FreeLists / az-observing / [AZ-Observing] Re: The Orion 120mm ED Refractor

[Roger Ceragioli <rogerc@xxxxxxxxxxxxxxxxxxxx>]

Hi, Jack.

Dwight's post on various colored fringes with different eyepieces was a 
very good one.  Eyepieces almost all have considerable aberration:  
astigmatism, distortion, and lateral color are the most common.  And 
it's impossible (if I recollect correctly) to remove astigmatism and 
distortion simultaneously.  Al Nagler got rid of the astigmatism of 
earlier Ploessl, orthoscopic, and Erfle designs by introducing the 
little negative "Smyth" lens at the front of his design (although he 
wasn't the first person to do this; Zeiss had done it in the 50s, if I 
recollect correctly), but the cost was huge distortion of the pincushion 
variety.  This is very easily visible in the bigger Nagler eyepieces.  
My 31mm Nagler shows distortion and lateral color in spades!  But it's 
still a fun eyepiece.

And then there's your eye, which is a terrible optical device full of 
huge aberration.  The brain does a lot to filter that, and the 
construction of the retina helps out.  We only see sharply near the 
center of the retina in the fovea region, which covers a few degrees.  
Further out in the field, we all have very fuzzy (low resolution) 
vision.   Also, the eye is totally non-achromatic.  You can detect this 
rather easily by focusing on a bright white star and looking askance 
through the eyepiece.  Shift your eye to the side and you'll pretty soon 
start to see a red fringe on one side of the star.  This works best (the 
effect is most visible) on achromatic scopes, but should happen even on 
reflectors.  Having a large exit pupil feeding your eye should increase 
the effect.  I've often seen it on my achromatic refractors, even at 
high power.

Concerning the moon, I'm not sure why it happens, but in an achromat you 
always see a blue fringe around the limb and nothing much around the 
terminator.  But a yellow fringe around the limb would be from lateral 
color error, and probably even secondary lateral color, since you can 
achromatize lateral color too and be left with a residue of secondary 
lateral color.

Hope this may help,
Roger
> I think this may answer my next question of why is there a thin yellow fringe 
> around the curve of the Moon but on the terminator I don't see any fringe. I 
> think that's right; I'd go check that, but now it's full!
>
>   

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