Second in the series profiling eyepieces that have been around the block and for one reason or another are held in high esteem by the community. The subject of this second post in the series? The 28mm Edmund RKE. You know the basics most likely: 28.7mm focal length, AFOV of 45-degrees, 24.5mm of eye relief, designed for Edmund Scientific by Dr. David Rank to replace the more expensive to manufacture eyepieces (Clave Plossls and others) that Edmund was bundling with its run-away hit Astroscan telescope.
Sometimes the design is erroneously dubbed the "Reverse Kellner Eyepiece" due to its resemblance to a Kellner design stacked in the housing upside down. But that's not correct. RKE stands for "Rank-Kellner Eyepiece" acknowledging that its designer, David Rank, derived his design from the Kellner. Like a Kellner the RKE features three elements in two groups with four air-to-glass surfaces. The coatings used are not listed, but to my eye RKEs of all vintages have ordinary MgFl single coatings on the exposed surfaces. I haven't taken one apart to see whether the internal air-to-glass surfaces are coated or uncoated, but given the Kellner's high propensity for internal ghosting I'm going to guess that internal surfaces of the RKE are also coated.
One of the mystical properties of this eyepiece is known as the "floating in space" effect. That is, the image formed appears to reside on the very surface of the eye lens, and due to its convex nature, the formed image can be viewed both on and extremely off axis, in the latter case making it appear that the image will spill over the edge of the eyepiece. The 28mm RKE is by no means unique in this regard, but it does display the characteristic very obviously.
I'm a cards on the table kind of guy. I am not a fan of the RKEs. I've owned multiple sets over the years and have always ended up comparing them to other simple/traditional designs and promptly selling them when they don't measure up. The 8mm RKE is one of the worst 7mm to 8mm eyepieces I have ever used, in fact. But as you can see I do still have a 28mm RKE. If I don't like RKEs, then why do I have the 28mm?
For me, to a large degree "gimmick value". By the numbers clearly it doesn't measure up a top choice for 1.25" low power finder eyepieces. 32mm Plossls have larger AFOVs and TFOVs and better throughput on account of modern full multicoatings. A 22-24mm modern superwide (Panoptic, LVW, etc.) likewise nips the RKE in FOV and throughput, and manages to do so at a larger image scale on account of the higher magnification. But the floating image effect is COOL. The eyepiece, used, is dirt cheap. While not a top choice as a finder eyepiece, it does work pretty well in that role, and for public observing sessions my pulse rate wouldn't even budge were a glop of mascara or a child's candy-stickied fingers to encounter the eye lens. There's also a nostalgia component. When doing re-enactment observing (i.e., using mid-70s scopes) it's nice to have a period-correct eyepiece for authenticity.
But I cannot help but feel that the passage of time and the reminiscing of old men (myself included) have clothed the humble RKEs with a greatness they never actually possessed. Edmund commissioned the design to be cheap first and foremost, to improve margins when bundled with their new cheap Astroscan telescopes. By using common inexpensive optical glasses, just 3 elements, simple coatings and a famous local optics academic as designer, Edmund achieved both cheap and near brand exclusivity, the latter freeing the marketeers to paint all kinds of mental images regarding the uber-specialness of the RKEs.
That's the way she looks from my side of the (floating-in-space) field of view.
Next up, next week probably, the 48mm Vernonscope Brandon...
Edited by jrbarnett, 23 January 2015 - 01:13 PM.