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Televue plossl patent - what we can learn on paper?


YKSE

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Al Nagler has patent on Televue's plossl

http://www.google.com/patents/US4482217 (The copy of original patent can be downloaded as PDF as well).

Understanding the technical details are plainly too difficult for me, but I'd still like to make out something from more general information in the patent, here're the parts I'm thinking about:

Quote from the patent:

"FIELD OF THE INVENTION

The invention relates to an eyepiece for astronomical instruments, and more particularly to a Plossl type eyepiece having improved coma and astigmatism correction.

BACKGROUND OF THE INVENTION

As is well known in the art, a Plossl type eyepiece is a relatively wide field eyepiece comprising two achromatic doublets in which the crown elements usually face each other. Such eyepieces are capable of good performance, i.e., acceptable degrees of aberrations, to about a 50° field. Generally, in order to minimize aberrations at the exit pupil and distortion, all air glass surfaces of the eyepiece are made convex. However, thre have been Plossl type eyepieces used commercially in astronomical instruments in which the external flint surfaces are plano.

GENERAL DESCRIPTION OF THE INVENTION

For astronomical viewing, pupil aberrations and geometric distortions are not as important as the correction of coma and astigmatism which control image sharpness at the edge of the field.

Accordingly, it is an object of the present invention to provide an improved Plossl type eyepiece for use with astronomical instruments.

It is another object of the invention to provide a Plossl type eyepiece having improved coma and astigmatism aberrations at the edge of the field of view.

In carrying out the invention, there is provided a symmetrical eyepiece comprising two achromatic doublets in which the external surfaces of the flint elements are concave. Such a lens configuration provides a significant improvement in the correction of astigmatism and coma at the edge of the field. This results in a sharper image for large field angles with a relatively small undercorrected field curvature."

Any suggestions?

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All he did was put a concave curve to the external faces of the flint elements?  :icon_scratch:

I'm sure it's more complex than that as one presumes all the other radii are altered as a result, but that's my guess.

Russell

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Yong,

My English is not too bad and I don't understand it either, however I am content in the fact I have two and they work.

Alan

I'm trying to improve my English by reading more, and trying to understand things seems to be a better way for me to learn. :smiley:

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Patents are an odd game.

Your aim in writing a good patent isn't to describe exactly what your invention is doing, it's to claim as broad a range of invention as possible, so that people cannot copy your ideas without infringing the patent.

So, if the Nagler patent claims 'a symmetrical eyepiece comprising two achromatic doublets in which the external surfaces of the flint elements are concave', then you can't make one of those without infringing the patent. The details of things like the strength of the various components of the doublets don't need to be part of the patent, because any design of that type with any strengths would be covered by the patent.

You could write apply for a patent that specified both the layout of the components and their strengths, but that wouldn't be as good - all a copycat would have to do would be to tweak the strengths slightly and then they wouldn't be infringing the patent.

Of course, the of a broad patent application is that you can't (or shouldn't be able to) patent something that has been patented already or is 'in the prior art' - ie is already being made, sold or has been described in public already.

cheers,

Robin

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I knew that the TV's were different in some way and some recollection of the curvatures came into it. Does no one make a non-symetric plossl?

As in where Lens1 is not identical to Lens 2 ?

Would it still be a "plossl" or revert as in AN's patent to Plossl type.

Would mean 2 different achromats in the eyepiece so additional manufacturing cost and a slightly more complex construction. Would have thought that it would produce some possible improvement as the eyepiece does not perform a symetric action - the object plane is close to the eyepiece whereas the image plane is at infinity. So the same lens symetrically arranged does not seem ideal. Equally would assume this thought has wandered through AN's mind at some time.

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Robin,

Very valid point, I've seen patent experts specialized in helping inventros writting patents.

I knew that the TV's were different in some way and some recollection of the curvatures came into it. Does no one make a nhon-symetric plossl?

I would have thought these are basic knowledge for an experienced optical engineer? :smiley:

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From the diagram I can see L2 being the way it is which then made me wonder what if L1 were at same orientation such that the concave flint was facing L2, not the human eyeball.

The "standard" plossl achromat (single) follows the Fraunhoffer doublet principles used in refractors. Likely I suspect a similar ratio of radaii (about 6:1).

Although I do suspect that the simplicity of 2 identical lens symetrically arranged will cancel out some aberrations to an extent, as in they are sort of "self optimising" and perhaps 2 inline ones add aberrations. Therefore giving an overall worse result.

Plossl's are a nice simple lens and their production costs would be minimal especially when the achromats are identical (just have to make a big box of one sort). So production cost has to be a factor.

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From description of "Field of invention" and the first 3 paragraphs of "GENERAL DESCRIPTION OF THE INVENTION" , I'd think we can draw following inferences:

1. TV plossls improve BOTH astigmatism AND COMA correction.

2. These improvements are done by giving up some corrections on aberrations on exit pupil and corrections on distortions.

The improvement means tighter stars in the edge, As usual, we can't eat a pie and have the pie at the same time, what the consequence of no. 2?

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Does no one make a non-symetric plossl?

Would have thought that it would produce some possible improvement

Equally would assume this thought has wandered through AN's mind at some time.

Ronin,

The history is the other way around. "True" Plössls - the original design - did indeed have different achromats. There is a price to pay though inasmuch as the sharpest images were not produced on-axis, but 30-degrees off-axis.

To my knowledge, no "True" asymmetric Plössls are produced today. The nearest asymmetric equivalent is probably the Brandon - although these use non-standard glass choices compared to a Plössl

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Yes - your points 1&2 pretty much have it covered.

Thanks Jeremy, would you care to explain what it means (last sentence in my quote above)?

"This results in a sharper image for large field angles with a relatively small undercorrected field curvature"

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It means sharper further to the edge compared to a standard symmetrical, and a flatter (but not completely flat) field.

Field flatness becomes increasingly important as one gets older...

(I got rid of my 16mm William Optics UWA because the field was too curved for me to cope with)

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Jeremy,

Does it mean that all plossls har some degrees of under-corrected field curvature? If that's the case, then curvature must be very minor since almost no-one sees it? And if no re-focus is needed to cope with this minor curvature, can we see it in some way?

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Field curvature is not an issue if the degree of curvature is so small the eye automatically refocuses as it scans the FOV.  Giving up distortion correction is often need to increase correction of coma and astigmatism. What the invention states is that for visual observation, rather than astrometry (measuring positions and angles), distortion is not much of an issue, whereas the classic Plossl was designed with astrometry as much as observation in mind. 

You can actually see that the surface of the eye lens of the TV Plossl is concave.

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Thanks for the clarification Michael.

When referring to the extreme minor curvature, I was thinking of day-time test of eyepieces (with or without a telescope), my perception is that na eyepiece with under-corrected curvature will show a thin red/pinkish ring near the field stop, while an over-correted one shows green/yellow/blue ring instead. Do you think I've got it right?

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Me collection from an optical designer was to look at patents for "inspiration", see what other people have tried and go from there. If you dot tell people your secret sauce you don't need to spend money on a patent....

Cheers

Peter

PS patent speak is a weird kind of speak... "Those skilled in the art"....

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Me collection from an optical designer was to look at patents for "inspiration", see what other people have tried and go from there. If you dot tell people your secret sauce you don't need to spend money on a patent....

Cheers

Peter

PS patent speak is a weird kind of speak... "Those skilled in the art"....

But someone else might patent your invention and then sue you for infringement. You then have to show you were first, which is not easy

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I think getting the patent is one thing but having the resources to sue for infringement is entirely another. I understand that it's a really expensive business for a smaller company, for example Tele Vue.

Anyway, it's pretty easy for a competitor to get some samples of your product, dissasemble them then broadly copy the design with a couple of small changes to avoid the risk of litigation. I guess it happens all the time.

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From Wikipedia: "In the United States, the cost of defending against a patent infringement suit, as of 2004, is typically $1 million or more before trial, and $4 million or more for a complete defense, even if successful"

For this reason, "protecting your property" is not the only goal for patents - a common reason for them is a pre-emptive strike to stop someone with bigger pockets from stopping you going about your business in future. This is called (I believe) a "defensive patent".

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Patent is a complicated matter, the patent inventors needs to pay maintenance fees to keep their patents valid too, let along driving the legal processes when needed. These are no small costs, no wonder Al Nagler stopped paying the patent fees.

I'd hope that we can pay more attension to the technical parts of this patent.

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The issue is that keeping the sauce secret is a bad idea. Publishing it is actually better, as it proves you were first and should prevent anybody else claiming a patent. In Europe this works quite well, because before granting a patent, a thorough search for prior art is conducted. In the US, this is often not the case, and the USPTO just grants patents on things assuming the lawyers will fight it out in courts, which they do with glee. In the US it is not uncommon for patents to be declared invalid after a (costly) challenge.

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....I'd hope that we can pay more attension to the technical parts of this patent.

I'm no optical expert but I can see that the patent includes details of the refractive indexes and Abbe numbers of the crown and flint glass of the lens elements as well as the radii of each lens surface with the 26mm focal length being used as the model. The major innovation claimed seems to be the concave external surfaces of the outward facing flint elements. The cement and coatings used, lens mounting and light baffling approach are not discussed.

Perhaps more could be gleaned if we had the equivilent information relating to a conventional plossl eyepiece ?

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