I got a N2 purged 18mm ES 82 eyepiece in a bundle and when I checked it out in the daytime, all the way round the outer 5% of the field of view I saw a brown/blue ring. I wondered if it might be some form of CA. The unit was in VG cosmetic condition and as far as I could tell in a casual daytime inspection, the rest of the FOV looked fine. I got the same effect in two different scopes (an F5 dob and an F7 Achro)
Does anyone have ideas what might be the cause of that and if there's anything I can do to remedy it (element reversal, Field stop missing/displaced, etc etc....)?
If I can't fix it, it's not a disaster since the rest of the bundle was easily worth what was paid and it may well be invisible at night but I haven't been able to do any night viewing with it yet (new gear, rain; you know the drill). However I would like to have an ES 18mm in the quiver if I can fix it.
I'd appreciate any suggestions.
Thanks
You have 2 issues:
--the blue ring at the very edge is because blue and red are bent differently by the negative field lens and the red is interrupted by the internal baffle, leaving blue at the very edge.
That is extremely common in eyepieces--to the point where eyepieces without it are a distinct minority. You don't see it at night unless the moon crosses that point.
It is an extremely thin ring right at the field stop.
--the brown color at and near the edge, which may extend inward from the field stop several degrees, is chromatic aberration of the exit pupil, or CAEP.
It is caused by different colors having different eye reliefs at the eyepiece, such that when your eye is at the exit pupil, the outer area in the eyepiece has a tint.
Changing the distance to the eyepiece can mitigate the effect, but may also cause blackouts.
It is a sign that not all colors are refracted equally through the eyepiece.
Brown usually indicates there is some inherent vignetting in the eyepiece that dims the coloration or is a mix of different colors.
In the "patron saint" of CAEP eyepieces, the 31mm Nagler, the outer field is tinted orange-red, hence the name "Ring of Fire".
Lots of eyepieces have this issue--Baader Morpheus, Pentax XW, Tele Vue Nagler Type 5, Explore Scientific 82°, United Optics 82° eyepieces, etc.
The issue is not seen at night unless the Moon crosses that part of the field, and it is most noticeable in long focal lengths of eyepieces.
The cure is to add more lenses to the eyepiece to fully correct color, but that adds cost and weight, and since it is not an issue at night, it's considered a reasonable optical compromise.
Imagine the 31mm Nagler's cost and weight with 3 more internal lenses!
It is similar to (but not identical to) longitudinal chromatic aberration, except it is not produced by the telescope's objective, but the eyepiece.
