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Thread: 32mm GSO 'RK' Kellner Experiment

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    Default 32mm GSO 'RK' Kellner Experiment



    I’ve been looking at 2“ eyepieces to use with my Sky-Watcher Evostar 72ED DS Pro . I already have 36 mm and 31mm Baader Hyperion Aspherics, a 28mm Sky-Watcher LET and a 19mm Celestron Luminos. Any 2“ eyepiece has to fulfil certain criteria for me. Overall weight being the prime consideration as I’m physically disabled and have limited range and dexterity in my right arm and hand. The Aspherics work quite well in the f/5.8 Evostar although I originally acquired them for my ST80 (after receiving some great advice from this forum). The 28mm LET is a relatively inexpensive reversed Kellner. I believe some of the Evostar refractors come supplied with them. It can suffer from an amount of off-axis astigmatism in the 72ED. Although, like most Kellners, it is lightweight and delivers sharp and bright on-axis views. The Luminos is similar but with an 82 FOV the distortions are mainly peripheral. It has other problems, principally the fact that it’s not far off half a kilo (460 grammes) in weight. So, my cunning plan was to use a 2“ Barlow element to increase the range of my present eyepieces. By threading a removable element directly into the eyepiece the magnification should be increased by a factor of about 1.5x. What could possibly go wrong?



    Well, the threads on this 2“ GSO Barlow could be incompatible with my eyepieces for a start!



    So, ‘Plan B’ was to acquire a GSO 2“ eyepiece that could be used both with and without a Barlow element. After some time I managed to acquire a 32mm GSO ‘RK’ (Reversed Kellner?) Kellner eyepiece. These are marketed in 2“ kits as Celestron and Meade. TS Optics also sell the 26mm focal length version.



    I swapped the entire RK drawtube with the GSO Barlow as it had an undercut. The 32mm RK is very similar in size to the 31mm Baader Aspheric. It is well made with a large eye lens and a soft plastic eye guard that crumples to the shape of your face when you use it. I make the field stop about 40mm and the EP itself weighs 240 grammes. The FOV is about 56 although it is sometimes stated these are 65 EP’s.



    The Barlow element increases its weight but not so much that it doesn’t handle well in the 72ED.



    I used it both with and without the GSO ED Barlow element. Despite poor seeing and transparency the reversed Kellner performed pretty well on axis in the Evostar giving a bright, clear view of about 13x for a 4, 16“ TFOV. However there was noticeable astigmatism off axis and towards the edge of field. The Pleiades looked pretty good though. I then tried it with the Barlow element directly threaded into the drawtube (giving 19.6x, 2, 51" TFOV). I thought this might improve the edge of field ‘seagulls’ somewhat. It did a bit, but not as much as I'd have liked. I'll probably use the RK more with the Barlow threaded into it than without. The eyepiece definitely needs more of a proper field test, as unfortunately cloud cut the session short, but it might be OK. It was worth a try anyway.
    Last edited by Shorty Barlow; 10-21-2018 at 06:43 PM.
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    Default Re: 32mm GSO 'RK' Kellner Experiment

    Hello Shorty Barlow,

    this is a very interesting choice you have made!

    RKE has been designed by David Rank, and it can be called "reversed Kellner", or "Rank Kellner" or whatever people like.
    Even if the RKE (2 - 1) looks on paper like a reversed Kellner (1 - 2), the both eyepieces are totally different, resulting into the generous eye relief of RKE.

    The classical microscopy Kellner can't be beaten on the transmissivity of light, and in this respect it is the king.

    The RKE can be viewed as a budget alternative to the still overprised Zeiss E-Pl, Leica HC Plan and Leica L eyepieces for the binocular and stereo microscopes, all of them having the (2 - 1) lenses sequence.

    The old Zeiss Kellner has been achromatic, like the CZJ A 12,5x (f=20mm), the Zeiss and Leica (2 - 1) EPs (f=25mm, and eventually 32mm) are very well corrected with special glass materials.

    Hence, it will be very interesting to read your report on the RKE.

    Thank you for sharing,

    JG
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    Default Re: 32mm GSO 'RK' Kellner Experiment

    Quote Originally Posted by j.gardavsky View Post
    Hello Shorty Barlow,

    this is a very interesting choice you have made!

    RKE has been designed by David Rank, and it can be called "reversed Kellner", or "Rank Kellner" or whatever people like.
    Even if the RKE (2 - 1) looks on paper like a reversed Kellner (1 - 2), the both eyepieces are totally different, resulting into the generous eye relief of RKE.

    The classical microscopy Kellner can't be beaten on the transmissivity of light, and in this respect it is the king.

    The RKE can be viewed as a budget alternative to the still overprised Zeiss E-Pl, Leica HC Plan and Leica L eyepieces for the binocular and stereo microscopes, all of them having the (2 - 1) lenses sequence.

    The old Zeiss Kellner has been achromatic, like the CZJ A 12,5x (f=20mm), the Zeiss and Leica (2 - 1) EPs (f=25mm, and eventually 32mm) are very well corrected with special glass materials.

    Hence, it will be very interesting to read your report on the RKE.

    Thank you for sharing,

    JG
    I need to try it more really. Tomorrow night looks promising. There is definitely off-axis astigmatism, but with such a large field stop it might be tolerable. Otherwise I'll use it in slow scopes lol.
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    Default Re: 32mm GSO 'RK' Kellner Experiment

    I have a used 26mm on the way I picked up for good price over at that other forum. My upgrade of both my refractors to crayford focusers is coming soon, I thought this would make a good outreach EP after I do. Especially after last event when someone's kid tried to stick his finger into a bit more expensive EP -- though, nothing I have is really all that expensive....

    The 2" Celestron/Meade kits seem overpriced to me.

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    Default Re: 32mm GSO 'RK' Kellner Experiment

    I still occasionally use an old Edmund 25mm RKE from the 1980's that has survived since then - it is in a 1.25" barrel, performs OK at slower F ratios.
    Michael
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    Default Re: 32mm GSO 'RK' Kellner Experiment

    Interesting exercise SB. Any particular reason why you going for 2" EP + 2" barlow combo vs stand alone EP?

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    Default Re: 32mm GSO 'RK' Kellner Experiment

    Quote Originally Posted by Don Alvarez View Post
    I have a used 26mm on the way I picked up for good price over at that other forum. My upgrade of both my refractors to crayford focusers is coming soon, I thought this would make a good outreach EP after I do. Especially after last event when someone's kid tried to stick his finger into a bit more expensive EP -- though, nothing I have is really all that expensive....

    The 2" Celestron/Meade kits seem overpriced to me.
    Take a look at Russell Optics offering. Good quality 2" EPs of simpler designs at reasonable prices. I have 56mm Plossl and very happy with performance and built quality, light weight to.

    Russell Optics & Machine

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    Default Re: 32mm GSO 'RK' Kellner Experiment

    Don't forget the sweet GSO ' Superviews ' , these are very good in f7 and above telescopes with the light thru put of a good Kellner and great coatings .
    Very light weight as well .

    Brian.

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    Default Re: 32mm GSO 'RK' Kellner Experiment

    Quote Originally Posted by Bigzmey View Post
    Take a look at Russell Optics offering. Good quality 2" EPs of simpler designs at reasonable prices. I have 56mm Plossl and very happy with performance and built quality, light weight to.

    Russell Optics & Machine
    I've seen these, I hear good things, but, I don't know...... I can't get over the way they look. Looks like a car horn I saw once. I shouldn't judge based on appearance like that. Does that make me an EP racist?
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    Default Re: 32mm GSO 'RK' Kellner Experiment

    Quote Originally Posted by Don Alvarez View Post
    I've seen these, I hear good things, but, I don't know...... I can't get over the way they look. Looks like a car horn I saw once. I shouldn't judge based on appearance like that. Does that make me an EP racist?
    What do you mean? They look nice.



    But I know that some peeps prefer their EPs blue short and chubby.
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