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What's causing brown EOF rings on an ES 82 deg. 18mm?

Explore Scientific Eyepieces
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#1 Taosmath

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Posted 09 May 2024 - 11:09 PM

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



#2 astropgr

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Posted 09 May 2024 - 11:40 PM

If it's new, then it's under warranty and ES should take care of you. If not, then seperating the elements could damage the waterproofing and the argon purge. It could have already happened though. Other than that, I'm unsure what the cause would be without seieng it myself or seeing a picture. 



#3 Ernest_SPB

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Posted 10 May 2024 - 01:06 AM

Hm...

 

What's difference! In TeleVue eyepieces it is named "fire ring", in Chinese ES - "brown"... smile.gif

 

18mm ES82 is astronomical eyepiece with relatively tight exit pupil and do not try it in daytime when interaction of SA/CA in exit pupil of the eyepiece and small pupil of observer produces quite known set of artifacts... In night time it works fine. .     


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#4 Jon Isaacs

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Posted 10 May 2024 - 04:08 AM

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

 

As Ernest said, this is quite normal for certain 82 degree and wider eyepieces.  

 

Jon



#5 Astrojensen

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Posted 10 May 2024 - 04:15 AM

As Ernest said, this is quite normal for certain 82 degree and wider eyepieces.  

 

Jon

Yes. And it shows up because light at the edge of the field of view have to pass through the lenses at such a steep angle that the coatings can't handle it and acts like a filter that blocks certain wavelengths, depending on the incident angle, creating the rainbow effect. There is no cure for this, except removing the coatings... 

 

 

Clear skies!

Thomas, Denmark


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#6 SeattleScott

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Posted 10 May 2024 - 08:26 AM

Hm...

What's difference! In TeleVue eyepieces it is named "fire ring", in Chinese ES - "brown"... smile.gif

18mm ES82 is astronomical eyepiece with relatively tight exit pupil and do not try it in daytime when interaction of SA/CA in exit pupil of the eyepiece and small pupil of observer produces quite known set of artifacts... In night time it works fine. .

Correct, we have to make it sound more exciting for the Naglers.

And correct, it is invisible at night, unless viewing the Moon. Which one typically doesn’t use a 2” ultrawide eyepiece for.
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#7 Starman1

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Posted 11 May 2024 - 11:49 AM

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.


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#8 Jon Isaacs

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Posted 11 May 2024 - 12:46 PM

Just to add to Don's excellent post:

 

For terrestrial viewing, I find eyepieces like the 30 mm UFF and 35 mm Panoptic preferable to the 31 mm Nagler types. More eye relief and a field that is plenty wide and no ring of fire.

 

Jon



#9 j.gardavsky

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Posted 11 May 2024 - 01:04 PM

Color fringes, an rings of fire

 

DOCTER, Nikon NAV (5mm), short focus Pentax XWs: Thin blue fringe

TV Delos: Wider blue fringe

Pentax O Abbe orthos: Thin orange fringe
Spotting eyepieces Swarovski and Zeiss: Thin orange fringe

 

Most of my eyepieces for my telescope have no visible color fringes on the field stop at all.

 

A pronounced "ring of fire" indicates the color magnification difference (CMD): For the different wavelengths of light the eyepiece delivers a different magnification.

Such designed eyepieces have been used to compensate the opposite CMD of the apochromatic microscope objectives in the past.

And these eyepieces have been called "compensating", and eventually marked with a red dot.

 

Best,

JG




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