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#1 Dreamon

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Posted 17 March 2013 - 08:49 PM

Bright night, well I dropped my RKE, then gave it a shake or two. Something came loose and rattled abit.

So, I disassembled it to find the rattle. The retaining ring needed some minor tightening is all.

Looking at the lenses, the doublet surface facing the obj is "concave". Does this mean that the doublet might have a "negative" property? Does the concave surface, over plano for instance, add to the performance of the RKE?

#2 izar187

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Posted 17 March 2013 - 09:11 PM

It is not performance adding.
It's just part of the chosen curves for the types of glass and their refractive idicii, intended to send the light where it needs to go, in a three element 45 degree field design. They've somewhat better eye relief, and handle somewhat faster focal ratio's somewhat better than kellners. Therefore a definitive improvement in a three element by most criteria.

#3 Dreamon

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Posted 17 March 2013 - 10:46 PM

I understand. would you say the doublets concave surface (facing the obj) has a "negative" value?

#4 izar187

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Posted 18 March 2013 - 02:14 AM

I'd say it is just the right shape in the right place.
Comparing the design of the RKE to other three element eyepieces makes it look like a derivation of one of Albert Konig's numerous designs. Trimmed down to a manageable sized field of view, for better astigmatism suppression, in more focal ratio's.

Apologies if you have already seen the images in this link. But it's still pretty illustrative of the RKE's non kellner-esque pedigree, in spite of Edmunds ad copy.
http://www.google.co...nt=safari&am...

#5 Brian Albin

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Posted 18 March 2013 - 04:36 PM

The doublet in an RKE is positive. The concave surface is on a negative lens if that lens is considered by itself, but once it is cemented to the biconvex, the doublet lens as a whole is positive.

Yes, using a concave here instead of planar does improve it's performance. I believe the correction they were going after with this curve was the spherical aberration. But all the curves work together to reduce all the aberrations.

#6 Dreamon

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Posted 20 March 2013 - 09:39 PM

IZAR, now thats a link to some eyepieces!!!!!!!!!

Brian, thank you, just the answer I was looking for. Thought the Barlow concept was at work here.

#7 Dreamon

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Posted 20 March 2013 - 09:42 PM

This might be arbitrary, but this place is sure busy lately!!!

Trying for a couple evenings to get back here. I would think everyone would be outside instead of slowing this place down. Ha Ha

#8 Brian Albin

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Posted 21 March 2013 - 04:39 PM

Yes! I was surprised that access was unavailable recently. I wonder if the site was under attack by one of those invader programs.

I was once at a forum that was several times attacked by some sort of robotic visitor which specializes in visiting forums. I never did learn it's purpose, but it made it look like the forum had far more visitors than it actually had.






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