile to spend significant $ on eyepieces: the price of a truly high quality refractor is hard to justify unless one is also willing to spend enough money on quality eyepieces which can bring out the best performance of the scope.

 

CS,

Daniel


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#3 Migwan

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Posted 18 April 2019 - 02:47 PM

Yup.  I have two short tube refractors.  You will definitely see CA increase at higher powers.  As I don't use high power much on these two scopes, the CA is not really noticeable.   I once noticed some serious CA with a 22T4 on the ST120.  That episode turned out to be lose screw on the tube that held the focuser.  I tightened them down and problem solved.  However, if I apply enough lateral tension on that focuser, the CA kicks up.  So a weak out of line focuser can cause problems too.

 

jd    


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#4 Ernest_SPB

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Posted 19 April 2019 - 01:37 AM

"CA" is too generic.

We have at least two basically different CA: axial (symmetric bluish halo around a star image) and chromatic defect of magnification (bluish tail from a star image - on edge FOV). 

The first is very unusual for eyepieces (and most probably coming from main scope lens), the second is in opposite - usual and can be observed in most eyepieces (closer to FOV edge or even in center when observer eye has strong displacement relative scope exit pupil).


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#5 JIMZ7

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Posted 19 April 2019 - 07:27 AM

I once purchased a Baader 8mm Hyperion for my Meade 826C  8" f/6 equatorial reflector. Never thought a eyepiece on a reflector would cause any CA. It was even worse. A spectrum of colors on 1st & 2nd magnitude stars. And it wasn't even raining.

Jim




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