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 be 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:
  
Click for a larger view

  
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. Click on the image below to see this happen in an animation:
  
Click to run this animation

  
As a consequence of the compensating chromatic aberration of the Smyth lens, 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 thin cyan-blue ring can be seen at the edge of the field. This is known as '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!

[j.gardavsky]

Hello Ruud,

this is a very nice explanation on how the negative achromatic doublet compensates the lateral CA of the eyepiece main lenses group.

Thank you for sharing,
JG

[Ruud]

Thanks, JG.

I should say, Smyth used an achromat but he was correcting just for curvature. These days the first group is almost always more complex. I believe the first group of the Delos consist of four lenses and the second group too.

[Bigzmey]

Very nice article Ruud!

I would like to add, that many reports of the ring of fire come from daylight testing of wide field EPs. I don't find it much of concern, because they work just fine at night.

[Ruud]

Very nice article Ruud!

I would like to add, that many reports of the ring of fire come from daylight testing of wide field EPs. I don't find it much of concern, because they work just fine at night.

  
You are absolutely right, there is confusion. I suppose the situation arose from a metaphor for one phenomenon becoming associated with another. Such things happen in language. In the end, the usage of the phrase will determine what it refers to. As it is 'ring of fire' effectively has two disparate meanings and to use it one must narrow down which one is refered to.

I didn't exactly do a frequency count, but 'ring of fire' seems actually more often used in the sense of the thin coloured rings that are the topic of my post. Maybe this is because the other rings of fire are noticed less frequently. As you say, they only show up during the day when the observer's pupil is small. Also, they appear as a fat brownish discoloration that gradually fades toward the centre of the field of view, more of a zone or a band than a ring.

I just went with the flow. The rings discussed in my post are thin, well defined rings, that are often blue and show up with any combination of observers' and exit pupils.

My idea is that these thin, coloured rings are caused by two groups of lenses separated by an internal field stop. If both groups are free of false colour, no coloured ring will show. If the first group corrects for a small amount of chromatic aberration caused by the second group, a thin ring will show. The ring gets wider as more correction is needed.

[j.gardavsky]

Thanks, JG.

I should say, Smyth used an achromat but he was correcting just for curvature. These days the first group is almost always more complex. I believe the first group of the Delos consist of four lenses and the second group too.
[/quote]

Hello Ruud,

I have the 17.3mm Delos, and it is a bit of surprizing design, and now I understand why TV has been silent about what's inside.
You can easily unscrew the Delos into 2 parts,

so it is not completely waterproof or fogproof.
The excessive surface curvatures of the lenses let me believe that they have saved costs on the high refractive index and lower dispersion lens materials.

The ray tracing with a green laser reveals a quasi symmetric concept, repeating the lens groupings (1 - 2 - 1), even if with different glass materials.
And this is a second surprize,

Otherwise, this eyepiece works fine,

JG

[Ruud]

Wow, great Job JG!

[Makuser]

Hello Ruud. A superb report on the Barlow and Smyth lens construction. Very well written and very informative.

I would like to add, that many reports of the ring of fire come from daylight testing of wide field EPs. I don't find it much of concern, because they work just fine at night.

And Andrey. I too can see the orange ring that occurs with my 2" Explore Scientific 9mm 100° eyepiece at the very outer perimeter when looking through it in daylight, but does not interfere at all with night observing.

Thanks very much Ruud, Andrey, and JG for your useful information.

[j.gardavsky]

Hello Ruud. A superb report on the Barlow and Smyth lens construction. Very well written and very informative.

I would like to add, that many reports of the ring of fire come from daylight testing of wide field EPs. I don't find it much of concern, because they work just fine at night.

And Andrey. I too can see the orange ring that occurs with my 2" Explore Scientific 9mm 100° eyepiece at the very outer perimeter when looking through it in daylight, but does not interfere at all with night observing.

Thanks very much Ruud, Andrey, and JG for your useful information.

Hello Marshall,

and thank you very much for your input!

The blue ring in the ES, is it shining from the black field stop into the field?
The ultrawide EPs do have it as a rule,

JG