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Reversed Kellners and Pterodactyls


Nightspore

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Last October I acquired a 32mm GSO ‘RK’ Kellner. Why? I hear you all say. Well, possibly temporary insanity. Seriously though, I had a cunning plan. We all know the *Kellner was invented way back by ‘wiggy blokes’ to use in aerial telescopes or something. OK, I made that up, but even in the Reversed Kellner format it is technically an old design. It is reviled by many because of its edge of field astigmatism and comparatively limited field of view compared to modern eyepieces.

 

XgYjpcU.jpg

 

But I digress, as I said, I had a cunning plan. I regularly use 31mm and 36mm Badder Hyperion Aspherics in my small refractors. These are nice, physically light rich field EP’s with a 72° FOV. The weight factor is important for me as I am partially paralysed in my right arm and hand. This complicates themanipulation of both OTA and mount. It also limits the weight of an eyepiece I can comfortable use in a 2” diagonal. The plan was to get a 2” Barlow with a detachable cell and thread it onto either of the Aspheric eyepieces. This then gives me more magnification choices for a minimum of weight increase. Unfortunately, the Baader threads were incompatible with the 2” GSO 2x Barlow. Which is why I ordered the 32mm GSO RK. Most retailers claim these are now discontinued although both Meade and Celestron still market these GSO 2” eyepieces and the GSO 2” Barlow in kits.

 

vIHI0Io.jpg

 

I managed to try the 32mm RK in my 72ED Evostarlast year a couple of times. It was not very successful in the f/5.8 refractor and edge of field seagulls were more akin to pterodactyls. Not only that they seemed to be flying ever nearer the on-axis view. The addition of the Barlow didn’t appear to ameliorate the astigmatic avian menace either. 

 

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However, as Kellners tend to fare better in scopes of f/8 and slower I chanced the RK in my f/7.5 ED80 Evostar a couple of times. The 32mm RK with 56° FOV doesn’t really compare with the 72° of the 31mm Baader. The RK does have about a 35mm field stop though and is ergonomically pleasant to use. With the Barlow threaded directly on to the eyepiece GSO reckon that it magnifies by 1.6x. Therefore technically turning the 32mm into a 20mm EP. This now gives almost exactly one arc degree, fifty two minutes of true field with a magnification of 30x and a 2.6mm exit pupil. 

 

RO1y1Ph.jpg

 

Idecided to test this combination on a few open clusters. Then out of curiosity I directly compared it with a 15mm Altair ULTRAFLAT on IC 4665 aka the Summer Beehive. Both eyepieces were used in a 2” Altair dielectric mirror diagonal. The ULTRAFLAT obviously gave a flat, sharp field right to the edge. The RK plus Barlow accounted for itself pretty well however. There is still some EOF avian activity but the large field stop makes on-axis views quite pleasing as it gives the sense of a much larger eyepiece (which it is). Not bad for a 2” EP that cost under fifty quid! I suppose I should have tried the RK in the Barlow for 2x as well but I was distracted by a rising Saturn. The weight of the combination is fairly easy to handle as the ED80 is very nicely balanced on the AZ5. I shall experiment further, I have a cunning plan.

 

y2G1llL.jpg

 

 

*The original Kellner orthoscopic eyepiece was invented in 1849 by Carl Kellner an optician from Wetzlar (now in Hesse, Germany).

Edited by Nightspore
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