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What's the different in ED glass

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#1 George Methvin

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Posted 13 October 2020 - 06:23 AM

Just wondering what's the different between the makes if Ed glass used in low cost Ed refractor? I noticed that Astro Tech uses FK-61 glass and Orion uses Fpl 53 glass is there a big different between the two if any?.



#2 gnowellsct

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Posted 13 October 2020 - 06:29 AM

Oh dear he has asked the glass question
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#3 beggarly

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Posted 13 October 2020 - 06:33 AM

There is a search button at the top of the page.

 

https://www.cloudyni...-61-and-fpl-53/

 

Cross reference refractive index and Abbe-number:  http://www.hoya-opti...sreference.html


Edited by beggarly, 13 October 2020 - 08:10 AM.


#4 db2005

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Posted 13 October 2020 - 06:35 AM

Not exactly being an expert on the subject, but I have being following thread like this for quite some time, and I have owned several low-cost and several high-end optics. Judging from this, the differences may boil down to: better grades of optical class, better figure and polish, choice of better mating element for the ED glass. One important aspect is that the difference between cheap vs expensive scopes doesn't just boil down to the choice of glass, but involves many more aspects, such as the workmanship put into making the lens, design of the lens cell, stray light control, choice of coatings, etc.

 

To my eyes, expensive optics perform noticeably better than cheap optics. I'm not too bothered with the actual choice of glass, Vixen doesn't even officially disclose the glass types used in their SD line of scopes, but their optical quality is IME noticeably better than the entry level EDs I've owned.



#5 sg6

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Posted 13 October 2020 - 07:37 AM

I believe technically that the difference is the Abbe Number, this measures the difference between dispersion of a glass at 3 different wavelengths.

 

The dispersion being defined as the difference in refractive indicies at each wavelength.

 

The 3 wavelengths are Short end, Long end and Middle. Basically Red, Blue, Green.

 

Formula is V = (nGreen - 1)/(nBlue - nRed) Where n is the refractive index.

 

More general formula for none visible:

 

V = (nCenter - 1)/(nShort - nLong)

 

In multiple lens design Low Dispersion is better, you can get "better" results, does depend on what you call better.

 

FLP-53 is Lower Dispersion then FPL-51.

In general:

FPL-53 = FCD-100.

FPL-51 = FCD-1 = FK-61.

 

Never quite that simple but a fair guide.

 

Seems that for the reason of making a fast scope you really need FPL-53, if however you have no great concern for a fast scope say f/7 or f/8 will do you then FPL-51 is very likely to perform the same as far as looking through a scope is concerned.

 

Problem occurs when imaging - you can build up say 1 hours worth of the aspect that the eye just would never see. Mainly in  doublet. Even an idiotically fast triplet will show CA. And some are idiotically fast. Seems at times that "fast" is preferred over "quality". Maybe because it is easier to compare f/5 to f/5.5, and for reasons beyond me f/5 wins. I would take f/7 over an f/5.

 

So the difference is Abbe Number. Which I am guessing means absolutely nothing to you. But FPL-53 has a lower Abbe Number then FPL-51. That is "the difference".

 

Suppose the reason FPL-53 is preferred is simply - you should be at least starting with the best glass properties available to you. Then it is design and manufacturing that enters the ring.


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#6 George Methvin

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Posted 13 October 2020 - 08:45 AM

Thanks everyone I think I kind of understand now.



#7 gnowellsct

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Posted 13 October 2020 - 06:34 PM

Thanks everyone I think I kind of understand now.

Only when you understand that you do not understand, do you understand


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#8 stevew

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Posted 13 October 2020 - 07:36 PM

Just wondering what's the different between the makes if Ed glass used in low cost Ed refractor? I noticed that Astro Tech uses FK-61 glass and Orion uses Fpl 53 glass is there a big different between the two if any?.

As was mentioned, there is quite a fair difference in refractive indexes between FPL51 or HFK61, and FPL53 or FCD100, however all are very good glass. The real key is in the design and execution.

I'd rather have an FK61 based lens with an excellent figure, than an FPL53 based lens with a poor optical figure. But typically FPL53 gives a better color correction than FK61.

However I have refractors with both, and have had many mesmerizing views with both.

If you stick with a reputable name brand, like Astrotech, Orion, Stellarvue etc, I'm sure you will get a good scope that the company will stand behind.


Edited by stevew, 13 October 2020 - 07:41 PM.

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#9 Wildetelescope

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Posted 13 October 2020 - 08:39 PM

I believe technically that the difference is the Abbe Number, this measures the difference between dispersion of a glass at 3 different wavelengths.

 

The dispersion being defined as the difference in refractive indicies at each wavelength.

 

The 3 wavelengths are Short end, Long end and Middle. Basically Red, Blue, Green.

 

Formula is V = (nGreen - 1)/(nBlue - nRed) Where n is the refractive index.

 

More general formula for none visible:

 

V = (nCenter - 1)/(nShort - nLong)

 

In multiple lens design Low Dispersion is better, you can get "better" results, does depend on what you call better.

 

FLP-53 is Lower Dispersion then FPL-51.

In general:

FPL-53 = FCD-100.

FPL-51 = FCD-1 = FK-61.

 

Never quite that simple but a fair guide.

 

Seems that for the reason of making a fast scope you really need FPL-53, if however you have no great concern for a fast scope say f/7 or f/8 will do you then FPL-51 is very likely to perform the same as far as looking through a scope is concerned.

 

Problem occurs when imaging - you can build up say 1 hours worth of the aspect that the eye just would never see. Mainly in  doublet. Even an idiotically fast triplet will show CA. And some are idiotically fast. Seems at times that "fast" is preferred over "quality". Maybe because it is easier to compare f/5 to f/5.5, and for reasons beyond me f/5 wins. I would take f/7 over an f/5.

 

So the difference is Abbe Number. Which I am guessing means absolutely nothing to you. But FPL-53 has a lower Abbe Number then FPL-51. That is "the difference".

 

Suppose the reason FPL-53 is preferred is simply - you should be at least starting with the best glass properties available to you. Then it is design and manufacturing that enters the ring.

Lower dispersion has higher Abbe number:-). 

 

JMD


Edited by Wildetelescope, 13 October 2020 - 08:48 PM.


#10 SandyHouTex

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Posted 16 October 2020 - 06:59 PM

I posted this in another thread, but it’s applicable here as well:

 

If you really want to know why different glasses matter when designing a doublet or triplet, get the book “Telescope Optics” by Rutten and van Venrooij.  Chapter 21.13 talks about designing doublets, and chapter 21.16 explains the triplet design process.

To sum it up for a doublet, you need to look at Fig. 21.14.  It shows why Fluorite (and FPL-53) are the best choice for controlling SA.  The second paragraph under the figure explains that you need two glasses with the smallest delta P on the y-axis (a horizontal line), and a maximum delta V on the x-axis.

With triplets, you need to look at Fig. 21.21.  To create the best triplet you need three glasses that create the widest triangle on the P-V diagram.

Simple.  Back in the 90s I used this book to design a great doublet using FPL-53.  Then I priced it.  $1000 for a 6 inch blank 1 inch thick.  Needless to say I decided to pass.


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#11 vdog

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Posted 17 October 2020 - 12:44 PM

Only when you understand that you do not understand, do you understand.

Sounds like something Master Kan would say to the young Kwai Chang. lol.gif


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#12 j.gardavsky

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Posted 17 October 2020 - 03:10 PM

Hello all,

 

here is a short explanation of the color correction in optical systems from the SCHOTT publications 2014.

Please, pay attention to page 17, where the mating of the PK and FK low dispersion glass with the short flint (KZFS) glass materials is explained,

 

https://www.google.c...rwWmERYUfz_yUz-

 

Best,

JG


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#13 SandyHouTex

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Posted 17 October 2020 - 07:01 PM

Hello all,

 

here is a short explanation of the color correction in optical systems from the SCHOTT publications 2014.

Please, pay attention to page 17, where the mating of the PK and FK low dispersion glass with the short flint (KZFS) glass materials is explained,

 

https://www.google.c...rwWmERYUfz_yUz-

 

Best,

JG

N-FK58 sounds like it could be a hit.  Thanks for the reference.


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#14 Simoes Pedro

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Posted 21 October 2020 - 04:53 AM

Read section 9 in: https://www.telescope-optics.net/


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#15 Jon Isaacs

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Posted 21 October 2020 - 05:18 AM

Just wondering what's the different between the makes if Ed glass used in low cost Ed refractor? I noticed that Astro Tech uses FK-61 glass and Orion uses Fpl 53 glass is there a big different between the two if any?.

 

Astro-Tech sells scopes with FPL-53/FCD-100 and Orion sells scopes with FPL-51/FK-61.  

 

A simple way to look at is this:

 

For a given focal ratio, aperture and design (number of elements etc), FPL-53 is able to provide significantly better color correction than FPL-51.  

 

Jon


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#16 BFaucett

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Posted 21 October 2020 - 09:08 AM

Astro-Tech sells scopes with FPL-53/FCD-100 and Orion sells scopes with FPL-51/FK-61.  

 

A simple way to look at is this:

 

For a given focal ratio, aperture and design (number of elements etc), FPL-53 is able to provide significantly better color correction than FPL-51.  

 

Jon

 

Some of Astro-Tech's scopes use FK-61. Examples:

 

"This Astro-Tech AT102ED refractor has:
• 102mm f/7 fully multicoated doublet optics using FK-61 ED glass"

https://www.astronom...ractor-ota.html

 

"This Astro-Tech AT80ED refractor has:
• 80mm f/7 fully multicoated doublet optics using FK-61 ED glass"

https://www.astronom...ractor-ota.html

 

"This Astro-Tech AT70ED refractor has:
• 70mm f/6 fully multicoated doublet optics using FK-61 ED glass"

https://www.astronom...ractor-ota.html

 

The Orion ED80 (f/7.5) has FPL-53:

 

"Orion ED80 80mm f/7.5 Apochromatic Refractor Telescope

• 80mm refractor objective doublet includes one element of high-quality FPL-53 ED or extra-low dispersion glass, which virtually eliminates false color"

https://www.telescop...2160/p/9895.uts

 

Bob F. smile.gif


Edited by BFaucett, 21 October 2020 - 09:11 AM.

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#17 Jon Isaacs

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Posted 21 October 2020 - 09:39 AM

Bob:

 

I didn't mean to suggest that Orion didn't sell scopes with FPL-53 or that Astro-Tech only sold scopes with FPL-53.

 

Rather, I was point out that in the context of first post, both vendors sold both.

 

In general, Astro-Tech is more upfront. They'll tell you a scope is using FK-61. Orion only tells you when they're using FPL-53, if theyre not using FPL-53 class glass, they just saw ED, extra low dispersion..

 

Your 72 mm Astro-tech ED-ll is an example of an Astro-Tech scope with FPL-53.

 

"72mm f/6 fully multicoated FPL-53 and Lanthanum doublet refractor optics"

 

https://www.astronom...fpl-53-f-6.html

 

Jon


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#18 Wildetelescope

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Posted 21 October 2020 - 09:49 AM

Some of Astro-Tech's scopes use FK-61. Examples:

 

"This Astro-Tech AT102ED refractor has:
• 102mm f/7 fully multicoated doublet optics using FK-61 ED glass"

https://www.astronom...ractor-ota.html

 

"This Astro-Tech AT80ED refractor has:
• 80mm f/7 fully multicoated doublet optics using FK-61 ED glass"

https://www.astronom...ractor-ota.html

 

"This Astro-Tech AT70ED refractor has:
• 70mm f/6 fully multicoated doublet optics using FK-61 ED glass"

https://www.astronom...ractor-ota.html

 

The Orion ED80 (f/7.5) has FPL-53:

 

"Orion ED80 80mm f/7.5 Apochromatic Refractor Telescope

• 80mm refractor objective doublet includes one element of high-quality FPL-53 ED or extra-low dispersion glass, which virtually eliminates false color"

https://www.telescop...2160/p/9895.uts

 

Bob F. smile.gif

I believe the EDT triplet lines also use FK-61 glass, where as the AT 92 uses the FK-100.  I think that another important part of this question is what level of color correction is acceptable to the user.  That is a more difficult question to answer.   I can tell you that the 80 EDT is a nice scope.  You will find a small amount of CA on bright objects if you really go looking for it.  But I have seen barges and texture in cloud bands on Jupiter at ~130X with, to my 52 year old eyes, no real evidence false color.  It has WAY better color correction than my long focal length achromats(which are not bad at all either).   A friend has the AT92.  It is a REALLY nice little scope.  It is faster than my 80 mm, and I have seen NO false color on any target.   It costs twice as much as my 80 EDT, and a bargain at that price.  But for me and what I do, I am perfectly happy with the 80 mm.  Other folks might come to a different conclusion based on their expectations.  If you REALLY want to understand what the practical difference is between different scope designs, go look through them.  What works for you may be quite different than what works for someone else.

 

Cheers!

 

JMD


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#19 Wildetelescope

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Posted 21 October 2020 - 01:58 PM

I believe the EDT triplet lines also use FK-61 glass, where as the AT 92 uses the FK-100.  I think that another important part of this question is what level of color correction is acceptable to the user.  That is a more difficult question to answer.   I can tell you that the 80 EDT is a nice scope.  You will find a small amount of CA on bright objects if you really go looking for it.  But I have seen barges and texture in cloud bands on Jupiter at ~130X with, to my 52 year old eyes, no real evidence false color.  It has WAY better color correction than my long focal length achromats(which are not bad at all either).   A friend has the AT92.  It is a REALLY nice little scope.  It is faster than my 80 mm, and I have seen NO false color on any target.   It costs twice as much as my 80 EDT, and a bargain at that price.  But for me and what I do, I am perfectly happy with the 80 mm.  Other folks might come to a different conclusion based on their expectations.  If you REALLY want to understand what the practical difference is between different scope designs, go look through them.  What works for you may be quite different than what works for someone else.

 

Cheers!

 

JMD

Just a couple further thoughts.  The OP asked basically what the difference would be between similar scope designs using different glass types, and if that difference is big.  Implicit in the question is whether or not it is worth the extra money to get a refractor with a  lower dispersion glass.  Based on my own personal experience looking through different scopes, the following is sort of my list of things I consider when deciding on a refractor design to purchase.  

 

For visual, if I were in the market for a refractor of a given aperture and I intended to use it for high magnification applications like splitting doubles and looking for fine details on solar system objects I would GENERALLY favor a longer focal length, with the objective using lowest dispersion glass combination I could afford.   A nice FPL 53 or Fluorite doublet in the 100-120 mm range with a focal ratio of F7-F9 would rock the house.  If I wanted something for looking at wider FOV DSO's, and occasional casual viewing of solar system objects, I would be looking more at price range, quality of the mechanical components and a shorter focal ratio.  In that case, to ME, the glass type would be less important, relative to cost, as long as there is no grave problems with execution of the figure of the lens. 

 

For astrophotography, a good friend likes to make the distinction between imaging and snapping pictures, which I think is accurate:-).  If my goal is to grab some images of a number of DSO targets over the course of an evening, just to keep record of what I looked at and share with friends, then I have found my 80 EDT to perform quite satisfactory.  It collects good enough data that I can process a relatively nice looking image to share with minimal effort.  It is also I believe adequate for someone like myself that is early on the learning curve for imaging.  It will be some time before the optics of my scope will limit the quality of my image:-).  If at some point in the future, should I decide that I really want to step up my game, and spend a large number of hours collecting data on a single object over the course of multiple nights, etc..  then I will probably start looking to invest in a refractor with the best color correction I can afford.  

 

In any event, that is how I look at this whole glass question.  It is one perspective and others may, and often do come to a different conclusion.  For my part, I think the take home is that today we have access to a wide variety of 3-5 inch doublet and triplet refractors that range from pretty darn good to excellent in a price range from ~$400 to 2000 dollars, depending on what you want accomplish. When I started doing this hobby seriously 20 years ago, that was just not imaginable.    

 

Cheers!

 

JMD


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#20 Jon Isaacs

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Posted 22 October 2020 - 07:07 AM

For visual, if I were in the market for a refractor of a given aperture and I intended to use it for high magnification applications like splitting doubles and looking for fine details on solar system objects I would GENERALLY favor a longer focal length, with the objective using lowest dispersion glass combination I could afford.   A nice FPL 53 or Fluorite doublet in the 100-120 mm range with a focal ratio of F7-F9 would rock the house.  If I wanted something for looking at wider FOV DSO's, and occasional casual viewing of solar system objects, I would be looking more at price range, quality of the mechanical components and a shorter focal ratio.  In that case, to ME, the glass type would be less important, relative to cost, as long as there is no grave problems with execution of the figure of the lens.

 

 

The thing of it is, a top notch apo/ed can do it all.  It's not necessary to separate fine details on the solar systems, double stars and wide field viewing. 

 

If one is going after an all around performer at the highest level, then using FPL-53/FCD-100/Fluorite is part of the equation. 

 

Jon 


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#21 gianluca

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Posted 22 October 2020 - 08:41 AM

OK: we are back to one of the most debated topic.

As just recently I am reading many posts referring to FPL 53 vs 52 vs Fluorite vs... allow me:

 

Once more: it is NOT the single glass type that grants better color correction in itself.

A low cost "ED reported telescope" will be surely enjoyable but, if you need a quality instrument allow me to suggest: it is VERY important not to stick only to the color correction claims: an optical designer must consider many other optical aberrations, fabrication related issues, and also the instrument's intended use (maybe this is the first point that is weighed...).

 

Nonetheless and in any case want to stick to bare color correction evaluation ?

Well: as already written in a previous post of mine: when it comes to bare color correction, it is NOT the single glass, but the combination of glasses that must be considered.

So: just pick two glasses that are "orizzontally as far apart as possible, and "vertically" as near as possible on the partial dispersion chart, and you have done.

I Mean: together to the "FPL" number, you MUST have also the "flint side" glass name: the "FPL" in itself means little.

Not only: to it you should add also a spherocromatism graph (an objective glass with three colours crossings could be a poor performer if the crossings follow meandering paths...).

 

Obviously I think that any Firm that takes the step of using FPL 53 is also bringing on the effort to execute a high performances design, and that its optical lab is able to build it with the needed (tight) tolerances and an adequate finish grade. Even if I have been a bit surprised at some DPACs report: apart from under/overcorrection in red/blue, some zones even in green light are not all that rare,and this is a pity for a telescope "ED claimed".

 

So: I assume that whoever takes the path of highly corrected refractors has (or is building) both: theoretical and technology knowledge, plus experience to design and build top notch optics; not only figuring tolerances are much more tight, but ED glass are also costly and fragile (claiming for a blank high rejection rate, if the Firm is serious).

 

Under this point of view, I am sure that today it is difficult to get an "apo lemon", and that most telescope offer enjoyable view and good imaging performance.

 

Still: there is a difference between large scale built telescopes that just put a "FPL53 inside" sticker, and those Firms that maybe do not declare the design specs/glasses types, but that clearly publish at least about Strehel ratio and spherocromatism graph.

Someone grant about RMS and roughness value, and/or add asphere surface tuning.

Telescopes that I consider are those for which, added to the above, also DEDICATED focal reducer / field correctors are available.

 

My suggestion to someone wishing to buy a well built and well performing apo without going to high price points: ask yourself what you are going to do with it, and search for (not hardcore elaborated) images from amateurs like you; have a look to their rigs, and put a check mark on instruments that are in your budget. Ask about them on forums like this very one.

 

Consider that known Firms and instruments that are on the market from some time could be a more interesting choise than otas "FPL sticked" with undisclosed specs and build in small batches.

 

I do not like instruments that anyone is selling (with minor differences like an added handle or differently coloured trims) and those that are short-lived.

 

Remember that a quality optic needs a same-quality OTA, a same-quality focuser, a same-quality mount, a camera well tuned to the telescope iteself.

 

For imaging I think that it's way better to take a small step back and sticking to a well done and well equipped quality 90mm instead that going for a 130mm pushing the rig's envelope.

 

Visual observing asks for diameter until it's possible: less correction in IR and violet could be an accaptable compromise.

 

My suggestion is to continue asking (good suggestions are already in the previous replies), better if evaluating personal experiences than sticking to the glass type in itself.

 

Clear Skies.

 

G.


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#22 Wildetelescope

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Posted 22 October 2020 - 02:00 PM

The thing of it is, a top notch apo/ed can do it all.  It's not necessary to separate fine details on the solar systems, double stars and wide field viewing. 

 

If one is going after an all around performer at the highest level, then using FPL-53/FCD-100/Fluorite is part of the equation. 

 

Jon 

I completely agree, Jon.  If your goal is to get the best performance, then pay the premium, it is worth it in my opinion.   The technical aspects of the differences in glass and design have been address thoroughly and accurately by you and others many times.   My point was that for ME personally, these are where I draw lines regarding cost/benefit with my own purchases.  It is entirely subjective(the cost/benefit part), which is really what these questions always try to get at.  It is a tough question, do I delay gratification and get the best of the best, or do I get something more modest that I can start using sooner.   How much will the difference performance impact my FUN factor:-)  I have gone both ways on that question, it depends on the moment I am in.   I suspect that is true for many folks.  

 

Cheers!

 

JMD


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