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Legendary Eyepiece Profiles: 28mm Edmund RKE

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#1 jrbarnett

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Posted 23 January 2015 - 01:11 PM

IMG_0391_zps727886e9.jpg

 

IMG_0506_zpsf1726734.jpg

 

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.   :grin:

 

Next up, next week probably, the 48mm Vernonscope Brandon...

 

- Jim


Edited by jrbarnett, 23 January 2015 - 01:13 PM.

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#2 Scott in NC

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Posted 23 January 2015 - 01:21 PM

Thanks for sharing that nice report, Jim.  This is one eyepiece that I've still never had the opportunity to look through, but it's on my list. :)


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#3 Starman81

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Posted 23 January 2015 - 02:53 PM

Yes, nice report. But do people do 're-enactment observing'? Something like civil war re-enactments? Were there some great observations that took place with the RKE 28 in the '70s that I am unaware of?

 

Anyways, the RKE 28 was the first eyepiece I bought new and my only remaining eyepiece from when I got back into the hobby just 3 years ago. So I will always keep it. 


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#4 GOLGO13

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Posted 23 January 2015 - 03:45 PM

Nice writeup...but I still love RKEs! While I would agree that Orthos and Plossls are probably better in general, I still think RKEs do a pretty good job.

 

I think they do pretty well...especially for solar observing. I also seem to remember them showing just a bit more of dim stars...but I will need to check my comparison on that one.

 

And yes...the 28mm RKE's effect is really neat. Makes it seem like the diagonal has dissapeared and your peering into space. Quite immersive.

 

Non astronomy folks often really like the 28mm RKE I find. Probably because they don't have to cram their eye into the eyepiece like they may have to with other designs.



#5 Kent10

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Posted 23 January 2015 - 04:12 PM

I also have the RKE 28mm and no others from that line.  Bought it based on all the enthusiasm for it, of course.  It also is one of the few eyepieces I have bought new.  I tried the bargain Surplus Shed ones at first but they were very dirty with lots of dust.  Perhaps that didn't matter with the views but I sent them back and bought them from Edmund before the price hike.  These look good. 

 

I bought 2 for my binoviewer.  I don't use them very often.  I found them hard to center in the binoviewer because of the long eye relief.  But the floating effect is neat.  I use it from time to time in mono and do enjoy the effect on clusters.  I tried it last night on the Horsehead Nebula but I don't recall it being special compared to other eyepieces I was using.  I won't sell mine either because they aren't expensive and I do enjoy them from time to time.  I haven't evaluated them critically to see if they are sharp or contrasty etc compared to others I own.



#6 rowdy388

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Posted 23 January 2015 - 05:04 PM

This eyepiece is fun to use and since I only observe for pleasure, I find it worth having.  I have never seen another eyepiece with my limited experience that gives the same effect.  I'll never sell it but it doesn't get a whole lot of eye time these days.

Dave Y



#7 Pezdragon

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Posted 23 January 2015 - 06:08 PM

The 28 RKE is one of those quirky ones. The floating presentation is akin to the meade research grade 20mm erfle which I also keep tho it's edge correction is a joke compared to today's offerings. But.....the small form factor of those guys are one of their strengths...wide field and tiny body..yes...it's just my eyes complain after awhile of the loopy back in the 60's space zoom trip.....now there's nostalgia for ya.....


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#8 jrbarnett

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Posted 23 January 2015 - 08:50 PM

Yes, nice report. But do people do 're-enactment observing'? Something like civil war re-enactments? Were there some great observations that took place with the RKE 28 in the '70s that I am unaware of?

 

Anyways, the RKE 28 was the first eyepiece I bought new and my only remaining eyepiece from when I got back into the hobby just 3 years ago. So I will always keep it. 

Dunno if others do observing re-enactment, but I sure do.

 

:grin:

 

- Jim



#9 jrbarnett

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Posted 23 January 2015 - 08:55 PM

Here are a few more "Legendary Eyepiece Profile" candidates I have in mind:

 

35mm Celestron Ultima

10mm CZJ Orthoscopic

22mm Televue Panoptic

22mm Vixen LVW

3.8mm Pentax XP

5mm TMB Supermono

6mm Pentax SMC Ortho

14mm Meade Series 4000 UWA

8.8mm Meade Series 4000 UWA

7mm TMB Supermono

 

I'll probably come up with a few more, too.

 

- Jim



#10 daniel_h

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Posted 23 January 2015 - 09:16 PM

excellent series



#11 Jeff Morgan

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Posted 24 January 2015 - 02:31 AM

 

 

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.  

 

Probably true. I have fond memories of it with the aforementioned Astroscan during the drought summer of 1988. The combo saw heavy use and the RKE was certainly enjoyable if not impressive.



#12 BillP

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Posted 24 January 2015 - 12:13 PM

 

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.  ...

 

Next up, next week probably, the 48mm Vernonscope Brandon...

 

Context is everything in language...and with just about everything IMO.  Divorce things from their context and that are nothing more than a collection of elemental particles.  So without context and in comparison to more modern fare, you are right, the RKE does not at first blush appear to have anything endearing.  However, when used properly and place in a longer focal ratio telescope, like an f/8, it does present the rather unique presentation of its FOV with little edges of the housing visible.  When mated with a strong Barlow it also curiously produces an exceedingly crisp and defined planetary view...better than modern counterparts I have found.  And when strongly Barlowed, extending its already large ER, it becomes a stand-off eyepiece which for sketching is a dream as you can stand off and glance at the planet, then glance at the sketch pad and just move your eye's focus back and forth as it you were looking at the subject instead of through the scope.  So the 28 RKE doesn't do all the dance steps well, but there are a few that it does better than most any.  All you gotta do is enjoy those particular dances :grin:

 

On the 48 Brandon....holy cow I got rid of that way fast as it's off-axis was horrible even in my f/8 scopes!!  That one IMO has zero legendary attributes and should not be on the list. 


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#13 ibase

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Posted 24 January 2015 - 12:56 PM

Legendary it is. The RKE 28mm, as simple

 

RKE28dis_zpscd846b44.jpg

RKE 28 exploded view

 

and as inexpensive as it may be (<$100) but with its vaunted magical floating view and EP disapearing act, stands tall among premium expensive EP's costing hundreds of $'s more like Ethos, Pentax, Delos, Brandons, Naglers etc., garnering mentions as among the top favorite EP's of all time.

Best,


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#14 stevew

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Posted 24 January 2015 - 03:22 PM

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. 

 

- Jim

I have an old version of this RKE 28mm, and while it does show some minor astigmatism it's enjoyable to use due to the "floating in space" effect as mentioned above.

Can someone tell me if there are other more modern eyepieces that have this effect.

 

Steve



#15 Dave Mitsky

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Posted 24 January 2015 - 03:29 PM

But the floating image effect is COOL.

 

There's simply no denying that fact.

 

Dave Mitsky



#16 waso29

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Posted 24 January 2015 - 06:54 PM

my edmund 3" f/10 newt came with the neat rke 28.

great outreach eyepiece indeed.



#17 socialoutkast

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Posted 24 January 2015 - 08:11 PM

I have had the RKE 28 and 8mm for over 30 years. My wife bought me an Astroscan for my birthday in 1980. The Astroscan has seen better days (focuser shot) and I have since upgraded to a C8,SLT 130 and 12XXI, the RKEs are still in my lens case. Not great in the 12xxi because of the 46 degree fov., but great in the other scopes.

 The 8mm performs well in the C8 on Jupiter,although I don't get all the moons in the narrow FOV. I bought the 21.5mm before the price hike, its not anything special though.



#18 bgi

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Posted 24 January 2015 - 08:55 PM

I have a pair of Russel 15mm Konigs that were custom-ordered without the eye guards.  They have a similar effect, but not quite as pronounced because the eye relief is fairly short so you need to get up close to them.  Short eye relief brings you in close enough so that you don't see much of the body - rendering the effect to a lesser degree.  They work well in a binoviewer.  If you back off (as the RKE 28 permits) then the effect is lost.



#19 amicus sidera

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Posted 24 January 2015 - 10:09 PM

 

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.  t, 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.   :grin:

 

Next up, next week probably, the 48mm Vernonscope Brandon...

 

- Jim

 

A few clarifications, if I may:

 

1. Edmund didn't design the RKE to replace the "more expensive to manufacture eyepieces (Clave Plossls and others) that Edmund was bundling with its run-away hit Astroscan telescope"; those premium oculars were merely expensive gap-fillers that enabled shipment of the Astroscan, as the supply of 1-1/4" f.l. Kellners (an Edmund staple for many years) had dried up due to unprecedented short-term demand for the scope. The RKE line had been planned since 1975, designed to replace their range of Kellners and Ramsdens; this was well prior to the Astroscan's arrival in the late summer of 1976.

 

2. While both "Rank Kellner Eyepiece" and "Rank Kellner Edmund" were used by Edmund in advertising as decryptions of the RKE acronym, the patent documents that Bill discovered a while back prove that the official descriptor is "Rank Kaspereit Erfle", after its three loosely-ancestral eyepiece forebears.

 

3. Edmund did not "commission(ed) the design to be cheap first and foremost"; they designed them to be well-performing eyepieces that were affordable to people of average means - this is an important distinction that should be made, as use of the term "cheap" in this context seems needlessly derisive.

 

4. "I am not a fan of the RKEs."  I sensed that.  :grin:


Edited by amicus sidera, 24 January 2015 - 10:20 PM.

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#20 Pezdragon

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Posted 24 January 2015 - 10:38 PM

The 28 RKE states a 45 degree afov but when I compare it to a nag 2-4 or 3-6 zoom which states a 50 degree afov they are the same. I did this by superimposing the fields. Which one is correct ? 



#21 pga7602

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Posted 25 January 2015 - 12:59 AM

IMG_0391_zps727886e9.jpg

 

IMG_0506_zpsf1726734.jpg

 

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.   :grin:

 

Next up, next week probably, the 48mm Vernonscope Brandon...

 

- Jim

 

Darn good write up Jim.  I just ordered the RKE after reading this to see what everyone is raving about. 



#22 Mariner@sg

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Posted 25 January 2015 - 08:37 PM

Here are a few more "Legendary Eyepiece Profile" candidates I have in mind:

 

35mm Celestron Ultima

10mm CZJ Orthoscopic

22mm Televue Panoptic

22mm Vixen LVW

3.8mm Pentax XP

5mm TMB Supermono

6mm Pentax SMC Ortho

14mm Meade Series 4000 UWA

8.8mm Meade Series 4000 UWA

7mm TMB Supermono

 

I'll probably come up with a few more, too.

 

- Jim

 

I'd thriw in the suggestion for a 7mm Meade RG Ortho.



#23 BillP

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Posted 25 January 2015 - 09:20 PM

 

 

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.

 

A few clarifications, if I may: ...

 

2. While both "Rank Kellner Eyepiece" and "Rank Kellner Edmund" were used by Edmund in advertising as decryptions of the RKE acronym, the patent documents that Bill discovered a while back prove that the official descriptor is "Rank Kaspereit Erfle", after its three loosely-ancestral eyepiece forebears.

 

 

Very true.  The only "paper trail" on the name shows that at time of trademark of RKE, the trademark request specified that the acronym stood for Rank Kaspereit Erfle, the individuals on which the design was based.  But popularisms are hard to fight...heck even employees from Edmund will say it means one thing vs another.  But even their employees have conflicting opinions as the engineering department believes it means one thing, while the marketing department believes it means another :lol:   Good thing we have the definitive documentation in the US Trademark Office as filed by Edmund's that RKE = Rank Kaspereit Erfle.


Edited by BillP, 25 January 2015 - 09:22 PM.

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#24 Vondragonnoggin

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Posted 25 January 2015 - 09:35 PM

The floating effect is so obvious even if you've never heard it mentioned. Several years ago I started a thread on why I hate the 28mm RKE as tongue in cheek. The only reason for the hate was that it sat proudly with eyepieces costing hundreds and I just happened to get the 28mm with a $30 used Astroscan. I immediately saw the floating effect in my 8" dob at the time. Liquid space like it's going to spill over the edge of the barrel. Still a favorite, but now I have 4 of them. Two from surplus shed after I got a second new one and loaned out the pair. I received the pair back after I had the surplus shed ones. Now there are 4. Surplus shed ones just needed cleaning out. Dust in between lenses, but it's so easy to take apart and clean and reassemble.



#25 Lew Chilton

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Posted 25 January 2015 - 09:47 PM

When I got my Astroscan in 1978, it came with a 32mm Brandon.  I still have both. Twenty years earlier, in 1958, when I purchased my Edmund 4.25-inch Palomar Jr. (black tube, wooden tripod legs), it came with the Edmund 28mm Kellner. At the time, it was my favorite eyepiece.


  • terraclarke likes this


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