What are Skywatcher included eyepieces?

[Jbwz]

Hey people.

Finally got my first telescope (130p) few days ago.

While skies are 100% cloud covered, decided to go a little deeper into optics (learning what lens does, eyepiece construction, etc etc).

 

Can't find a definitive answer on what those stock Super 25mm and Super 10mm are. Some websites say they are Plossl, others swear they're Kellner. Who's right?

Based on anything I read so far (Kellner having only 2 lenses vs 4 in Plossl design) I am leaning towards those provided eyepieces are Kellner?

 

P.S. if anyone has any recommendations on optics books please feel free to share. Not knowing anything about optics (to get quick core intro) I started reading Optics for Dummies. Thinking to progress to Optics by Eugene Hecht (looked at few pages and thought it looks a little too steep for an absolute beginner, even with mathematical background)

 

Thanks. 

[Ian McCallum]

They're not Plossls, but modified achromats.

Edited November 29, 2022 by Ian McCallum

[Jbwz]

2 minutes ago, Ian McCallum said:

They're not Plossls, but modified achromats.

Thank you

[Ian McCallum]

1 minute ago, Jbwz said:

Thank you

This may help clarify a little more...

http://www.swindonstargazers.com/beginners/eyepieces.htm

[vlaiv]

50 minutes ago, Jbwz said:

P.S. if anyone has any recommendations on optics books please feel free to share.

This is very good source of info:

https://www.telescope-optics.net/

 

[Louis D]

The definitive optics book for amateur astronomers is Telescope Optics : A Comprehensive Manual for Amateur Astronomers by Harrie G. J. Rutten and Martin A. M. Van Venrooij.  I find myself going back to my copy every so often to reread certain sections.

[Louis D]

The best page for comparing eyepiece types on TelescopeOptics.net is 12.4. TELESCOPE EYEPIECES: COMPARATIVE RAYTRACING.

Two other eyepiece diagram charts floating around the web for years are below:

 

You can see that the Achromatic Ramsden is indeed very similar to the Kellner in design.

[allworlds]

Kellners have has three elements - a single "field lens" and a cemented doublet "eye lens". The Skywatcher stock eyepieces are "modified achromat"; same general layout but a bit different on the lens shapes. Both designs are OK. Then Konig and RKE are also three-element designs but the other way round, doublet field lens and singlet eye lens.

The main 2-element designs are the Huygens and Ramsden and they're kind of rubbish. It doesn't help that when made nowadays they are virtually always made as cheaply as possible.

[Don Pensack]

König actually designed 38 different eyepieces during his career, including a design later used by Clavé in their "Plössl" line.

It's unclear whether Clavé changed some glass types to yield better chromatic correction.

The 3-element one is well-known, but the ones produced under his name in Japan and sold in the '80s and '90s had 4 and 5 elements,

and were not the 3 element design for which he is best known.  The first ones sold by University Optics were the König design in the top chart, not the bottom chart.

[Jbwz]

Thank you for the info and advice. Much appreciated 

[Piet Le Roux]

Although this is an old topic it is one that is of interest for me now.  The "super"and  MA or even Super MA eyepieces that I have come across and have inspected, I believe are revesed kellners because the original Kellner had a doublet eye lens and single field lens these have a doublet field lens and single eye lens. I suspect that the revesed Kellner design are propriety and for this reason they do not want to name them as such. Edmund RKE has the patent rights and made this design famous, they are still selling a 1.25" range of these which includes their famous 28mm. The Celestron E-Lux range of 2" Kellners, 26, 32 and 40mm, are also reversed Kellners but are sold as just Kellners. My interest in these started with the 40mm that I use in my 8" F10 SCT and although it has a little more distortion as my 27mm Panoptic it is notably brighter which helps when you are looking for faint galaxies with 8"  SCT. I then compared a Super 10mm with my 7 and 13mm Naglers, the objects were notably brighter in the 10mm than the 7mm Nagler but about the same as my 13mm Nagler, my conclusion is simply that  less glass means more light and although  the Super or MA has less FOV and do scatter with bright objects, my main goal are faint galaxies and planetary nebulae. Resolution is not bad with these cheap eyepieces and I could split any double that I can split with the Naglers. I think these eyepieces has the best glass VS performance ratio. 

[Don Pensack]

6 hours ago, Piet Le Roux said:

Although this is an old topic it is one that is of interest for me now.  The "super"and  MA or even Super MA eyepieces that I have come across and have inspected, I believe are reversed Kellners because the original Kellner had a doublet eye lens and single field lens these have a doublet field lens and single eye lens. I suspect that the reversed Kellner design are propriety and for this reason they do not want to name them as such. Edmund RKE has the patent rights and made this design famous, they are still selling a 1.25" range of these which includes their famous 28mm. The Celestron E-Lux range of 2" Kellners, 26, 32 and 40mm, are also reversed Kellners but are sold as just Kellners. My interest in these started with the 40mm that I use in my 8" F10 SCT and although it has a little more distortion as my 27mm Panoptic it is notably brighter which helps when you are looking for faint galaxies with 8"  SCT. I then compared a Super 10mm with my 7 and 13mm Naglers, the objects were notably brighter in the 10mm than the 7mm Nagler but about the same as my 13mm Nagler, my conclusion is simply that  less glass means more light and although  the Super or MA has less FOV and do scatter with bright objects, my main goal are faint galaxies and planetary nebulae. Resolution is not bad with these cheap eyepieces and I could split any double that I can split with the Naglers. I think these eyepieces has the best glass VS performance ratio. 

The RKE is not a reversed Kellner.  It is, according to Edmund and the Patent Office, a "Rank-Kaspereit-Erfle", and it uses different curves, spacing, and glass than the Kellner design, even reversed.

There have been many different eyepieces from several companies made with a doublet field lens and singlet eye lens (one of the most common is the König design).

[Louis D]

I searched around the internet and the best diagrams of 3 element doublet-singlet eyepieces were on Telescope-Optics.net.

I cropped and combined together four Kellner or "Reversed Kellner"-like designs below for easy comparison:

There was no mention of a true Reversed Kellner design on that site.

Edited April 4, 2023 by Louis D

[Piet Le Roux]

Yes the reversed Kellner or type II Kellner does  have a different lens configuration as the Original Kellner or type I Kellner, as I seen it being referred to in some literature, the same literature referred to a Plössl as a type III Kellner!  But  I suppose we can refer to it as we see fit  as long as we are talking about the same diagram, but the most used term is reversed kellner. However I have no interest in the different terms used to refer to the RKE, reversed Kellner, type II Kellner or long spaced König, but only to bring to the attention, of those who are interested, that it is a fine eyepiece when used in F10 to F15  telescopes , which I have, to observe dim doubles, nebula or galaxies and that most of  the design, that are still being made, are not called RKE or reversed Kellner! 

[Don Pensack]

12 hours ago, Piet Le Roux said:

Yes the reversed Kellner or type II Kellner does  have a different lens configuration as the Original Kellner or type I Kellner, as I seen it being referred to in some literature, the same literature referred to a Plössl as a type III Kellner!  But  I suppose we can refer to it as we see fit  as long as we are talking about the same diagram, but the most used term is reversed kellner. However I have no interest in the different terms used to refer to the RKE, reversed Kellner, type II Kellner or long spaced König, but only to bring to the attention, of those who are interested, that it is a fine eyepiece when used in F10 to F15  telescopes , which I have, to observe dim doubles, nebula or galaxies and that most of  the design, that are still being made, are not called RKE or reversed Kellner! 

What is a reversed Kellner, and who makes one? 

It is not the RKE (Rank/Kaspereit/Erfle) and it is not a König.

It is merely someone looking at a 3-element design and calling it that, despite it not being that.

Edmund, I think, coined the name in their literature to describe a generic eyepiece with a doublet field lens and singlet eye lens.

There is no evidence, however, that Kellner designed an eyepiece like that.

Edmund might have made one and called it that, but did they use the same glass types or curves?

I think such a design may never have existed, but just had that name assigned, just like "Kellner Type II".