86 replies to this topic

[Dave Mitsky]

Count me among those who enjoy the floating effect of the 28mm RKE.

[CeleNoptic]

The *actual* AFOV of the RKE eyepieces is right at 50-degrees, and not the 45-degrees stated by Edmund.

 

I've recently measured the effective FOV in my 15mm RKE and it was exactly 45° as claimed by Edmund.

[Thomas_M44]

I've recently measured the effective FOV in my 15mm RKE and it was exactly 45° as claimed by Edmund.

That’s strange.

 

My 21mm RKE measures just over 50-deg, and stars clean to the field stop.

 

If one looks back to the earliest RKE advertisements, Edmund does claim 50-deg AFOV. Later, after Edmund released a line of Plossls, they downgraded the RKE series claimed AFOV to 45-deg.

 

 

Btw: My current-production 21mm and 12mm RKEs, with either 5X Powermate or 3X TV Barlow, delivered a good margin of superior contrast and fine detail retrieval when viewing Mars as compared to my current-production TV Plossls and KK Fujiyama orthos.

 

I need to perform some further eyepiece comparisons with my recently acquired 150mm f/6 Newtonian once Jupiter and Saturn return to convenient viewing hours.

Edited by Thomas_M44, 02 July 2021 - 01:15 PM.

[lylver]

That’s strange.

 

My 21mm RKE measures just over 50-deg, and stars clean to the field stop.

 

If one looks back to the earliest RKE advertisements, Edmund does claim 50-deg AFOV. Later, after Edmund released a line of Plossls, they downgraded the RKE series claimed AFOV to 45-deg.

 

Edmund Optics spoke about unvignetted field.

RKE is a "45° class" EP

In fact 47° unvignetted and not taking account of distorsion.

It is an asymetric design based on a "magic match" of two glass and optical maths.

It sits on the middle of the optimisation for about every modern scope and eye aberrations

Main strengths are

- the comfortable eye relief ratio to focal length.

- lower aberration due to high index glass and compensation of inner aberrations between themselves.

- superior eyecup

- superior optic fabrication and adequate "simple" coating.
 

[Dave Mitsky]

Here's a photograph of the venerable 15 and 28mm RKEs that the Astronomical Society of Harrisburg owns. 

Attached Thumbnails

• 

[JIMZ7]

Liked the RKE 28.7mm & didn't like it. It gave you a "floating" effect, but floated the images into my light pollution so I sold it. Oh well.

Jim

[Nippon]

In a thread by BillP he came to the conclusion the they are actually 50 degree AFOV. The floating effect is neat but I agree with the poster who said it is a hindrance unless your are at a dark site. The floating field seems to get all the attention but to me the beauty of the series is their sharpness and contrast. Plus they are truly parfocal. I have the entire series and have found if you focus with any one your done with the focus knob for the session.

[Terry Smith]

I own a complete set of current-production Edmund RKE eyepieces (except for the now discontinued 15mm).

 

These current Edmund RKE eyepieces all use high quality Japanese lenses.

 

The unique presentation of the 28mm is enjoyable, but ironically I feel this distracts attention away from the fact that the Edmund RKE’s in general are remarkably sharp and high-contrast eyepieces which are truly exceptional for high-magnification planetary viewing.

 

During the last Mars opposition, I had several types of quality eyepieces which I utilized with my TV85 refractor, including sets of current production KK Fujiyama Orthos; TV Plossls and Edmund RKE’s.

 

The overall best results I obtained were using eyepieces in the 28mm to 15mm range in conjunction with a TeleVue 5X Powermate, which gave effective eyepiece  FL’s in a range between 5.6mm to 3mm. I also could insert a simple 1.5” long extension tube between the 5X Powermate and the eyepiece which would increase the Powermate magnification to approximately 6.2X,  and so an effective FL of 2.42mm when using a 15mm eyepiece.

 

Surprisingly, the most detailed and highest contrast views I was able to obtain of Mars was via using the 21mm RKE with the 5X Powermate with the 1.5” extension tube (6.2X) and so an effective FL of 3.4mm which gave me 177X in the 600mm FL TV85.

 

I also used various combinations of 15mm to 9mm range KK Ortho, TV Plossl and RKE eyepieces with a TV 3X Barlow,  and the results were VERY good, but this did not seem superior to the results using the longer FL eyepieces with the 5X Powermate, which also gave the considerable benefit of much more comfortable eye-relief.

 

Another nice combination was the 28mm RKE plus 5X Powermate with 1.5” extension tube for an effective eyepiece FL of  4.5mm, producing 133X in the TV85.

 

A few other surprising findings:

 

The *actual* AFOV of the RKE eyepieces is right at 50-degrees, and not the 45-degrees stated by Edmund.

 

The RKE eyepieces tolerate moderately fast f-ratios below f/6 much better than is generally assumed. The RKE is much more versatile in this respect than a Keller.

 

And so, the Edmund  RKE eyepieces in general are surely worthy of respect and serious consideration as high-resolution planetary eyepieces and also purportedly very good for double star viewing, which I hope to experiment with soon in comparison to my Abbe Orthos, Plossls and Keller’s

 

I really like the RKE's myself. They are my favorite eyepieces in my collection. Super sharp and contrasty. I had two sets before and sold them...big mistake. I just recently acquired another complete set. I'm keeping them this time. Also the AFOV seems much closer to 50 degrees than the stated 45 degrees.

 

The 21.5mm and 12mm I purchased new from Edmund Scientific. The 28mm, 15mm and 8mm were included in an Astroscan purchase. What a deal. Sold the Astroscan, kept the eyepieces basically for free.

 

I will also say the TeleVue 3x barlow works fantastic with the 21.5mm, 15mm and 12mm.
 

Edited by Terry Smith, 07 August 2021 - 10:24 PM.

[jeffmac]

How do you measure spot size? The smaller the spot size the better, right? On the Edmund site, it lists the 8mm RKE spot size as 7/8. They give the spot size for each of the RKE's. The spot size for another focal length is 3/12. What do these measurements mean?

[Ihtegla Sar]

When reading about eyepieces, a lot of people speak passionately about RKE, with many mentioning it in the thread on the 10 best eyepieces of all time (specifically the 28mm RKE). Can someone explain to me what the big fuss is with this one? And if I'm curious to try one for my 8" Dobsonian, which one would you recommend?

My first scope was an Astroscan and my first three eyepieces purchased 30 years ago were the 8mm, 15mm and 28mm RKEs. I remember them being good eyepieces back in the day but eventually I moved on to other eyepieces, including initially some Orion ortho and ultrascopic designs, which according to Orion were supposed to be better, but in retrospect I don't think were any better and may not have been as good as the RKEs.

I haven't used the RKEs much in years, but last week I pulled them from storage and was surprised that they compared favorably to my brand new Baader Classic Orthos in my FC100DL on the Moon. The RKEs seemed to provide better contrast on the dark shaded areas in and around the Sea of Vapors, and the overall views in the RKE were at least as sharp as the BCO on axis and seemed sharper off axis.

After researching the RKEs a bit, I found a wide divergence in the view points. Some people seem to dismiss them as a "gimmick" due to the floating effect, or consider them a cheap outdated design no better than a slew of other eyepieces, or only suitable for nostalgia. Others find the RKEs to be very good eyepieces, some comparing them favorably to Japanese abbe orthos.

A number of eyepiece experts seem to hold the RKEs in high regard. Bill Paolini, author of Choosing and Using Astronomical Eyepieces (The Patrick Moore Practical Astronomy Series) ranks the RKEs among the best planetary eyepieces currently in production.

https://www.cloudyni...ion/?p=10761176


I also found this article on Planetary Eyepieces written by Daniel Mounsey back in 2004 where a group of experienced observers tested a wide variety of eyepieces (including eyepieces made by Televue, Pentax, Takahashi, Brandon and Zeiss, among others) using some very high quality apo refractors as well as some newtonians, a Tak Mewlon and at least one C14.

https://www.cloudyni...yeyepieces.pdf

The article concluded that the RKEs performed equally or better than any of the other eyepieces tested, except for the ZAOs.

The RKE only has three elements, so that's less glass than most other "minimal glass" planetary eyepieces, like plossls or abbe orthos that have four elements. They were designed by Dr. Rank, who seems to be highly regarded as optical designer. I even came across one poster saying that Dr. Rank was the second person to have produced heavy water, which he handed in as a sample, along with a PhD thesis. https://www.cloudyni...-rke/?p=6414662

I am going to have to spend more time with my RKEs.

Edited by Ihtegla Sar, 31 August 2021 - 10:23 PM.

[vtornado]

After swapping out the RKE 8 and televue plossl 8 on the moon, I could see no material difference.

This is in a 12 inch f/5 scope.

Same with Jupiter.   

In the day time comparing the field of view they are very close.

 

The rke has a retro coolness factor.  The recessed eye lens decreases the potential eye relief but ...

It does stay cleaner than the televue 8.

[jeffmac]

After swapping out the RKE 8 and televue plossl 8 on the moon, I could see no material difference.

This is in a 12 inch f/5 scope.

Same with Jupiter.   

In the day time comparing the field of view they are very close.

 

The rke has a retro coolness factor.  The recessed eye lens decreases the potential eye relief but ...

It does stay cleaner than the televue 8.

Which was more comfortable to use?

[vtornado]

Which was more comfortable to use?

Hard to say both have short eye relief once the recess is taken into account.   maybe the RKE slightly but it is not

a tipping factor.  The RKE site lists the ER at 7mm.   That might be from the top of the eyepiece and not from the lens.

Televue lists ER as 6.

[Bill Weir]

I’m going to go in the opposite directing of the RKE worshippers. I just don’t get this eyepiece. Years ago I pulled a 21.5 out of the junk eyepiece drawer at the school observatory. Heard so much raving about them. Tried it in my f/8 150mm dob. Very unpleasant view. Then tried it in my f/5 315mm dob. Same. Then my little f/6.8 80mm refractor. Nope. Guess I like my whole FOV to be distortion free. 

 

So what kind of scope is this eyepiece good for? I still have the eyepiece in my cupboard waiting for info on how to witness the “Magic” this eyepiece is supposed to provide. 

 

Bill

[Starman1]

My first scope was an Astroscan and my first three eyepieces purchased 30 years ago were the 8mm, 15mm and 28mm RKEs. I remember them being good eyepieces back in the day but eventually I moved on to other eyepieces, including initially some Orion ortho and ultrascopic designs, which according to Orion were supposed to be better, but in retrospect I don't think were any better and may not have been as good as the RKEs.

I haven't used the RKEs much in years, but last week I pulled them from storage and was surprised that they compared favorably to my brand new Baader Classic Orthos in my FC100DL on the Moon. The RKEs seemed to provide better contrast on the dark shaded areas in and around the Sea of Vapors, and the overall views in the RKE were at least as sharp as the BCO on axis and seemed sharper off axis.

After researching the RKEs a bit, I found a wide divergence in the view points. Some people seem to dismiss them as a "gimmick" due to the floating effect, or consider them a cheap outdated design no better than a slew of other eyepieces, or only suitable for nostalgia. Others find the RKEs to be very good eyepieces, some comparing them favorably to Japanese abbe orthos.

A number of eyepiece experts seem to hold the RKEs in high regard. Bill Paolini, author of Choosing and Using Astronomical Eyepieces (The Patrick Moore Practical Astronomy Series) ranks the RKEs among the best planetary eyepieces currently in production.

https://www.cloudyni...ion/?p=10761176


I also found this article on Planetary Eyepieces written by Daniel Mounsey back in 2004 where a group of experienced observers tested a wide variety of eyepieces (including eyepieces made by Televue, Pentax, Takahashi, Brandon and Zeiss, among others) using some very high quality apo refractors as well as some newtonians, a Tak Mewlon and at least one C14.

https://www.cloudyni...yeyepieces.pdf

The article concluded that the RKEs performed equally or better than any of the other eyepieces tested, except for the ZAOs.

The RKE only has three elements, so that's less glass than most other "minimal glass" planetary eyepieces, like plossls or abbe orthos that have four elements. They were designed by Dr. Rank, who seems to be highly regarded as optical designer. I even came across one poster saying that Dr. Rank was the second person to have produced heavy water, which he handed in as a sample, along with a PhD thesis. https://www.cloudyni...-rke/?p=6414662

I am going to have to spend more time with my RKEs.

Light scatter, though, is determined by air-to-glass surfaces, not lens count, so there would be no difference in that regard between 1:3 design, 2:2 design or 1:2 design.

Other differences would apply in a bigger way, like lens polish, coatings, internal suppression of scattered light, etc.

Glass only loses 1% of light per inch of glass, so the actual lens count is not relevant for these small eyepieces.

Edited by Starman1, 01 September 2021 - 11:38 PM.

[Ihtegla Sar]

Light scatter, though, is determined by air-to-glass surfaces, not lens count, so there would be no difference in that regard between 1:3 design, 2:2 design or 1:2 design.

Other differences would apply in a bigger way, like lens polish, coatings, internal suppression of scattered light, etc.

Glass only loses 1% of light per inch of glass, so the actual lens count is all that relevant for these small eyepieces.

Okay, so three elements vs four elements only translates to a fraction of a percent less light loss, probably imperceptible.  The RKEs must have many of those other differences you mentioned that puts them among the top tier of the "minimal glass" eyepieces, at least among the various eyepiece experts listed in my initial post.  My own evaluations of the RKE are still at an early stage to form any solid conclusions myself, and I am far from any kind of eyepiece expert.

Edited by Ihtegla Sar, 01 September 2021 - 09:33 PM.

[Ihtegla Sar]

I’m going to go in the opposite directing of the RKE worshippers. I just don’t get this eyepiece. Years ago I pulled a 21.5 out of the junk eyepiece drawer at the school observatory. Heard so much raving about them. Tried it in my f/8 150mm dob. Very unpleasant view. Then tried it in my f/5 315mm dob. Same. Then my little f/6.8 80mm refractor. Nope. Guess I like my whole FOV to be distortion free.

So what kind of scope is this eyepiece good for? I still have the eyepiece in my cupboard waiting for info on how to witness the “Magic” this eyepiece is supposed to provide.

Bill

The scope I used was an f/9 Tak doublet (FC100DL). What eyepieces are you comparing it to? I don't have the 21.5 (yet) but I have compared the RKE 15 and the 8 to my ES 82 degree 14 and 8.8. I found the ES 82s to be much better corrected off axis and overall more comfortable to use, but the RKEs were sharper on axis with better contrast on Lunar shading than the ES 82s.

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.

If I was looking for a wide field eyepiece that was well corrected off axis, I would not be looking for an RKE, but an ES 82 or 92, Nagler, Delos, Ethos, APM UFF or XWA, etc. or one of the many other modern eyepieces that is designed to be well corrected off axis.

The RKE is more of a niche "minimum glass" eyepiece and with only three elements does not provide the same level of off axis correction as a modern widefield design. You might try barlowing (or better yet Powermating) the RKE 21.5 to see how it performs on axis for planets at higher magnification. On axis planetary is where the "magic" of the RKEs seems to be.

Edited by Ihtegla Sar, 01 September 2021 - 09:49 PM.

[izar187]

Short ramble...

 

The 21.5mm was my least used, by a wide margin.

 

After eventually getting a used GSO 32mm plossl, the 28mm saw far less use.

Often relegated to UHC use, with and without barlow, so I didn't have to futz with moving the filter.

 

They are all of 'perceived' high contrast, yup.

Sharp on axis too... so honestly best with a mount that tracks.

 

When used in a very short focal length scope... well, everything looks sharper at lower power.  ; D

 

For me, a 6mm VT ortho is easier to look thru then the 8mm RKE.

On gas giants, a UO 16 Konig II plus Silvertop 2x was very much the equal of the 8 RKE, plus larger field and way way easier to look thru.

 

Rather than barlowing up one of the two longer focal length RKE's to a 5, 6 or 7 element long eye relief combo in the focuser, I'll take a 60 to 70 degree FOV with that please, and just use a bit better long eye relief wide field ep instead.

 

They can fit in ok between missing ortho focal lengths.

[Thomas_M44]

I’m going to go in the opposite directing of the RKE worshippers. I just don’t get this eyepiece. Years ago I pulled a 21.5 out of the junk eyepiece drawer at the school observatory. Heard so much raving about them. Tried it in my f/8 150mm dob. Very unpleasant view. Then tried it in my f/5 315mm dob. Same. Then my little f/6.8 80mm refractor. Nope. Guess I like my whole FOV to be distortion free. 

 

So what kind of scope is this eyepiece good for? I still have the eyepiece in my cupboard waiting for info on how to witness the “Magic” this eyepiece is supposed to provide. 

 

Bill

Must have been a flawed sample.

[bremms]

I was talking about this to a friend a while back. Had probably three RKE's over the years. Four if you include that early 1 1/8" FL, but that was a Plossl IIRC.

Always liked the RKE and all but one of mine were given with scopes that were sold or gifted. searched through all my stuff and I don't have an RKE currently. I have a surplus eyepice that is about 28mm that gives a similar effect.

[Nippon]

Must have been a flawed sample.

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

[Tony Cifani]

My very first eyepiece was an RKE 15mm, back in the early 80's when I was a kid. I was told that the eyepiece was designed using a computer, and this really impressed me at the time. I really don't know how much truth there was to that story but I thought the eyepiece was very special at the time. I never owned the 28mm.

[Thomas_M44]

During the last Mars opposition,  swapping repeatedly between a TeleVue 20mm Plossl, 18mm KK Fuji ortho and a 21mm RKE (all used in conjunction with a 5X Powermate), I found the RKE 21mm delivered the greatest amount of detail and boundary contrast. The superiority was not necessarily “ night and day” but it was nonetheless very apparent and useful.

 

Both the 20mm TV Plossl and 18mm KK ortho provided excellent,  enjoyable views, it’s just that the RKE was performing on a somewhat higher level. My spouse came to the same conclusion herself.

 

The results on Saturn were similar to the Mars viewing.

 

Note:  the TV85 scope was used, with an APM Amici diagonal, and also straight-through, with 5X Powermate but without diagonal. Going straight-through increased the clarity and sharpness of all views by a modest measure, but still showed similar differences between the three eyepiece types as compared to viewing through the diagonal. All eyepieces and equipment are current production, purchased within the last 2 years.

 

The 21mm RKE plus 5X Powermate has become my current reference eyepiece setup for planetary viewing with the TV85.

Edited by Thomas_M44, 03 September 2021 - 04:37 PM.

[greenjuice]

Glad to hear it Thomas. I recently purchased the 21mm and I do have a 4x powermate and a TV 85, so looking forward to checking it out tonight BECAUSE we seem to be having an actual clear night here in beautiful downtown NW CT !

[213Cobra]

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