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Edmund RKE Eyepiece Overview



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Date: 9-6-2001
Price: $39.95 ea.
Design: Multi-coated three element Kellner eyepieces
Description: High quality Kellner eyepieces from a good name in optics.


I bought a pair of Edmund Scientific RKEs, 28 and 15mm, for my 90mm RFT (identical to the Orion ST90) to augment the supplied brand X Plossls. I wanted something inexpensive, well made and light weight. I felt that the 32 Pl. that I sometimes used bordered on being too heavy for the plastic drawtube. Also, a 32 is about at the exit pupil limit for this f5.6 scope. I thought the three-element RKE would not have too much distortion for f5.6 since Edmund supplies them with the Astroscan f4.2. In the past I might have passed over the RKE because, frankly, Edmund was charging more than a three element eyepiece is worth. Now they are $39.95 and competitive with other discount vendors.

The internal configuration of an RKE is essentially the reverse of a Kellner. There is an achromat doublet, a space and then a single element eye lens. Some people speculate that "RKE" is short for "reversed Kellner." I have been told, however, that they are the initials of its inventor (whose name escapes me). The barrel seems to be aluminum and it is thoroughly painted in and out. The barrel has the trademark red rubber finger grips.

The 15mm has a recessed eye lens which blocks some stray light. The 28mm has a huge eye lens that is flush with the top of the eyepiece. Consequently, a rubber eye cup is supplied. The eye cup is fresh from the factory and covered with mold release powder. The eye cup is made of thick material and does not easily stretch over the top of the eyepiece. I swapped it with a similar Orion eye cup which is much more pliable and a lot cleaner. The eye cup itself came with its own warning label cautioning against latex allergies.  On the reverse, was a general disclaimer against warranties essentially saying that if the buyer unwraps the product then he or she agrees that there are no warranties of any kind. (It does not specify if it means the eye cup only or the eyepiece as well.) Of course in states that have adopted the UCC (almost all of them) this disclaimer is meaningless since it contains additional terms that were not present at the time of sale. I intend to send them a note saying that I reject all additional terms beyond those agreed to at the time of sale just to be sure.

I tested the new eyepieces last night. This is a suburban location with the Kmart Nebula not far to the south. It was mostly clear with a full moon. The moon made viewing conditions pretty lousy. I tested the eyepieces on my 8" f6 Dob. I tested the 28 RKE against a 32 Sirius Pl and a 25 U.O. ortho. The 15 RKE was tested against a 17 Sirius Pl and a 12.5 U.O. ortho.

I first viewed the retreating Mars just after dusk. Using a Celestron Ultima 2X Barlow, both the 15 RKE and the 12.5 ortho showed a shapeless, bright dot. I added an Orion orange filter to cut glare. The RKE threads accepted the filter easily. Will tube currents were still evident, both eyepieces revealed the gibbous shape, light patches at the poles and a hint of a dark surface feature. They were similar in sharpness and light transmission. Even though they are both nominally 45 apparent f.o.v., the RKE appeared to have a substantially wider picture than the ortho.

Moving to Sagittarius, I examined M-8 with the 32 Pl, 25 ortho and 28 RKE.  With the proximity of the full moon, viewing conditions for deep sky objects were pretty bad. Unfiltered, all three oculars showed very similar light transmission. Filtered with an Orion Ultrablock, all three showed very slight nebulosity. The finish on the RKE threads prevented the nebula filter from being screwed-in all the way. Nevertheless, it screwed in far enough to be secure. Of the three, the ortho had slightly better contrast than the other two. Both the Pl and the 28 RKE had similar apparent f.o.v., each noticeably wider than the ortho. Both the ortho and the RKE were picky about eye placement. Too close or too far or too far off axis and blacked-out spots will appear. The Pl has less sensitive to that. Also the eye cup for the RKE is not entirely adequate. If extended far enough to cover the user's eye, it ill vignette the field of view. Consequently, I retracted it somewhat.

With or without a filter, none of the three eyepieces could detect the Veil in the full moonlight. All three had nice images of the Milky Way. Edge distortion on the 28 RKE was similar to the 32 Pl: very slight, though maybe just a very tiny bit worse. The 25 ortho, by the way, has no edge distortion. Again, contrast on the 28 RKE was similar to and maybe a little better than (owing to the higher magnification) the 32 Pl. Once again, the ortho had slightly better contrast than the other two.

Moving to the Double Cluster, still low in the sky, the 28 RKE gave similar performance to the 32 Pl. Again, upon close scrutiny, both had similar edge distortions, though neither was objectionable. The ortho was sharper than either with slightly better contrast. Again, the ortho presented a narrower apparent field than either of the other two. In fact the apparent f.o.v. on the RKE was very close to the big Pl. Switching to the 15 RKE and 17 Pl, each gave comparable views of the individual clusters. Light transmission was similar as was the tiny edge distortions and contrast.

Later that night, about midnight, I tested the RKEs on the 90mm RFT refractor (using an Orion mirror diagonal) against the supplied 25 and 10mm Plossls. By this time a hazy, intermittent cloud cover had rolled in. The Double Cluster area was pleasing [but not spectacular due to the crappy transparency] in the huge true field of view supplied by the 28 RKE. The performance was very close to the supplied 25mm Pl. Edge distortion, light transmission and contrast were very similar with perhaps the RKE having a slight edge on light transmission and the Pl being a bit better on contrast (probably owing to the lower magnification). Comparing the 10 Pl with the 15 RKE (not exactly fair with a 50% focal length difference) both gave good views of the clusters. The RKE appeared to have a little better contrast, light transmission and lack of distortion than the Pl. The eye relief on the 10 Pl was, in fact, very tight. Swinging over to M31, both the 25 Pl and the 28 RKE gave similar views (which were not very good given the bright, hazy viewing conditions). Seeing Saturn as a target of opportunity, both the 25 Pl and the 28 RKE showed the 6th planet as a teeny, tiny, yellow football with Titan plainly visible. The 15 RKE barely revealed the rings, but the image scale was too small for any ring structure, including the Cassini Division, to visible. Only with the 10 Pl were the rings clearly present (but still not ring structure). Even with the short eyepiece, it was still pretty tiny (still only 50x).

I hope to test these RKEs again when it is dark. All in all, on a scale from A to F (with A being perfection, F being unusable and C being average performance), I give the RKEs a solid "B" on the telescopes I used for the test. For comparison, I give the Orion 32 Pl a B and the 25 ortho a B+ for deep sky. I suspect that on big, fast scopes, the edge distortions might be more noticeable than on mine. Removing crappy came-with-the-scope eyepieces from the grading curve probably knocks the RKE down to a C+. To conclude, the RKE performance was all that was advertised and at $40 each they are a steal. In fact, a beginner with his or her first decent telescope (a 6 or 8 Dob for example) would do well to replace the supplied SMAs or Chinese Plossls with a whole set of RKEs. The whole line of 5 eyepieces and a good Barlow runs $300 with the case. (Keep the old ones for star parties). Good stuff!

Submitted by Russell Hopkins - - USA

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