Celestron 10mm Stock EP Disassembled

[Rocket_the_Raccoon]

I have two of these sets sitting around, so I decided to have some fun with them... :-)

Many entry-level telescopes (Celestron, Orion, Skywatcher) include eyepieces like the above, economical 3-element of unknown designs. Most people simply called them "modified achromatic", "modified Kellner", or just "Kellner". So what actually are they?

A "true" Kellner consists of a singlet plano-convex field lens and a achromatic doublet plano-convex eye lens, it has an AFOV of 40 degrees:
 (by Tamaflex, used under CC-BY SA 3.0)

High quality classic Kellners are widely available in the used market with some being semi-collectables. At least one online dealer is still selling brand new ones: http://www.stjarnhusetonline.se/prod/Sky-Watcher/ kellner/6101525 Eyepieces.html

The name "Modified Achromatic" actually came from the MA series of eyepieces from Meade which has been discontinued. They resemble the original Kellner with the eye lens replaced by a bi-convex doublet. They have AFOV of 40 degrees. It seems that Meade aimed for improvements in image qualities rather than increasing AFOV. 

The RKE from Edmund Scientific reverses the orientation of the field and eye lenses of the Kellners, replacing the eye lens with a bi-convex singlet, and field lens with a concave-convex doublet. The AFOV increases to 45 degrees with image quality praised by many.

Another 3-element design, unknown to many, is actually the original Königs, surficially resembles the RKE but actually a close relative to the Abbe Orthoscopic. AFOV of this version of Königs is 55 degrees:
(by Tamaflex, used under CC-BY SA 3.0)

Well, how about the entry-level set mentioned at the beginning? Let's disassemble one, the 10mm in particular, to find out its "secret formula"...
*lens edges blackened by myself using fine tip felt-pen

As shown above, the eye lens is a singlet plano-convex with the field lens being a thick plano-convex doublet. It looks like a Kellner in reverse but in fact, the field and eye lenses are of the same size and more important, they almost touch each other. Thus, this 3-element Celestron stock 10mm is actually a slight modification of the Königs. Moreover, from what I have read from the Internet, this is in fact the same design of the Celestron SMA series from the past.

 

[ronin]

An example of the RKE (Reverse Kellner) no idea what the "E" stands for.

THis: http://www.billsastro.com/Astronomy/Equipment/Eyepiece Folder/eyepiece_design.htm  says they are slightly better then the standard Kellner

Reverse Kellner is slightly better, though not many manufactures offer it.  Edmonds Scientific is the last offering I saw of it (RKE).

Below that it mentions the SMA:

Another variation is the Super Modified Achromat (SMA).  Another slight improvement on the basic Kellner design, they are standard equipment on many lower cost scopes.

Just doesn't say what the slight variation is. I think most Meades are the MA type eyepiece, just not "Super". 

When are you going to start on a Nagler ? 

[Merlin66]

A popular misconception....

The RKE has no direct link to the Kellner.

It is just a trade name given to the Edmund Optical eyepiece.

In an amendment to their trademark application on January 16, 1979 it was given as "Rank, Kaspereit, Erfle", the three designs from which the eyepiece was derived.

[triton1]

Something else I've learned today thx.

[John]

I took some Vixen orthoscopics apart a few years ago to clean the lenses (they were really filthy !) and was a little suprised to find the the optical design was a 2+2 rather than the traditional 3+1 abbe orthoscopic layout. The Vixen design looked very like a plossl or symmetrical with 2 pairs of lenses with their curved surfaces facing each other, but not touching.

The eyepieces were good performers (when clean !) though.

[Rocket_the_Raccoon]

13 hours ago, Merlin66 said:

A popular misconception....

The RKE has no direct link to the Kellner.

It is just a trade name given to the Edmund Optical eyepiece.

In an amendment to their trademark application on January 16, 1979 it was given as "Rank, Kaspereit, Erfle", the three designs from which the eyepiece was derived.

Thanks for the clarification. Both Kaspereit and Erfle are early 20th century wide field designs. "Rank" is the last name of Dr. David Rank who developed the RKE.

11 hours ago, John said:

I took some Vixen orthoscopics apart a few years ago to clean the lenses (they were really filthy !) and was a little suprised to find the the optical design was a 2+2 rather than the traditional 3+1 abbe orthoscopic layout. The Vixen design looked very like a plossl or symmetrical with 2 pairs of lenses with their curved surfaces facing each other, but not touching.

The eyepieces were good performers (when clean !) though.

I have a Celestron Circle-V 6mm Ortho. which gives superb contrast and sharpness despite its short eye relief. These classics, like yours and mine, are truly gems.

[John]

I don't have mine any more. They came with a Vixen 4" refractor that I bought back in the 1980's. I don't have that now either but it was a very good scope.