13 replies to this topic

[werewolf6977]

Are these Kellners? Or RKE's? The reason I need to know is I'm thinking about getting a 20mm to use/barlow for planetary/lunar observation. They sure are inexpensive you know... Help..... Pete

[Bill Grass]

I believe they are Kellners. Don't quote me, though!

[Dave Mitsky]

IIRC, the shorter focal lengths are orthos (4 lens elements) and the longer ones Kellners (3 lens elements). Years ago they were marketed separately.

I bought a 12.5mm Explorer ortho years ago at a star party swap meet and used it for some time. It worked well enough considering how little I paid for it. The same is true of an Edscorp ortho that I purchased for 3 bucks.

Dave Mitsky

[Tim2723]

I though the 25mm was prtty good for $27. They're cheap enought to try, that's for sure. As for ortho EP's, the U.O. orthos are more expensive but are getting lots of good press, and would probably be easier to get your money back if you ended up selling it.

[erik]

i've had several of the explorer ep's (even some .965" way back when!). they're all pretty good for the price. if you've got a few more bucks, i'd recommend some plossls, but you can't go wrong with the explorers considering the price. orion even sells them in sets at further reduced prices...

[Dave Mitsky]

BTW, I don't recommend Kellners for planetary and lunar observing since they are prone to ghosting. In fact, the Kellner is sometimes called the ghost eyepiece. I think a University Optics ortho would be a far better choice.

Dave Mitsky

[Mike Hosea]

I think the 20mm and shorter focal lengths are simply symmetrical Plossls. At least this was true of the one that I had several years ago (17mm, IIRC). A triplet-singlet design is going to tend to be more expensive.

[erik]

yes, the short ones are very similar to plossls, but the explorers seem to have a narrower fov and less eye relief. things get really tight with the 6mm version. but for cheap ep's, you could do a lot worse...

[Don W]

Are these Kellners? Or RKE's?

RKE is a Rank Kellner Enhanced or something like that. They are basically Kellners but the order of the lenses has been changed. Supposedly they have better views than Kellners. RKEs are a proprietary brand sold only by Edmund Scientific or whatever they are calling themselves these days.

[Scott Beith]

I have the 6mm and it is a nice little EP. Short eye relief does not bother me and the EP gives sharp images. Very inexpensive for the performance provided.

[Mike Hosea]

The letters "RKE" are simply trademarked by Edmund. The design per se has long since become public domain, if indeed it was ever patented, and since the trademark is for "RKE" alone, the original inspiration as an acronym is, in addition to being obscure and variously interpreted at this late date, of no lingering importance. Although if you think it means "Rank modified Kellner (or Konig) for Edmund", then it at least reminds you that the designer's last name is "Rank" and that the design was originally created for and marketed by Edmund Optics. If one thinks it means "Reversed Kellner Eyepiece" or some such, then it reminds one that the singlet and doublet are in the reverse order as they are in a Kellner, as if that meant anything at all useful to the average Joe.

Anyway, there's no reason why the 25mm Explorer II couldn't have that design, but they wouldn't want the trouble of legally call it an "RKE" in the US because the trademark is still active. There have been some cheap Chinese eyepieces and some Rini's labeled RKEs, despite the active trademark, however.

[Don W]

That's what it is, Rank Kellner Eyepiece. It's definitely not a Konig. I had a couple years ago and they performed pretty much like a Kellner, what a surprise. For a time, Kellners had a bad reputation. They worked fairly well with a long focus scope, but not with fast wide field scopes. Various companies marketed Kellners under different names, Meade was MA (Modified Achromat), I think Celestron was SMA (Super Modified Achromat) and Orion has their Explorer and Explorer II series. Oh yeah, and Edmund switched the lense order and called them RKE. Well a Kellner by any other name is still a Kellner. Celestron, Meade and Orion include them with their low end scopes that accept 1.25" oculars. If the scope has a short f/ratio, they won't do well. If it's long, it will do just fine.

[Mike Hosea]

That's what it is, Rank Kellner Eyepiece. It's definitely not a Konig.

Well, they're neither Kellners nor Konigs, they're RKEs, of course. But one of Konig's designs has just 3 elements with a field doublet and an eye lens singlet, which is why I wonder whether David Rank started with a Kellner or a Konig and modified from there to produce the RKE design. The RKEs do have three advantages over Kellners that I can see. 1. They don't suffer from as much ghosting, 2. their focal plane is is not as close to the field lens surface, and 3. they have greater eye relief.

[erik]

i believe the 25mm, 20mm, and 17mm explorer ep's have 4 elements, while the shorter versions have 3. whether or not they're technically kellners, they perform decently for the price.