24 replies to this topic

[Astro Chimp]

While looking for more TeleVue eyepieces to acquire, I came across in my reading that some of them exibit the Smyth Lens phenomenon, or ''ring of fire'', while daytime using or moon viewing. The 31 Nagler T5 is notoriously one of them. That got me wondering, which of the other Tele Vue eyepieces are known for exhibiting that effect ? Is this ''aberration'' inherited from the design (so all N31 T5 would have it), or is it differently present within the eyepiece family ?

 

[triplemon]

I doubt that is specifically from the Smyth element - its lateral chromatic aberation caused by a number of lenses in the eyepiece. The more refraction, the more to correct for.
More or less all ultra wide eyepieces from all manufacturers have it, some more, some less.

Edited by triplemon, 18 February 2025 - 03:26 AM.

[SeattleScott]

It seems to me like the issue is with eyepieces that don’t have a Smyth lens.

In general, it is an issue with longer focal length ultrawide eyepieces. The 31T5 tends to get mentioned to show that it doesn’t make an eyepiece a bad eyepiece. Even the best can have it. The moral of the story seems to be, don’t use 2” ultrawide eyepieces for terrestrial or lunar viewing. Not too hard. I mean, just try to find a 45 degree erect image diagonal that won’t severely vignette a 31 Nagler. That thing wasn’t designed for terrestrial viewing.

It seems the Orion LHD series might not suffer from this effect, but they only go up to 20mm.

[Astro Chimp]

It seems to me like the issue is with eyepieces that don’t have a Smyth lens.

In general, it is an issue with longer focal length ultrawide eyepieces. The 31T5 tends to get mentioned to show that it doesn’t make an eyepiece a bad eyepiece. Even the best can have it. The moral of the story seems to be, don’t use 2” ultrawide eyepieces for terrestrial or lunar viewing. Not too hard. I mean, just try to find a 45 degree erect image diagonal that won’t severely vignette a 31 Nagler. That thing wasn’t designed for terrestrial viewing.

It seems the Orion LHD series might not suffer from this effect, but they only go up to 20mm.

I get that these eyepieces are meant for night time viewing, and  that it would be unfair to judge them with a use it was not meant to be. But I won't hide that if I can use a widefield Televue (say Panoptic 41) also on a terrestrial setup, or for Moon viewing, I will rather choose that one. You got a point with the fact that a 2'' 45 degrees diagonal are less common. That's why I use my 2'' 90 degrees for terrestrial use, with a lowered tripod. With 45mm being its shortest internal diameter, no vignetting to be expected with the 42mm field stop of the N31 T5.

 

Thanks for the input. I'll look into the Orion LHD series.

[Astro Chimp]

I doubt that is specifically from the Smyth element - its lateral chromatic aberation caused by a number of lenses in the eyepiece. The more refraction, the more to correct for.
More or less all ultra wide eyepieces from all manufacturers have it, some more, some less.

Does that mean no one really use ultra wide eyepieces for neither Moon or terrestrial use ?

[K-night]

If you willing to accept compromise in the ultrawide aspect, Sky Rover's UF 30mm does not exhibit "ring of fire", and its daylight performance is fantastic.

[davidgmd]

Does that mean no one really use ultra wide eyepieces for neither Moon or terrestrial use ?

  
Not necessarily. Depends on the scope and the eyepiece focal length. Short- and medium- focal length ultra-wides can be great on the Moon. 

[Starman1]

While looking for more Tele Vue eyepieces to acquire, I came across in my reading that some of them exhibit the Smyth lens phenomenon, or ''ring of fire'', while daytime using or moon viewing. The 31 Nagler T5 is notoriously one of them. That got me wondering, which of the other Tele Vue eyepieces are known for exhibiting that effect? Is this ''aberration'' inherited from the design (so all N31 T5 would have it), or is it differently present within the eyepiece family?

It is not from the Smyth lens, but a characteristic of the exit pupil called chromatic aberration of the exit pupil.

All the T5's had/have it, though the effect diminishes with shorter focal lengths.

Some other eyepieces that also have it: 30mm ES 82°, Astrotec/Stellarvue 28mm 82°, and there are others that have the issue to a lesser extent (Pentax XWs, for instance)

To correct it would require more lenses, and these large eyepieces are already very heavy.

 

Tons of other eyepieces with Smyth lens lower assemblies do not have CAEP.

Edited by Starman1, 18 February 2025 - 02:29 PM.

[Starman1]

Does that mean no one really use ultra wide eyepieces for neither Moon or terrestrial use ?

I've done a lot of Moon viewing, but it really starts about 100x.  You'd need a 3000mm focal length for a 30mm eyepiece to yield 100x.

So most people are not going to be using eyepieces this long in focal length to view the Moon.

[Starman1]

If you willing to accept compromise in the ultrawide aspect, Sky Rover's UF 30mm does not exhibit "ring of fire", and its daylight performance is fantastic.

an excellent suggestion for a 30mm eyepiece.

[Starman1]

Does that mean no one really use ultra wide eyepieces for neither Moon or terrestrial use ?

Not at all, I love using the 6mm Ethos on the Moon.

And the 21mm Ethos doesn't exhibit this issue.

It's primarily the longer focal lengths with smaller numbers of lenses, like 6 elements.

As I said, add more elements and it can be controlled, but at the cost of increased weight and cost.

[triplemon]

Does that mean no one really use ultra wide eyepieces for neither Moon or terrestrial use ?

No, nearly everybody does.

The lateral color does not increase linearly from the center. Same as with most other aberations. Even at 80% out from the center you will not see any color fringes on the moon. Have a look for yourself.

 

So this focus on the field stop, not being able to "unsee" some in practice irrelvant effect is a mental aberation, not fixable by nearly flat glass.

 

If its a problem for you, seek medical treatment from trained professionals. If you ask us amateurs, you'll get treatment as in the old days: rip out the tooth, only anesthesia is whiskey, but it has green lettering on the shot glass.

Edited by triplemon, 18 February 2025 - 05:26 PM.

[SeattleScott]

I haven’t noticed issues with a 7T1 on the Moon. It just seems to be an issue with the longer focal length ultrawides, that wouldn’t tend to be used for lunar viewing anyway.

[SeattleScott]

I get that these eyepieces are meant for night time viewing, and that it would be unfair to judge them with a use it was not meant to be. But I won't hide that if I can use a widefield Televue (say Panoptic 41) also on a terrestrial setup, or for Moon viewing, I will rather choose that one. You got a point with the fact that a 2'' 45 degrees diagonal are less common. That's why I use my 2'' 90 degrees for terrestrial use, with a lowered tripod. With 45mm being its shortest internal diameter, no vignetting to be expected with the 42mm field stop of the N31 T5.

Thanks for the input. I'll look into the Orion LHD series.

I can vouch for the 14LHD. It seems I heard the 20mm doesn’t have the ring of fire either, but I don’t have that one.

The Nikon SWs have distortion better suited for terrestrial if you view straight lines like buildings and roads. And they still work fine at night because stars aren’t in straight lines. Some people who pan around with them don’t like their distortion because the stars at the edge move a little funny, but I only pan around at low power to find stuff.

[j.gardavsky]

The "ring of fire" along the field stop is an overcorrection of the eyepiece design, not an aberration of the exit pupil.

This sort of overcorrection has been used for the eyepieces compensating the chromatic magnification difference of the old style APO microscopy objectives.

 

My ultrawide (85°) f=30mm, Carl Zeiss West Germany, Erfle II design, is free from the ring of fire.

 

I have checked all of my eyepieces for the color fringe along the field stop, even if it does not play role when observing the DSOs.

 

For some people, the visible color fringe indicates of how much attention has been paid to the optics design, glass materials choice, manufacturing costs, etc.

 

Best,

JG

[Astro Chimp]

 

No, nearly everybody does.

The lateral color does not increase linearly from the center. Same as with most other aberations. Even at 80% out from the center you will not see any color fringes on the moon. Have a look for yourself.

 

So this focus on the field stop, not being able to "unsee" some in practice irrelvant effect is a mental aberation, not fixable by nearly flat glass.

 

If its a problem for you, seek medical treatment from trained professionals. If you ask us amateurs, you'll get treatment as in the old days: rip out the tooth, only anesthesia is whiskey, but it has green lettering on the shot glass.

 

 

I think the mistake on my part was to understand the effect based on a dramatic picture of the phenomenon. This is from Louis D. from a discussion on Stargazers Lounge :

 

 

From this picture alone, and from reading about it, it would seem that such an effect would render the eyepiece unusable in any daytime situation, or even in certain full moon nights. But having read more about it today, and from all your responses, I suspect that the effect is not as "in your face" as I first thought... Does eye placement play a role in the severity of it ? (Just like dark spots appear if bad eye placement relative to eye relief).

 

I have played with basic kelleners / Plossls with carefree pleasure for the most part of a decade... Is the ring of fire not as scary as it sounds, and is a matter of just not looking for it ?

[Starman1]

The "ring of fire" along the field stop is an overcorrection of the eyepiece design, not an aberration of the exit pupil.
This sort of overcorrection has been used for the eyepieces compensating the chromatic magnification difference of the old style APO microscopy objectives.

My ultrawide (85°) f=30mm, Carl Zeiss West Germany, Erfle II design, is free from the ring of fire.

I have checked all of my eyepieces for the color fringe along the field stop, even if it does not play role when observing the DSOs.

For some people, the visible color fringe indicates of how much attention has been paid to the optics design, glass materials choice, manufacturing costs, etc.

Best,
JG

J G,
We're not talking about the narrow ring of color at the field stop here. We're talking about a 10° wide strong color band that comes in from the edge and lends a strong coloration to the moon when the moon passes through it. It's usually yellow orange to orange in color.

[triplemon]

That picture from the lounge is not the ring of fire.

 

Its chromatic aberation of the exit pupil, CAEP, entirely different thing. Interaction of a small camera iris with a small exit pupil. somewhat similar in root cause than Kidney beaning. Your eyes can't contract this much, nor can your eyes crank up color saturation as much as you can do in Photoshop to illustrate some point. You also have to place the eye at a very specific, normally not used distance to see this effect.

 

There is a lot on the internet that should be rated PG18. As it confuses clueless kids a lot more than it helps.

 

This is caused by your eye relief essentially being a little bit different for each color. So normally, if you place your eyes too close or to far from the eyepice, the FOV gets smaller, you don't see the field stop any more. The field of view becomes dominated by the aperture stop. instead. Something outside of the eyepiece. Now in some cases this effect depends on the color, so thats why you see the color rings. Different fields of view at differenrt colors. The resulting "edge dimming" or vignetting is not grey, but colorful. The light that comes though is all in the right place, though - its not causing any loss of sharpness. If you would be able to see the fieldstop, it would have no color fringing. Louis explains all this nerdy detail in that post. I can see why such an discussin of very high order details can go six miles above many folks head.

 

If you place your eyes at the propper distance for eye relief its greatly reduced or goes away entirely. Just as kindney beaning does. If your scope focal ratio is faster, i.e. you get a wider exit puil it also goes way, even in daytime views when outside of the propper viewing distance and with fully contracted pupils.

Edited by triplemon, 18 February 2025 - 10:37 PM.

[Starman1]

Thats picture from the lounge not the ring of fire.

Its chromatic aberation of the exit pupil, CAEP, entirely different thing. interaction of a very small (sub-1mm) camera iris with the much larger exit pupil. somewhat similarr in root cause than Kidney beaning. Your eyes can't contract this much, nor can your eyes crank up color saturation as much as you can do in Photoshop to illustrate some point.

There is a lot on the internet that should be rated PG18. As it confuses clueless kids a lot more than it helps.

Excuse me, CAEP = "The Ring of Fire".
The thin line of CA at the fieldstop doesn't have a common name. Some person, some time in the past, mistakenly confused the two.
The name was coined because the CAEP in the 31mm Nagler had the color of fire.

Edited by Starman1, 18 February 2025 - 10:07 PM.

[triplemon]

Excuse me, CAEP = "The Ring of Fire".
The thin line of CA at the fieldstop doesn't have a common name. Some person, some time in the past, mistakenly confused the two.
The name was coined because the CAEP in the 31mm Nagler had the color of fire.

Well, anybody can see for himself if the above picture looks like a ring of fire of a whole rainbow of colors.

 

CAEP can cause a ring of fire effect, it does so in the 31 Nagler. But its not the same, a true synonym. CAEP can look very different in other eyepieces, or have no visible effects at all. But in the picture posted and what the OP question was about its not looking like a ring of fire, its not a 31 Nagler. Its rather a picture very deliberately taken in a extreme way to exceptionally well show what CAEP is and can do - its not limited to just a brownish/reddish ring. Its not a sharp delineated effect, not what is often called "color fringing" or lateral chromatic aberation. Exaggerated for a very good reason, but not to show a realistic looking view.

 

So over simplification like "CAEP = Ring of Fire" is what IMO caused the OP's confusion, to the point that he may have thought a 31 Nagler looks that way when used in the daytime, in every scope. That is very, very far from the truth.

Edited by triplemon, 19 February 2025 - 02:42 PM.

[PKDfan]

I think i might see a bit of it with the 16.5XW during daylight observing if i'm not correctly positioned in the exit pupil.

Its not really a terrestrial eyepiece IMO.

Its not really a lunar eyepiece either but on the stars its flawless.



CSS
Lance

[Starman1]

Well, anybody can see for himself if the above picture looks like a ring of fire of a whole rainbow of colors.

 

CAEP can cause a ring of fire effect, it does so in the 31 Nagler. But it's not the same, a true synonym. CAEP can look very different in other eyepieces or have no visible effects at all. But in the picture posted and what the OP question was about its not looking like a ring of fire, it's not a 31 Nagler. Its rather a picture very deliberately taken in an extreme way to exceptionally well show what CAEP is and can do - it's not limited to just a brownish/reddish ring. It's not a sharp delineated effect, not what is often called "color fringing" or lateral chromatic aberration. Exaggerated for a very good reason, but not to show a realistic looking view.

 

So, oversimplification like "CAEP = Ring of Fire" is what IMO caused the OP's confusion, to the point that he may have thought a 31 Nagler looks that way when used in the daytime, in every scope. That is very, very far from the truth.

Huh?  The 31mm Nagler's outer field is orange in my 4" f/7 apo, my 12.5" dob, my former 8" SCT, and every scope I've ever looked through, if used in daylight.

And it does mean the eyepiece is a poor choice for daylight use.

It is not visible at night, largely because our eye has very little sensitivity to the coloration with dark-adapted eyes, though it can be seen if the moon passes through that zone.

But the low power eyepieces that have this issue do vary in coloration, somewhat.

For want of a better term, and to differentiate it from the simple CA that puts a thin color ring right at the field stop (which I've seen in eyepieces from 50° to 100° and which is not CAEP), the term "Ring of Fire" will suffice.

I've only seen yellow, amber or orange for that aberration, by the way.  Even in the 30mm ES eyepiece pictured, I don't see a rainbow.

[briansalomon1]

While looking for more TeleVue eyepieces to acquire, I came across in my reading that some of them exibit the Smyth Lens phenomenon, or ''ring of fire'', while daytime using or moon viewing. The 31 Nagler T5 is notoriously one of them. That got me wondering, which of the other Tele Vue eyepieces are known for exhibiting that effect ? Is this ''aberration'' inherited from the design (so all N31 T5 would have it), or is it differently present within the eyepiece family ?

I've used N31 along with many other TV eyepieces for many years and have never noticed a "ring of fire". Mainly, I look at large nebulae with N31, and when I've used it for birding, I've never noticed anything unusual either.

 

I have noticed my simple TV plossl eyepieces give as good, and maybe better sharpness and contrast on The Moon, planets and The Sun than my Ethos set.
 

[Jon Isaacs]

The "ring of fire" along the field stop is an overcorrection of the eyepiece design, not an aberration of the exit pupil.

This sort of overcorrection has been used for the eyepieces compensating the chromatic magnification difference of the old style APO microscopy objectives.

 

My ultrawide (85°) f=30mm, Carl Zeiss West Germany, Erfle II design, is free from the ring of fire.

 

I have checked all of my eyepieces for the color fringe along the field stop, even if it does not play role when observing the DSOs.

 

For some people, the visible color fringe indicates of how much attention has been paid to the optics design, glass materials choice, manufacturing costs, etc.

 

Best,

JG

 

 

Is your Ultrawide 30mm free of off-axis astigmatism at F/5?  I think the design and materials choices of the 31mm Nagler are based on providing the sharpest stars off-axis.  The 31mm Nagler in the NP-101, it is an awesome view, a perfectly flat 4.5 degree field, stars as sharp at the edge as they are in the center. I see no CA.  

 

It was not designed for day time viewing.. The 35mm Panoptic excels during the day.  

 

I personally think the Ring of Fire is a design decision.  The 20mm Type 2 Nagler used the basic type 2 Nagler design, 8 elements and was massive, the heaviest production TeleVue eyepiece ever, 2.30 lbs, 1043 grams.  A 31 mm Nagler of the type 2 design would be even more massive, at least double the weight.  The 31mm Type required a new design..  The 6 element Type 5 design is much lighter.  The 20mm Type 5 only weighs 1.04 lbs, 472 grams.  The 16mm Type 2 and the 16mm Type 5 are very similar, the ratio for the 20mm are 2.21, the 16mm, 2.27.  

 

Apply that same scaling factor (2.21) to the 31mm Nagler and the type 2 design for a 31mm eyepiece would be 5.1 lbs, 2300 grams...,  Obviously not practical. 

 

The other well corrected, long focal length 82 degree eyepieces (30mm ES, 28mm UWA) follow the 31mm Nagler design, 6 elements and they also show the ring of fire during the day.

 

In my way of thinking, the reason it is not seen at night is that it is very low contrast, very dim.  It is seen during the day as a faint cast over the existing image in the very outer field, color vision has a much greater dynamic range during the day...

 

Jon

Edited by Jon Isaacs, 20 February 2025 - 04:01 AM.

[quilty]

I'd like to clear this up.
My ring of fire in the goldline 6 mm has nothing to do with CA at the field stop. It's a reflection of the scope's clear aperture somewhere, maybe in the Smyth group.
You find this easily in daylight at any big dark/bright boundary or at the terminator.
I wouldn't be surprised to find similar in other eyepieces but I definitely would be so in a TV one. It's really annoying and excludes the moon as target from the goldline 6
Other fire rings like that brown one near the field stop in the 20 mm goldline is a different thing.

Edited by quilty, 20 February 2025 - 05:01 AM.