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Hmph... Sorry I've been away from CN a while...

I had an original set of RKE's - full set - 8, 12, 15, 21 and 28mm when they first were made in the early 1980's and used them with a variety of scopes:

- 4" f/10 Meade SCT,
- 8" Celestron 8,
- 6" f/5 Newtonian,
- 12" f/3.7 Newtonian.

Originally Edmund designed and sold the RKE's for the little Astroscan telescope, a 4.25" f/4.2 Newtonian on a ball mount. There appeared to have been some attempt to optimise the RKE design to match the optical characteristics of this scope and it is quite noticeable that they perform very well for such a simple eyepiece when used in a Newtonian around f/4 to f/5.

Other budget eyepieces of that era were primarily orthos, Kellners, Erfles or Konigs and the RKE stood out against that crop for being admirably sharp in a Newtonian, with the apparent field of view being fairly typical for that era (we can leave Naglers out of this as they cost 10X as much at that time).

Characteristics:

The good:

- Excellent transparency and little colour thanks to the relatively short glass path and low number of glass-air surfaces compared to modern ultra wide designs, and they're coated.

The 28mm and 21mm had nice eye relief and fine for modest wide field views. The 15mm was ok but I didn't have much use for it. While the 8 and 12mm were OK for planetary, they were painful to use (impossibly short eye relief) and had a LOT of pincushion distortion.

The bad:

- Pincushion distortion - lots of it, and bad enough to distort jupiter or saturn quite badly when off-axis in the 8mm.

- Very limited field of view by modern standards.

- You must take care not to scratch the eye lens of the 28mm, it is convex and protrudes beyond the barrel - it is a shame Edmund didn't extend the barrel a few mm to protect it.

A few years ago I donated the RKE set to a worthy cause, and replaced it with a set Vixen LV's which are a whole lot better thanks to the consistent eye relief and better glass, with no regrets.

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I have to hold on to it for a while... I need to compare it to my new RKE when it arrives.

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Iluxo wrote:

(snip)Originally Edmund designed and sold the RKE's for the little Astroscan telescope, a 4.25" f/4.2 Newtonian on a ball mount. There appeared to have been some attempt to optimise the RKE design to match the optical characteristics of this scope and it is quite noticeable that they perform very well for such a simple eyepiece when used in a Newtonian around f/4 to f/5.

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(snip)While the 8 and 12mm were OK for planetary, they were painful to use (impossibly short eye relief) and had a LOT of pincushion distortion.

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(snip)You must take care not to scratch the eye lens of the 28mm, it is convex and protrudes beyond the barrel - it is a shame Edmund didn't extend the barrel a few mm to protect it.

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Perhaps this was a very early example? I've never owned an RKE with this issue... my early 1980's, early 1990's and 2012 versions of the 28mm RKE all have their eyelens recessed below the level of the surrounding barrel.

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My RKE doesn't quite protrude but it's very close:

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Good comments! If I may respond:

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Iluxo wrote:

(snip)Originally Edmund designed and sold the RKE's for the little Astroscan telescope, a 4.25" f/4.2 Newtonian on a ball mount. There appeared to have been some attempt to optimise the RKE design to match the optical characteristics of this scope and it is quite noticeable that they perform very well for such a simple eyepiece when used in a Newtonian around f/4 to f/5.

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The Astroscan came out in late 1976, and I believe was originally was equipped with the 1-1/8" focal length RKE precursor. The RKE line proper was introduced a bit later, around 1978 or so, coinciding with Edmund's introduction of their new 8" f/5 and 6" f/6 fork-mounted reflectors. I agree that the line performs very well indeed with relatively fast 'scopes such as the Astroscan and these two reflectors, and I'm sure that was the intention.

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(snip)While the 8 and 12mm were OK for planetary, they were painful to use (impossibly short eye relief) and had a LOT of pincushion distortion.

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Indeed, the eye relief is almost intolerable on the 8mm and only slightly better with the 12mm - certainly not an eyepiece for eyeglasses wearers! Much of this is due to the machined housing that rises much too far above the eyelens. Back in the 1980's I threw an 8mm I had on the lathe and turned down the housing considerably, to just a few millimeters above the eyelens... the eye relief was still a bit tight, but nowhere near as bad as it had been.

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(snip)You must take care not to scratch the eye lens of the 28mm, it is convex and protrudes beyond the barrel - it is a shame Edmund didn't extend the barrel a few mm to protect it.

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Perhaps this was a very early example? I've never owned an RKE with this issue... my early 1980's, early 1990's and 2012 versions of the 28mm RKE all have their eyelens recessed below the level of the surrounding barrel.

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Thanks for the additional info on the RKE's! I've been eying a red Astroscan can if one comes around at a good price.

Best,

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My RKE doesn't quite protrude but it's very close

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Same thing with my RKE28 "Edmund Scientific" where the surface of the eye lens is recessed just a teeny bit below the EP's top rim as shown below:



P.S. The news about the availability of the RKE28mm at Surplus Shed for only $35 had spread to 2 other threads in the forum; they will surely be cleaned off the shelf in no time now.

Best,

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Well... despite all the hype over you guys (re)discovering the RKE 28mm, I should add there is an EVEN BETTER eyepiece - if you have the scope to match it.

It is the Vixen LV50mm, its a 2" massive piece of glass weighing a pound, and you MUST have at least an 8" f/10 to f/15 scope to do justice with this beastie - it is NOT suited to shorter/smaller scopes as they cannot fill the field of view - this baby is basically TWICE THE SIZE of the RKE28mm.

To give you some idea, the eye lens on this beastie is bigger than most finders !

Like the RKE28mm, the LV50 provides a stupendous amount of eye relief, maybe 30mm, and you will get the 'stars floating in front of my nose' effect, and they are tack sharp edge to edge like diamonds on velvet.

Those with faster/smaller scopes need not apply.

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