86 replies to this topic

[MarkGregory]

RKE lost on me. Floating effect? What are you even talking about? It's just an image where it always is -- on my retina. Yup, minimal glass. So what? Views are still insipid & uninspiring. I've been taking a look through RKEs from time to time for 45 years, and the legendary view never shows up. Its stepson bailed long ago, thinking "I'm not pushing this charade anymore..." The purported view is AWOL in SCT, Newtonian, APO, refractor astrograph, you name it. I'm baffled by enthusiasm about this RKE family of glass. Hey, it was fine sitting in an Astroscan, but beyond that? No.

 

Phil

I don’t see it either. And not the sharpest ocular either. At least that has been my experience. Wish I could report otherwise, I was really hoping for the alleged space walk effect. 

[izar187]

Floating or space walk effect is only apparent in the 28mm, and sometimes 21 per a few folks.

There is a few other ep's from yester/when that do the same floating in space thing, and have been listed here before.

Haven't used them, and will screw it up if I were to try and list them. : )

It's supposed to be a combo of long eye relief, large eye lens, and thin wall barrel structure, that sometimes allows for it.

Nothing designed for.

15mm, 12mm and 8mm RKE'S never had it.

[jeffmac]

Well, talk about YMMV! Another eyepiece line that draws both very positive and very negative feelings. The more time I spend on the Eyepiece Forum, the more I am convinced that they are a very personal thing. One person loves 'em, another one hates 'em. I'm also convinced that sample variation is more widespread than I used to think. But I think that eyepiece characteristics such as FOV, ER, etc. are the main reason for most wide differences in opinion on eyepieces and may even cause some bias when evaluating other characteristics, such as sharpness and contrast....in theory at least. Not that any of us would ever do that, though!   

[Bill Weir]

Or maybe life was hard on the RKE in "the junk eyepiece drawer."

I don’t think so. The 16mm early 1960s Brandon I took out of the same drawer at the same time is brilliant. The 4mm Criterion ortho is also good. 

 

I understand there are many who like the RKE and I respect that. It’s a flavour that’s not for everyone.

 

I was just curious what scope it is best used in.  It came with the Astroscan and those are fast right? Maybe I should give it a go in my f/3.3 20” without the Paracorr to see what kind of craziness it gives.

 

Bill

[CeleNoptic]

But when I compared my RKE 15 and 8 to my Baader Classic Ortho 18, 10 and 6 (well regarded modern abbe orthos), I found the RKEs were just as sharp on axis, had better contrast on Lunar shading than the BCOs and were noticeably sharper than the BCOs off axis. This surprised the heck out of me as I had always heard that "ortho" meant "without distortion" but I was seeing noticeable distortion off axis in all three of the orthos. Off axis distortion in the RKEs was still there but was much less than the orthos.

 
Think, what part of that 45° FOV in RKEs is distorted compared to 50° FOV in BCOs. Typically,  an Abbe Ortho FOV restricted to ~40-42° to avoid off-axis distortions. Baader, and it's well known, widened FOV in BCOs up to 50° on purpose to make finding targets easier, e.g. for users with fully manual scopes. Some observers dislike this feature, others, me included, accept this trade off OK. If this feels annoying enough, one can install custom diafragms/Field Stops limiting FOV to regular 40° and BCOs will show images sharp to the edge like any Abbe Orthos. Unlike single-coated RKEs, which excel mostly on bright targets, BCOs with their superb Baader Phantom coatings are also pretty good on faint DSOs.
 
I, personally, almost always have in my EP case the 10 and 6mm BCOs and the 15 and 8mm RKEs which I use both directly and with 2x-3x Barlows on planets, the Moon, double stars. They all perform excellent in my SV102 when the seeing conditions support high magnifications.

[CeleNoptic]

I was just curious what scope it is best used in.

 
IMO, you need a really good quality refractor like APO triplet or ED doublet or maybe high Strehl mirror custom Dob. Most industrial grade Newtonians or Cassegrains are unlikely distinctive enough with their decreased contrast, IMO. For example, you may want to check out this excellent  BillP's review. BillP tested them on his f/7.7 Vixen 81S. But, of course the eyepiece factor is highly subjective, so no surprise if your mileage may vary   

[Thomas_M44]

RKE lost on me. Floating effect? What are you even talking about? It's just an image where it always is -- on my retina. Yup, minimal glass. So what? Views are still insipid & uninspiring. I've been taking a look through RKEs from time to time for 45 years, and the legendary view never shows up. Its stepson bailed long ago, thinking "I'm not pushing this charade anymore..." The purported view is AWOL in SCT, Newtonian, APO, refractor astrograph, you name it. I'm baffled by enthusiasm about this RKE family of glass. Hey, it was fine sitting in an Astroscan, but beyond that? No.

 

Phil

The 28mm RKE “floating image effect” is fun, but a sideshow, a distraction —forget about it.

 

Currently manufactured RKE eyepieces have very good Japanese-manufactured lenses.

 

The RKE is an excellent planetary EP —that’s the real headline.

[Muffin Research]

Doesn't the floating effect occur when you're not actually looking through the eyepiece? so really not anything useful but just cool.

I found it with Nagler T4s .. when you approach the telescope you can kinda see the image already from afar.

Usually you have to get your eye in there to see something but here's it's like it's projected on the rather large top lens, so it's like a tiny tv screen. 

[Ihtegla Sar]

Think, what part of that 45° FOV in RKEs is distorted compared to 50° FOV in BCOs.

Bill P (and others) have measured the AFOV of their RKEs at 50. There has been some discussion about why Edmunds advertised them as only 45. I haven't drift tested mine yet but they seem more like 50 than 45. The Pleiades (110 arc minutes) fit within the field of view in my 900mm focal length Tak using my 28mm RKE.

 

If you have measured your RKEs at 45, then I would guess that there maybe differences in the field stops used in the various versions of the RKEs over the years. Since the RKEs have been in production for about 50 years and the company has changed ownership structure over that time, it would not be surprising if there have been build differences over the years.  When I get time I will have to drift test mine. 
 

Typically,  an Abbe Ortho FOV restricted to ~40-42° to avoid off-axis distortions. Baader, and it's well known, widened FOV in BCOs up to 50° on purpose to make finding targets easier, e.g. for users with fully manual scopes. Some observers dislike this feature, others, me included, accept this trade off OK. If this feels annoying enough, one can install custom diafragms/Field Stops limiting FOV to regular 40° and BCOs will show images sharp to the edge like any Abbe Orthos.

 

I read once where Don Pensack has said that plossls out perform abbe orthos off axis once you get about 20% from center field.  That's confusing to me since I've also read that orthos are supposed to be without distortion within their 40-42 field. 

 

The Baaders are my only abbe orthos (other that a 30 year old Orion volcano top 9mm, which might be an abbe). On the Moon I found the Baaders to be sharp out until about 50% from center of field. That should be within the 40-42 AFOV of a traditional abbe design.

 

But your point is well taken about Baader widening the field out from 42 to 50 to provide a wider field for locating and holding objects in the field of view. I use a fully manual mount so that is something I appreciate and may help explain why I was initially disappointed in their off axis performance.
 

Unlike single-coated RKEs, which excel mostly on bright targets, BCOs with their superb Baader Phantom coatings are also pretty good on faint DSOs.
 
I, personally, almost always have in my EP case the 10 and 6mm BCOs and the 15 and 8mm RKEs which I use both directly and with 2x-3x Barlows on planets, the Moon, double stars. They all perform excellent in my SV102 when the seeing conditions support high magnifications.

I don't get my 100mm Tak out under a dark sky as often as I would like but I will have to take the Baader orthos out to see what they can do on DSOs under a dark sky. I found their off axis performance distracting when sweeping the Moon but it would probably not be an issue for deep sky, or planets that are kept in the central 50% of the field. I just got them recently so I need to try them on different targets and under a dark sky. The focal lengths of the Baaders do compliment the RKEs nicely.

[Starman1]

Doesn't the floating effect occur when you're not actually looking through the eyepiece? so really not anything useful but just cool.

I found it with Nagler T4s .. when you approach the telescope you can kinda see the image already from afar.

Usually you have to get your eye in there to see something but here's it's like it's projected on the rather large top lens, so it's like a tiny tv screen. 

The eye lens is pretty much the same diameter as the outer part of the eyepiece.

The thin aluminum edge surrounding the eye lens is beveled so that when you hold your head at the exit pupil, you are looking past the aluminum.

So the aluminum ring surrounding the field is very very thin, giving the sensation that the field of view is floating above the focuser and not in an eyepiece.

This only occurs when your eye is exactly at the exit pupil and your head is not bobbing back and forth.

 

It's not the only eyepiece that gives this impression, but that is an explanation of what some experience with the 28mm RKE.

[Starman1]

I read once where Don Pensack has said that plossls out perform abbe orthos off axis once you get about 20% from center field.  That's confusing to me since I've also read that orthos are supposed to be without distortion within their 40-42 field.

 

Outer field performance is affected by field curvature, astigmatism, chromatic aberration, light scatter, and distortion.

Distortion, in a tracking scope, is a normally invisible distortion as long as the object is not at or near the edge, and the eyepiece is not panned across a field of stars.

 

The contemporary Plössl designs are just as orthoscopic as the Abbe eyepieces, since most of the Orthoscopics currently sold are not completely orthoscopic.

The degree of distortion varies by brand, as it does in the Orthoscopics.

 

Different people have different prerequisites with regard to eyepieces.  Maximum axial contrast might give an edge to the Abbe orthoscopic design.

Maximum sharpness and acuity 20° off axis may favor the Plössl design.

If a factory tweaks either design, those statements could be invalid.  One contemporary "orthoscopic" line is essentially a tweaked Plössl.  Some Orthos are far from othoscopic.

 

When I said "outperform" in the outer field, I was referring to spot size and absence of chromatic aberration.  Control of astigmatism would also vary by the f/ratio of the scope.

Both types work fantastic at f/11, and neither is great at f/4, for instance.  If both are used in a tracking scope only to look at planets, I can see where the Abbe ortho might be preferred.

If both types are used in a non-tracking scope to look at deep sky objects like star clusters, I can see where a Plössl might be preferred.

 

At very short focal lengths, the Abbe design has a better eye relief, and that could be important.  If long eye relief is desired, neither design has the edge on some more contemporary designs.

 

Hope that clarifies the earlier comment.  The answer is often more nuanced than a simple statement.

[Muffin Research]

The eye lens is pretty much the same diameter as the outer part of the eyepiece.

The thin aluminum edge surrounding the eye lens is beveled so that when you hold your head at the exit pupil, you are looking past the aluminum.

So the aluminum ring surrounding the field is very very thin, giving the sensation that the field of view is floating above the focuser and not in an eyepiece.

This only occurs when your eye is exactly at the exit pupil and your head is not bobbing back and forth.

 

It's not the only eyepiece that gives this impression, but that is an explanation of what some experience with the 28mm RKE.

Ah okay, then I have not seen it I thought it was the effect I saw with the T4's which kinda feels a bit like you're looking at a hologram in a dome. 

[Mcloud]

The "floating" effect is not specific to RKEs. It seems the requisites include long eye relief (perhaps very long) and large eye lenses with moderately (or significantly) convex outer (eye-facing) surfaces. Nikon's C-W 10x/22 microscope ocular might fit the bill, but I'm just guessing (never tried one). Moderately/significantly convex eye-lens surfaces aren't very common among eyepieces.

Best wishes.
Dan

.

That actually makes sense.

[Thomas_M44]

I’m pretty vexed by all the attention that gets placed upon the 28mm RKE  “floating effect”.

 

One could be innocently lead by such endless chatter about the novelty of the 28mm, to lose sight of other significant attributes and particulars of the RKE eyepiece line.

Edited by Thomas_M44, 05 September 2021 - 08:42 PM.

[izar187]

Yeah, but that 28mm... it's floaty!

[Nippon]

I’m pretty vexed by all the attention that gets placed upon the 28mm RKE  “floating effect”.

 

One could be innocently lead by such endless chatter about the novelty of the 28mm, to lose sight of other significant attributes and particulars of the RKE eyepiece line.

I agree. Even Edmund did not see that everyone would be all gaga about the floating thing because they supplied a huge ugly accordion type eyecup with it that makes that effect disappear. Truth is if you want the best contrast from it it is better to use the eyecup. Even at very dark sites I find the eyecup or just cupping my hands around it helps low contrast deep sky objects.

[Lighthound]

Seeing since no one has explaned it yet, here:

 

 

 

[Starman1]

In several long threads (look them up in the search), Edmund themselves explained that RKE was "Rank--Kaspereit--Erfle"

even though many were saying "Rank Kellner Eyepiece" or "Reverse Kellner Eyepiece" and many personal opinions have been posted on the web.

 

Edmund's explanation was from David Rank, who pointed out the derivation for the design in the name.

That makes sense, actually, since no one else ever uses the word Eyepiece in the name of an obviously eyepiece product.

If it had been merely a Reverse Kellner, it would have been called an RK eyepiece, not an RKE eyepiece.

Earlier literature from Edmund called such a deign a "Kellner Type 2", with an eyepiece having 2 doublets as a "Kellner Type 3".

Note that Plössl designed his 4 element eyepiece BEFORE Kellner came out with his 3 element design, so it was probably incorrect to refer to a Plössl as a Kellner Type anything.

 

The RKE more closely resembles the 3 element König eyepiece:

https://www.telescop...berration_2.htm

Go down the page to the cross section charts.

 

But many similar designs were penned by optical designers of the era, so I'm inclined to tribute David Rank with the final say.

[John Rogers]

I have posted this in some previous threads and might be worthwhile to post it here as well.  It is the first reference to the RKE eyepiece that was announced in March 1978 by Edmund.  As you can see, the spec value of the apparent field of view was 50 degrees.  Note the claim that some users were reporting that it was even larger than that value.

 

 

 

 

[DeWynter]

In several long threads (look them up in the search), Edmund themselves explained that RKE was "Rank--Kaspereit--Erfle"

even though many were saying "Rank Kellner Eyepiece" or "Reverse Kellner Eyepiece" and many personal opinions have been posted on the web.

 

Edmund's explanation was from David Rank, who pointed out the derivation for the design in the name.

That makes sense, actually, since no one else ever uses the word Eyepiece in the name of an obviously eyepiece product.

If it had been merely a Reverse Kellner, it would have been called an RK eyepiece, not an RKE eyepiece.

RKE explanation as "Rank, Kaspereit, Erfle" is also stated in the Edmund Scientific's trademark registration:

http://tdr.uspto.gov...ion?sn=73173827

Need to click on "Unclassified" document and then scroll to page 9.

[j.gardavsky]

On a side line,

the Kellner (Kellner I) is an intrafocal eyepiece (field stop between the lenses), the RKE is extrafocal (field stop in front of the first lens).

There is no reversal of an intrafocal into extrafocal design. And even a computer would not help to make such reversal, unless the math comes from another universe. In plain language, flip would be better than reversal.

 

The predecessors to RKE are the Zeiss West designs from the former Zeiss-Winkel company in Göttingen, not in Jena.

The design has been developed in brains of the designers, before the computers. The roots of the design are believed to be due to the designs of König, even if the Göttinger school of mathematics may believe differently.

 

 

At left is the "Urvogel" - the first widespread form from about 50 years ago.

At right is the popular later version, also suited for the astronomy. The E-Pl (and W-Pl) versions are still manufactured.

 

The first two numbers in the product code indicate the compatibility with the specific microscopes, the last four numbers are the eyepiece codes.

 

The (field stop - 2 - 1) type of eyepiece design became the most popular one for the stereo microscopes (Zeiss West STEMI), and for the professional grade Zeiss West microscopes (Axioskop, and others). And also at Leica Microsystems, this design is in the Leica HC Plan.

So far I know, these eyepieces are still manufactured, both by Zeiss and Leica.

 

Best,

JG

 

 

[Thomas_M44]

I have posted this in some previous threads and might be worthwhile to post it here as well.  It is the first reference to the RKE eyepiece that was announced in March 1978 by Edmund.  As you can see, the spec value of the apparent field of view was 50 degrees.  Note the claim that some users were reporting that it was even larger than that value.

 

 

Edmund_RKE_Announcement_March_1978_1.jpgEdmund_RKE_Announcement_March_1978_2.jpg

I suspect that when Edmund came out with a line of Plossls, a few years after release of the RKE line, the marketing department “massaged” the RKE AFOV specification downwards, in order to provide market differentiation between the RKE’s and the new Plossls.

 

Remember, the Plossl became a very hot, popular eyepiece in the early 1980’s, with virtually all major eyepiece brands scrambling to release a Plossl line.

 

Being able to list Edmund Plossls at 50-degrees AFOV, and RKE’s at 45-degrees AFOV—this is a bit of product differentiation, helping highlight the new Plossl line.

 

This is only my conjecture 

[AdmiralAckbar]

I am an RKE fan.

 

This has been noted already, but it's worth reiterating: The only RKE to offer the famous "floating" effect is the 28mm. The 21.5mm does not create this effect, nor do the 15mm, 12mm, or 8mm. I know this from plenty of experience with these eyepieces. So, if you're using any of the aforementioned other than the 28mm and wondering where the floating is, that's a case of user error.

 

If you wish to understand the floating effect but do not currently have access to a 28mm Edmund RKE, I suggest finding a thin-walled cup without a handle--perhaps one of the stainless steel cups w/ a wall only a few millimeters thick--and filling it completely to the brim with water. Then, look directly down at it. Notice how you see the water almost overflowing from the cup, but you barely see the perimeter because it is so thin. Now, image that you're looking down at a luminous star cluster or nebula instead of water. Voila: The 28mm Edmund RKE.

 

As Thomas M44 has noted, this is not the only (or main) advantage of the RKE eyepieces. (And, again, the floating effect is peculiar to the 28mm RKE.) They also have wonderful transmission, (on axis) sharpness, and color rendition. I love the 28mm RKE on everything, and I happily use the whole set to observe double stars. If you add a 4x barlow (e.g., Siebert telenegative) to the set, you get a wonderful spread of focal lengths from 28mm to 2mm.

 

I'll offer an anecdote. This past summer, I was out observing with a few friends who had never before looked through a telescope. So, I had a Takahashi FC-100DC and Mewlon 180C out at their place, where the sky is relatively dark (Bortle 3). We looked at some of the greatest hits... Saturn & Jupiter, Wild Duck Cluster, Izar, Delta Cygni, Veil Nebula, Ring Nebula, Double Double, the richness of Sagittarius, and more. I used a variety of eyepieces, including 100* APMs and Nagler T6's. Undoubtedly, there were many oohs and aahs all night. But what stood out was that, without fail, every time the 28mm RKE was in the diagonal, every observer's immediate response was, "Holy ****. That's gorgeous."

 

In case you didn't know, Edmund Scientific still sells RKE eyepieces. It's curious that this meek set of eyepieces seems to be so divisive. So, I'll take one for the team. If you're one of the people who has an RKE and doesn't like it, PM me. I'll buy em off you. It's an open offer.  

 

Love,

AA

Edited by AdmiralAckbar, 10 December 2021 - 05:00 PM.

[Thomas_M44]

The Edmund  RKE is an unusually underrated and misunderstood eyepiece.

 

The current ones have very nice Japanese-made lens sets, and the optical quality is excellent.

 

The absolute best views I obtained of Mars with my TV85 this last conjunction were using the 21mm RKE plus 5X TV Powermate. The sharpness and definition bested my KK orthos and TeleVue Plossls by a useful margin.

Edited by Thomas_M44, 11 December 2021 - 11:26 PM.

[luxo II]

Funny you should mention that - the 21mm RKE was IMHO the best of the set I had and paired nicely with Barlows.