Smyth Lens - the Ring of Fire Explained

[Ruud]

A Barlow is a negative achromatic lens that can be inserted before an eyepiece. It elongates the light cones from the telescope’s primary mirror, effectively increasing the focal length of the telescope. With a Barlow lens in place the magnification of any eyepiece used in that telescope increases. Ideally, a Barlow affects magnification only and will not introduce any aberrations.
 

Smyth Lens

Charles Piazzi Smyth, 1819-1900, was an Italian-born astronomer and Astronomer Royal for Scotland from 1846 to 1888. He devised a new application for the negative achromat, using it as a field lens to correct for field curvature in an otherwise well corrected lens.

Smyth’s idea can de used for other corrections as well, and over the years Smyth lenses have been developed to correct for all sorts of undesired side effects in eyepiece designs. This has led to significant improvements of eyepiece performance. Many modern wide field eyepiece designs involve a negative lens group, followed by a positive lens group which forms the final image. Collectively these negative groups are called Smyth lenses and invariably their purpose is to correct for aberrations.

Unlike a Barlow, a Smyth lens is not an optional element: it is an inherent part of the eyepiece design. Removing it would result in a poorly performing eyepiece.

Eyepieces that have a Smyth lens are also called negative-positive eyepieces. Light from the objective enters the eyepiece through the negative Smyth lens and an image forms at the field stop which in these eyepieces is internally located between the negative and positive groups. The eye looks at the image through the positive lens group, the last lens of which is the eye lens.
 

Ring of Fire

In eyepieces, lateral chromatic aberration is caused by light of different wavelengths being focused at different positions.

This happens in the positive lens group of for instance the Delos eyepiece. If used without Smyth lens, the positive group in the Delos would cause the image in cyan-blue wavelengths to be magnified slightly more than in other wavelengths. This would cause some nasty lateral colour, adding a red-cyan fringe to all high contrast edges.

Of course the Delos is a near-perfect eyepiece. A Smyth lens corrects for the positive group’s lateral colour. It achieves this by projecting a slightly smaller image in cyan-blue light than in colours of other wavelengths:

This works well across the field, except for rays at the very periphery. There cyan-blue light can pass while the other colours are being blocked:

 

As a consequence of the compensating chromatic aberration of the Smyth lens in the Delos eyepiece, a star that drifts out of the field briefly turns cyan-blue at the edge of the view before it disappears. Also, during daytime observations or when the Moon fills the view, a hair-thin cyan-blue ring can be seen at the edge of the field. This is the Ring of Fire.

It's actually quite pretty when a bright star drifts out of the field with a tiny explosion of blue light.

Thanks for reading!

[andrew s]

Fascinating,  thanks for posting. Regards Andrew 

[John]

Very interesting.

I'm always intrigued how Tele Vue managed to control this effect so well in the Ethos eyepieces despite their hyper-wide field of view.

It is quite pronounced with the Naglers.

[JeremyS]

Thanks for the explanation

 Not come aCross this before so I learned a lot 

[Don Pensack]

Nope, that is not the "Ring of Fire".

That is the chromatic issue at the field stop seen in nearly all widefield or wider eyepieces.  It's even visible in eyepieces without a Smyth lens, i.e. positive systems.

I've seen red, amber, yellow, green, orange, blue and even violet in different eyepieces.

The so-called "Ring of Fire", which is present in only a very few eyepieces, like the 31mm TeleVue Nagler, is not just at the field stop but extends inward 5-10 degrees into the outer field.

The term "Ring of Fire" was coined because the 31mm Nagler's outer field, when used on the Moon or in the daytime, has a pronounced orange-red color, like a flame.

The 30mm Explore Scientific 82° also has this, though it is more yellow than the Nagler.  The 26mm nagler as well.  There are others I can't remember off the top of my head

Al Nagler called the 31mm Nagler issue "Chromatic Aberration of the Exit Pupil" (abbreviated CAEP) and it's cause is different than the explanation given by Ruud of the reason for the color ring right at the field stop.

Ruud's explanation also doesn't explain why a positive-only assembly of lenses would have the same CA right at the field stop, but that is an aside.

[Louis D]

I captured the CAEP of my ES-82 30mm pretty conclusively in my 127 Mak in the following image:

I've never captured anything close to this through any of my other 50+ eyepieces.  Notice there's no chromatic smearing of the individual tick marks or numbers, so there's no classic chromatic aberration as seen in many lesser wide field eyepieces such as the Kasai Super WideView 90° 30mm below:

Notice how the tick marks and numbers nearing the edge get progressively more blue-green shifted to the left and red to the right?  Also notice how the field stop is a rainbow of colors going from reg/orange/yellow to green/blue?  These are all classic chromatic aberrations.

Compare it to the minor SAEP of my 12mm ES-92 in the 127 Mak:

Notice the thin blue line at the ES-92 field stop but no rainbow spread of light in the outer field and no spread of colors within a detail?  The ES-92 has minor SAEP, no ring of fire (CAEP) and no classic chromatic aberration, the ES-82 has CAEP but no classic SAEP or chromatic aberration and the Kasai has classic chromatic aberration but no SAEP or CAEP (it's not there in the full images, take my word on that).  @Ruud is probably correct that if the negative group were to be removed from the ES-92, it would have pronounced classic chromatic aberration.

CAEP is caused by the blue end of the spectrum being focused further away from the eye lens and the red end closer.  At the best exit pupil, you get fairly close to white, but in a very broad exit pupil circle compared to an eyepiece without CAEP.  Notice there's no rainbow effect in the ES-92's SAEP ring?  That's because all the light wavelengths from any given ray bundle are focused at the same point.

[Don Pensack]

Excellent pix!  Especially the 30mm ES.  There is a very slight SAEP there, too. The ring at the edge extends farther into the field on the 31mm Nagler.

Over the years, the very thin colored line at the field stop that I've seen in eyepieces have also varied in width.  The line at the edge in the new TeleVue Apollo 11 is about the thinnest I've seen.

That's really never much of a problem at night, of course, but chromatic aberration in the form of lateral prismatic effects is an issue in some wide/ultrawide/hyperwide field eyepieces.

Photographing the issues can be hard when it is in the form of Edge of Field Brightening (EOFB), though Bill Paolini has captured some images that reveal it in at least one eyepiece.

It goes to show there is no perfect eyepiece.

[Ruud]

Gee, Louis, that looks awful! It's difficult to call that a ring of fire, it's more like a rainbow disk!

@Don Pensack

Google kind of fails me when I search for “chromatic aberration of the exit pupil” (with quotation marks to avoid partial matches). I find two categories of references:

The first is about chromatic aberration of a Galilean telescope using non-achromatic lenses. That’s not what we are talking about. (It’s odd, btw, that these links refer to exit pupils of Galilean telescopes. Galilean eyepieces do not form an exit pupil in the sense of a region behind the eye lens through which all rays pass).

The second category involves discussions on cloudynights.com. The links referred to are associated with fat and hideous brown rings inside the exit pupil. That is not what I’m addressing in my post here.

For a quick impression have a look at this search result:  link

Another thing I am not talking about is chromatic aberration in single positive rather than negative-positive eyepieces. Most of us already know that when an eyepiece has chromatic aberration, it tends to get worse toward the edge of the field, so the stop shows it more clearly than any other part of the field.

I briefly considered asking a mod to change the title of my post, but really the well defined coloured ring close to the field edge of neg-pos eyepieces is what we call a ‘ring of fire’. This may even be the case in most places.

I’d like to refer you to this thread, in particular the second post:

And yes, this ring does not always need to be cyan or blue. It really depends on what kind of chromatic aberration the Smyth lens needs to correct for. And of course, the less chromatic aberration the Smyth lens has to correct for, the thinner the ring will be.

 

Edited June 10, 2020 by Ruud

[Louis D]

Here is Ernest's post about CAEP on CN.  I've reproduced the image here:

That is the exact same version of the ES-82 30mm (mine is decloaked) that I shot my rainbow AFOV image through.

[Louis D]

1 hour ago, Don Pensack said:

There is a very slight SAEP there, too.

Yes, but I have never been able to detect it in use.

1 hour ago, Don Pensack said:

The ring at the edge extends farther into the field on the 31mm Nagler.

Naked eye, I was able to clearly see the orange/red ring, but not the other ring colors.  The camera was way more color sensitive.  I was flabbergasted when I reviewed the image.  Apparently, it's actually a ring of rainbow rather than fire, but the non-orange/red colors are harder for the human eye to detect.

[Don Pensack]

3 hours ago, Ruud said:

Gee, Louis, that looks awful! It's difficult to call that a ring of fire, it's more like a rainbow disk!

@Don Pensack

Google kind of fails me when I search for “chromatic aberration of the exit pupil” (with quotation marks to avoid partial matches). I find two categories of references:

The first is about chromatic aberration of a Galilean telescope using non-achromatic lenses. That’s not what we are talking about. (It’s odd, btw, that these links refer to exit pupils of Galilean telescopes. Galilean eyepieces do not form an exit pupil in the sense of a region behind the eye lens through which all rays pass).

The second category involves discussions on cloudynights.com. The links referred to are associated with fat and hideous brown rings inside the exit pupil. That is not what I’m addressing in my post here.

For a quick impression have a look at this search result:  link

Another thing I am not talking about is chromatic aberration in single positive rather than negative-positive eyepieces. Most of us already know that when an eyepiece has chromatic aberration, it tends to get worse toward the edge of the field, so the stop shows it more clearly than any other part of the field.

I briefly considered asking a mod to change the title of my post, but really the well defined coloured ring close to the field edge of neg-pos eyepieces is what we call a ‘ring of fire’. This may even be the case in most places.

I’d like to refer you to this thread, in particular the second post:

And yes, this ring does not always need to be cyan or blue. It really depends on what kind of chromatic aberration the Smyth lens needs to correct for. And of course, the less chromatic aberration the Smyth lens has to correct for, the thinner the ring will be.

 

You should not refer to the thin color ring at the field stop, blue or otherwise, as the "Ring of Fire".  This was a mistake made by some observers who had never seen it and assumed it was the same thing.  It is obviously not.

Your link does show the issue with a 31mm Nagler, which was the eyepiece that prompted the coining of the term a couple decades ago.  As Ernest pointed out, it is "chromatic aberration of the exit pupil".

 

The thin ring of color at the edge of the field shows up in some 50° all-positive eyepieces, as well as many 60-70° eyepieces without a negative field lens.  So it is not necessarily just caused by the use of a negative field lens in an eyepiece.

It is different than the lateral chromatic aberration in most eyepieces.

[Don Pensack]

Just had a long conversation with Al Nagler of TeleVue who confirmed some thoughts of mine:

There are 3 different ways chromatic aberration expresses itself in an eyepiece near the edge of the field:

1) Chromatic aberration of the exit pupil, as found in the 31 and 26mm Nagler and 30mm Explore Scientific (and others).  It is a ring of color extending well into the field.

This is what is termed "the Ring of Fire".  It will be different in different f/ratios of scope since the exit pupil will differ.  He suggested experimenting with aperture stops to see the difference.

2) lateral chromatic aberration that yields a thin ring of color right at the field stop.  It is produced by the upper section of the eyepiece and it is caused by chromatic aberration of the system, which is why it is seen more in wider eyepieces than narrow ones.  I asked if it would be covered completely if the field stop were made narrower, and he said the ring would grow thinner, but unless the field stop were substantially smaller, it would not be gone.  He suggested the experiment of putting a small tab of tape on the field stop of an all-positive eyepiece like a Panoptic (which is below the bottom lens) to see where the color fringe gets thinner and disappears.

3) Chromatic smear, or prismatic smear of a star image as the star gets closer to the edge of the field.  This is the most common form of chromatic aberration in eyepieces and it can be reduced or eliminated with additional lenses in the system.  That may or may not be practical if the eyepiece is already large and heavy.  This one is from all the elements in the eyepiece, whether positive or negative.

[Mr Spock]

A fascinating topic. As good as eyepieces have become they never really fix all aberrations, only 'mostly' fix.

[andrew s]

1 minute ago, Mr Spock said:

A fascinating topic. As good as eyepieces have become they never really fix all aberrations, only 'mostly' fix.

Yes indeed compared to an eyepiece objective design is trivial.

Regards  Andrew 

[Louis D]

2 minutes ago, andrew s said:

Yes indeed compared to an eyepiece objective design is trivial.

Regards  Andrew 

And yet actually successfully fabricating an advanced refractor objective design seems to be much more difficult and expensive.  It can't just be about the vast amount of expensive glass involved, or perhaps it is.

[andrew s]

7 minutes ago, Louis D said:

And yet actually successfully fabricating an advanced refractor objective design seems to be much more difficult and expensive.  It can't just be about the vast amount of expensive glass involved, or perhaps it is.

Getting large flawless banks is difficult and hence expensive.  Fabrication is very different to design.

Regards Andrew