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Yellow ring around the lunar limb.


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Have been out tonight with a Vixen SD81s observing the Moon and the eyepiece used was a Baader MkIV zoom in a Baader prism diagonal, no filters. The views were very crisp and clear with no false colour along the terminator, but the lunar limb was surrounded with a slight yellow ring. Is this something to do with the eyepiece or the atmosphere?

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Eyepiece probably. Zooms are less likely to be completely aberration free. You are most likely seeing some residual chromatic aberration.

It's not something I'd associate with a Vixen apo. They are usually well corrected.

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I think you could be right. I went back out and used a 6mm and 4mm SLV and the yellow ring was no more. Maybe the brightness of the moon coupled with the multi-element zoom was causing this yellow ring. In my achros the lunar limb is usually a purple colour but that's expected with those scopes. I also thought the prism diagonal may be having an effect as well. Will have to give the mirror a try.

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This I have seen before with my 102 depending on the eyepiece and the position of the limb within the FOV. On occasion I have seen this slight yellow border (very thin and faint) but I also noticed if I move the slow motion 

control so as to bring the limb closer to the centre of the FOV the line gradually disappears completely as the limb moves closer to centre. This does not happen with every eyepiece but because I was never too concerned

I didn't bother making note of the eyepiece. Don't let it bother you, there are so many variables in an optical train combined with atmospheric conditions it can split your head open trying to get to the root.

Edited by Sunshine
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