I donno....I have a Pentax 12.5mm Kellner, and in my f10 refractor on the moon, it has a clarity that isn't matched by most of my premium eyepieces...
I have a Zeiss 20mm Kellner from 6x30 Silvarems (~1915)
that is astoundingly clear and true-colored.
And....it is even "uncoated".
I put that in quotes because some glasses actually grow a coating.
That's how coating was originally discovered. Sometimes, later, lenses
were baked and/or chemically basted to grow that transitional layer
. They called that "luminized" sometimes.
Still, pretty impressive for contrast, from a 1915 EP.
The lining is the Zeiss standard fine threads with lampblack.
To address what was said about the field lens being nearly in focus;
that is true of the Kellner, the 1,2 Konig, and the Ramsden, if
a longer-fl Ramsden is spread to improve abberations
beyond the standard formula Ramsden.
(you can afford to do that when the eye relief is large to start)
But back to the (~1915) Zeiss 20mm Kellner and field lens smutz:
I did need to clean the glass, especially the field lens,
to maximize the contrast.
The RKE is a 1.2 Konig that has been stretched in spacing,
for less field with greater geometric accuracy. Strangely,
a stretched-1,2 Konig was used for many decades, maybe
since ~1930s, as an eyepiece to directly read instruments like
radiation dosimeters, pressure gauges, precision compasses,
monitoring meters, and sextants.
The RKE Seems like a parallel develop, made later on,
in ignorance of instrument Konig. The RKE also uses
some low-ED glass, of course.
The RKE drawing at Edmund is a bit coy....
the spacing is well off from the patent.
Since Edmund publishes extensive distortion data (to boast),
I wonder if it might be the most 'orthogonal' eyepiece.
As far as other properties of the RKE, I don't know about contrast
compared to an Abbe, for example.
A Plossl's advantage might be a bit less chromatics at the field edge,
which is not noticeable in daylight or for most planetary, but can be
noticed for those teeny-dot stars.
Edited by MartinPond, 06 July 2020 - 01:49 PM.