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Fluorite vs ED

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#51 BillP

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Posted 07 August 2015 - 01:30 PM

 

 

What sparks debate is that spending so much per inch makes most refractor owners insane.  It's like trying to talk an elephant out of a tree when you don't know what got him up there in the first place.  GN

I love refractors and when looking through them, I am constantly reminded why. You could try talking me down from the tree, but I am too busy looking at the universe to listen.  :grin:

 

 

Clear skies!
Thomas, Denmark

 

PS: But I also have a 12" dob. It does a fine job on deep-sky and is a fun scope, I just love refractors more. I spend five times more on my Zeiss Telemator than I did on the 12" and I don't regret it. 

 

 

+1 :waytogo:

 

I certainly love my Dob for what my Dob does best, show me fainter objects better.  But when it comes to the aesthetic of the star points and a myriad of other factors, the refractor easily wins.  Like anything, each does great at some things, not so great at others, so always a matter of what the task is that makes equipment shine or not.  If you demean a refractor only means you are not appreciating what it does better than others, and if you demean a Dob just means that you are not appreciating what it does better than others.  So never a matter with something being wrong with the equipment...instead something is wrong with the observer :lol:



#52 junomike

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Posted 07 August 2015 - 01:49 PM

Fluorite VS ED, for me that means doublet VS triplet. The Takahashi TOA 150 is more than 10lbs heavier and cost $5000.00 more than my Takahashi  FS 152 NSV for nearly identical views according to most reviews.

I now take out my 6" APO nearly every clear night without much drama. I still can't believe I have this scope after 9 years of wanting it.

CS

Jonathan

 

Fluorite VS ED, for me that means doublet VS triplet. The Takahashi TOA 150 is more than 10lbs heavier and cost $5000.00 more than my Takahashi  FS 152 NSV for nearly identical views according to most reviews.

I now take out my 6" APO nearly every clear night without much drama. I still can't believe I have this scope after 9 years of wanting it.

CS

Jonathan

Jonathan, Beautiful set up indeed, but I don't think your math adds up as your scope was acquired used, no?

If so, then it cost about the same as a used TOA 150.

(I'd love to have either but would most likely grab the TOA 150 If I had a preference).

 

Mike



#53 Jon Isaacs

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Posted 07 August 2015 - 03:00 PM

 

 

What sparks debate is that spending so much per inch makes most refractor owners insane.  It's like trying to talk an elephant out of a tree when you don't know what got him up there in the first place.  GN

I love refractors and when looking through them, I am constantly reminded why. You could try talking me down from the tree, but I am too busy looking at the universe to listen.  :grin:

 

 

Clear skies!
Thomas, Denmark

 

PS: But I also have a 12" dob. It does a fine job on deep-sky and is a fun scope, I just love refractors more. I spend five times more on my Zeiss Telemator than I did on the 12" and I don't regret it. 

 

 

I think one needs to evaluate scopes on an individual basis as Tom Davis suggests. I see comparisons between a 12 inch Lightbridge and a Zeiss Telementor, a 6 inch ED refractor costing several thousand dollars and a 10 inch Orion Dob.  I am not surprised that the refractors are more loved.  They're nicer scopes.  

 

My collection includes a 10 inch GSO Dob, similar to the Bill's Dob, similar to yours.  I love the scope because of all the good times the two of have had together, the difficult doubles we have split, the times high in the mountains of the Navajo reservation, the fun at star parties, the people who have borrowed it and gone on to become dedicated astronomers in their own right.  But, it has it's issues, it cost me $240 used, 12 years ago..  A heck of a bargain and a pretty decent scope.. 

 

Sitting in my garage, a gift purchased as a going away present for a good friend, is a 10 inch F/.6 Starmaster with a Zambuto mirror, a vanishingly small central obstruction and the rest of the stuff that can provide those views and experiences possible with excellent optics in a well executed design. So far, the views have been amazing and the scope is a true pleasure to use.  If it were mine, I could grow to love it in way that is not quite possible with the 10 inch GSO, it's just a very nice scope and more capable than a similar priced refractor.  

 

Somehow these threads end up comparing refractors and reflectors. I wish that didn't happen.  Refractors, whether FPL-53, Fluorite, OK-4, other ED glass or achromatic, they have their virtues, their limitations.  The same is true of reflectors. Personally, I find my Dobs more lovable because I am closer to them, I have put more of myself into them and too, they have provided me with more memorable views.   

 

FPL-53 versus Fluorite:  Both can provide essentially perfect views.  Discussions like these, they remind me of a joke about two wealthy matrons jousting over whose diamonds are brighter..  The brilliance of a diamond mostly depends on the cut and the setting.. sound familiar?

 

Jon



#54 Sasa

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Posted 07 August 2015 - 03:01 PM

3) There's what they say and there's what they do. At one time all of Tak's flagship refractors had a fluorite element, now their flagship refractors with the best color correction in Tak history are FPL53 scopes.

Once again, this need a clarification.

TSA-102 uses one (only one) ED. FCT-100 uses only ONE Fluorite. Both has same grade of real measured polystrehl ratio -as you know where check this-. The FCT is a f/6.4 but not a f/8 as TSA-102 is.
By the way, the better color corrected scope (and not only in Tak history ;) ) employs TWO ED in a triplet.

Maybe in future it can be measured other thing.
Just for record. When the lens is made out of only two types of glasses, the level of color correction is the same being it dublet, triplet, quadruplet,....

The trick with same two ED glasses in triplet, like in TOA, is that such lens has significantly larger lens radii compared to the case with one ED glasd. So you can either make such triplet faster, or if you keep the speed reasonably slow, like in TOA, you can make the lens essentially perfect, with minimal spherochromatism and higher order spherical aberration.

Edited by Sasa, 07 August 2015 - 03:03 PM.


#55 WesC

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Posted 07 August 2015 - 04:17 PM

This is all just confirmation bias.

 

I liken this to the "butt dyno" effect prevalent on car and motorcycle forums where people vehemently argue about how much more power they "feel" from whatever mod is being discussed--when there either isn't any or too little to really matter. Lots of stubborn righteous opinion. No hard evidence, or evidence that is so slim that it is barely even measurable--and completely imperceptible to human senses.

 

My dad always told me, "You can't argue with a closed mind. Once someone has convinced themselves they will defend their position to the death."

 

Yep!

 

I doubt anyone on this forum, in a blind test could pick the fluorite refractor out of a lineup 9 times out of 10. Of course, since this test will never happen, and even of it does there will always be someone who will arbitrarily invalidate and dismiss the test because of how it was carried out or whatever.... these pointless arguments will go on.

 

And FWIW, It's my opinion that the views through my TSA-120 are far better than the FS102s and FS152s I have looked though. That's why I bought one.

 

So there! :neener:



#56 Astrojensen

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Posted 07 August 2015 - 04:27 PM

Why do we always try to rationalize love?

 

 

Clear skies!
Thomas, Denmark



#57 Jon Isaacs

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Posted 07 August 2015 - 04:30 PM

 

My dad always told me...

 

In a similar vein:

 

"You can't depend on your eyes when your imagination is out of focus."

 

Mark Twain

 

Very appropriate for amateur astronomers, me included.. 

 

I personally think we should just all be happy that Fluorite exists and that FPL-53 and OK-4 exist and some of us at least can have a chance to look through telescopes made with these materials.  How lucky we are to be able to worry about such trivia.  

 

Jon 

 



#58 Kunama

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Posted 07 August 2015 - 04:46 PM

............................

 

I doubt anyone on this forum, in a blind test could pick the fluorite refractor out of a lineup 9 times out of 10. Of course, since this test will never happen, and even of it does there will always be someone who will arbitrarily invalidate and dismiss the test because of how it was carried out or whatever.... these pointless arguments will go on.

 

..............................

So there! :neener:

 

 I would go so far as saying twice out of ten, except by chance.  I know that with my FS128, TSA120 and TOA130 the differences were so small that they really did not matter for visual use.........

 

As for the durability of CaF2, the only time I have seen any deterioration of a CaF2 element was when it had been subjected to abuse and incorrect cleaning solvents.


Edited by Kunama, 07 August 2015 - 05:37 PM.


#59 rolo

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Posted 07 August 2015 - 09:37 PM

 Wow! I never knew there were so many optical experts on CN! I'm blessed to be here! :jawdrop:



#60 edif300

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Posted 08 August 2015 - 12:56 AM

 

Just for record. When the lens is made out of only two types of glasses, the level of color correction is the same being it dublet, triplet, quadruplet,....

The trick with same two ED glasses in triplet, like in TOA, is that such lens has significantly larger lens radii compared to the case with one ED glasd. So you can either make such triplet faster, or if you keep the speed reasonably slow, like in TOA, you can make the lens essentially perfect, with minimal spherochromatism and higher order spherical aberration.

 

 

The trick is there is no trick :)



#61 RichA

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Posted 08 August 2015 - 02:46 AM

Another one from the table referenced above.

 

100mm f/10--- 7KN7 and FPL53.   Polychromatic Strhel of .99.   

 

CaF2 and K5..  .984..   

 

And yet another one... 

 

Here is a glass most people would throw up on vs FPL53.   Another 100mm f/7 triplet   It is a ZNK7, FK51, ZNK7  with a polychromatic Strehl of .97.

 

Man, people tend to dismiss the magic of a triplet.  Even using non ED glass, a triplet can come very close to a Fluorite doublet.

 

Even an FK51 triplet can give outstanding performance in a 4" instrument.    Only the marketing and sales people at Tak would see a real difference in performance, and it would all be in the ledger.

 

This last one is perhaps a rant at the 120ED.  People think it is better than the ES because it uses FPL 53..

 

But a doublet is a doublet, and adding an extra piece of glass to the ED formula makes it possible to get superior color correction over what you can get with Fluorite or FPL 53 in a doublet.   Three pieces of glass rule...   To suggest that the 120ED has better color correction than the ES 127 because it uses FPL 53 instead of the FPL51 equivalent used in the ES ignores the awesome benefit of adding a third element.

 

Another one from the table referenced above.

 

100mm f/10--- 7KN7 and FPL53.   Polychromatic Strhel of .99.   

 

CaF2 and K5..  .984..   

 

And yet another one... 

 

Here is a glass most people would throw up on vs FPL53.   Another 100mm f/7 triplet   It is a ZNK7, FK51, ZNK7  with a polychromatic Strehl of .97.

 

Man, people tend to dismiss the magic of a triplet.  Even using non ED glass, a triplet can come very close to a Fluorite doublet.

 

Even an FK51 triplet can give outstanding performance in a 4" instrument.    Only the marketing and sales people at Tak would see a real difference in performance, and it would all be in the ledger.

 

This last one is perhaps a rant at the 120ED.  People think it is better than the ES because it uses FPL 53..

 

But a doublet is a doublet, and adding an extra piece of glass to the ED formula makes it possible to get superior color correction over what you can get with Fluorite or FPL 53 in a doublet.   Three pieces of glass rule...   To suggest that the 120ED has better color correction than the ES 127 because it uses FPL 53 instead of the FPL51 equivalent used in the ES ignores the awesome benefit of adding a third element.

Provided all six surfaces of the Chinese-made instrument are accurate.  Four is easier.



#62 Astrojensen

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Posted 08 August 2015 - 02:56 AM

 

Provided all six surfaces of the Chinese-made instrument are accurate.  Four is easier.

Not necessarily. Four steeply curved surfaces can be just as difficult - or more so - than six gently curved ones. 

 

 

Clear skies!

Thomas, Denmark



#63 Jeff B

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Posted 08 August 2015 - 11:41 AM

 

Another one from the table referenced above.

 

100mm f/10--- 7KN7 and FPL53.   Polychromatic Strhel of .99.   

 

CaF2 and K5..  .984..   

 

And yet another one... 

 

Here is a glass most people would throw up on vs FPL53.   Another 100mm f/7 triplet   It is a ZNK7, FK51, ZNK7  with a polychromatic Strehl of .97.

 

Man, people tend to dismiss the magic of a triplet.  Even using non ED glass, a triplet can come very close to a Fluorite doublet.

 

Even an FK51 triplet can give outstanding performance in a 4" instrument.    Only the marketing and sales people at Tak would see a real difference in performance, and it would all be in the ledger.

 

This last one is perhaps a rant at the 120ED.  People think it is better than the ES because it uses FPL 53..

 

But a doublet is a doublet, and adding an extra piece of glass to the ED formula makes it possible to get superior color correction over what you can get with Fluorite or FPL 53 in a doublet.   Three pieces of glass rule...   To suggest that the 120ED has better color correction than the ES 127 because it uses FPL 53 instead of the FPL51 equivalent used in the ES ignores the awesome benefit of adding a third element.

 

Another one from the table referenced above.

 

100mm f/10--- 7KN7 and FPL53.   Polychromatic Strhel of .99.   

 

CaF2 and K5..  .984..   

 

And yet another one... 

 

Here is a glass most people would throw up on vs FPL53.   Another 100mm f/7 triplet   It is a ZNK7, FK51, ZNK7  with a polychromatic Strehl of .97.

 

Man, people tend to dismiss the magic of a triplet.  Even using non ED glass, a triplet can come very close to a Fluorite doublet.

 

Even an FK51 triplet can give outstanding performance in a 4" instrument.    Only the marketing and sales people at Tak would see a real difference in performance, and it would all be in the ledger.

 

This last one is perhaps a rant at the 120ED.  People think it is better than the ES because it uses FPL 53..

 

But a doublet is a doublet, and adding an extra piece of glass to the ED formula makes it possible to get superior color correction over what you can get with Fluorite or FPL 53 in a doublet.   Three pieces of glass rule...   To suggest that the 120ED has better color correction than the ES 127 because it uses FPL 53 instead of the FPL51 equivalent used in the ES ignores the awesome benefit of adding a third element.

Provided all six surfaces of the Chinese-made instrument are accurate.  Four is easier.

 

 

Sorry but not necessarily true.  Generally, the six surfaces allow for shallower radii than those of a doublet for a given F ratio and correction level.  The shallower radii are easier to make and may be more tolerant of cell design than a doublet design.

 

That being said, based upon user and test reports, the 120ED FPL-53 based doublet seems to be a well thought out design that can actually be well made too.

 

Jeff



#64 RAKing

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Posted 08 August 2015 - 01:03 PM

Isn't the execution of the design at least as important as the number of elements or type of glass?

 

I have personally seen three ES 127 triplets that had to be returned to the factory for QC issues. I don't care how many elements that scope has, I would much rather have a well-made doublet (such as the SW 120ED) than a poorly made triplet.

 

As far as fluorite versus ED glass - I loved my Tak FS-128, but my Tak TOA-130 was just as good in most things and better with star colors. It simply weighed too much and I don't know how the marketing folks could talk their way around that. :lol:    

 

Cheers,

 

Ron



#65 Astrojensen

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Posted 08 August 2015 - 01:25 PM

Excecution and attention to detail is everything.

 

 

Clear skies!
Thomas, Denmark



#66 Lance1234

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Posted 08 August 2015 - 02:31 PM

Isn't the execution of the design at least as important as the number of elements or type of glass?

 

 

Except, within a price group, how much choice is there on execution?  Don't you kind of get what you get?  At the top end you've got the AP, TAK, TEC, LZOS etc. which are likely all very well executed.  But in the more moderate price groups, they're all made at the same few Chinese factories, i.e. United Optics, JOC, etc.  So you really have no idea how well executed they are, other than to get reviews and hope the QC is sufficient to weed out the dogs.  So assuming you're in the Chinese price bracket, and if you're lucky enough to know what at least one lens element is made of (which you usually aren't) do you go for the FPL-51 (or Chendu equivalent) or for the FPL-53 glass given that everything else is an unknown.



#67 Jeff B

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Posted 08 August 2015 - 08:39 PM

Excecution and attention to detail is everything.

 

 

Clear skies!
Thomas, Denmark

Yes but you have to have a good design that can actually be made too.

 

Good execution and attention to detail of a mediocre design will yield mediocre performance...at best.

 

Jeff



#68 LewisM

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Posted 10 August 2015 - 10:27 PM

I love fluorite. Most of my scopes have been fluorite.

 

BUT, ED - if well made - can be supremelly good too. I note negligable difference between an FSQ-106ED and FSQ-106N.

 

Incidentally, there IS NO SIZE DIFFERENCE between the FSQ-106N and FSQ-106ED focuser - both arte 4" diameter drawtubes. The difference is the LENGTH of the drawtube. Also, the ED's advantage at backfocus is negated if you fit the CA to alleviate the capstan wheel deficiency.

 

I recently made a choice to swap my as-new FSQ-106ED for an older FSQ-106N. I think I made a personal choice to the better, but only because of a pre-disposed bias I have :)



#69 Edd Weninger

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Posted 11 August 2015 - 12:30 AM

We have three pages and no one has mentioned coatings.  Roland seems to think them important enough to use what he considers the best.  The ones he specifies are expensive and are claimed to have a very high transmissibility.  



#70 Jon Isaacs

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Posted 11 August 2015 - 04:39 AM

We have three pages and no one has mentioned coatings.  Roland seems to think them important enough to use what he considers the best.  The ones he specifies are expensive and are claimed to have a very high transmissibility.


Weren't many of the older Fluorite elements left uncoated because of the difficulty coating them with the technology of the btimes?

Jon

#71 Markab

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Posted 11 August 2015 - 08:10 AM

 

We have three pages and no one has mentioned coatings.  Roland seems to think them important enough to use what he considers the best.  The ones he specifies are expensive and are claimed to have a very high transmissibility.


Weren't many of the older Fluorite elements left uncoated because of the difficulty coating them with the technology of the btimes?

Jon

 

 

True that. Takahashi began coating a few of their FC's in the early 1990s (I had a 1994 model FC-100 that had the fluorite multi-coated, a rarity at the time). The Vixen/Celestrons of that era also had an uncoated fluorite lens.



#72 Eddgie

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Posted 11 August 2015 - 08:37 AM

We have three pages and no one has mentioned coatings.  Roland seems to think them important enough to use what he considers the best.  The ones he specifies are expensive and are claimed to have a very high transmissibility.  

 

Roland does not think of his big refractors as telescopes.  He thinks of them as astrographs, and based on comments to me once, I think he might believe that using one of his scopes visually is a waste.   For planetary performance, he pointed me to a 10" f/6 reflector as being a better choice than a 155 EDF.

 

The purpose of the coatings is I believe to allow greater transmission into the near infra red and ultraviolet.  This is what rings his bell.  A quote:

 

"The acid test was, of course, imaging and for that I added the dedicated field flattener and my STL1100 CCD camera. The results were everything I had hoped for. All the colors came to the same exact focus - no focus tweek required for any of them. Stars showed similar size and full width half max resolution with all 8 filter positions (RGB, L, H-alpha, OII and SII)..."

 

For visual use, you rea​lly don't need to go very wide.  This is why the red Airy Disk is often allowed to be twice the diameter of the green Airy Disk in many designs.   When it is kept to this size, the human eye simply can't see it.   But the camera misses nothing.    And for many photos, having high transmission at these more exotic wavelengths increases the ability to image large H-alpha features.

 

For visual use, coatings simply don't need to be so wide.   For an astrograph though, it starts to become more important.



#73 Sasa

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Posted 11 August 2015 - 08:57 AM

 

 

Just for record. When the lens is made out of only two types of glasses, the level of color correction is the same being it dublet, triplet, quadruplet,....

The trick with same two ED glasses in triplet, like in TOA, is that such lens has significantly larger lens radii compared to the case with one ED glasd. So you can either make such triplet faster, or if you keep the speed reasonably slow, like in TOA, you can make the lens essentially perfect, with minimal spherochromatism and higher order spherical aberration.

 

 

The trick is there is no trick :)

 

 

Edif300, you have nice web page with lot of interesting information about Takahashi telescopes. One plot (relevant for the discuss topic) caught my attention. It is the one which is supposed to show how better is transmission of CaF2 with respect to traditional cworn BK7

 

http://www.astrosurf...2/FS-curves.jpg

 

If you look at it without too much thinking (or knowledge) you will immediately say, wow, CaF2 is indeed much better than BK7.

 

However, the plot is completely irrelevant in terms of discussion of internal glass scattering. As far as I can say, the plot describes the transmission of 10mm thick plate INCLUDING the reflection on two air-to-glass surfaces. This is dominant source in optical part of spectrum (in the part where the two curves are flat). This air-to-glass reflection is described by formula [(n-1)/(n+1)]^2. The reason why CaF2 transmits more light in this curve is its lower refractive index. From this formula, CaF2 should peak at about 0.94, BK7 at about 0.91. So mostly, the plot show differences between uncoated lenses from CaF2 and BK7. Proper coatings will makes this difference negligible.

 

In order to have a plot that describes the internal light loss, one has to subtract this air-to-glass reflections. Here, the differences are much smaller. OHara gives for their crown (S-BSL7)  internal transmittance for 10mm glass path between 99.5-99.8% for the 390-1200nm wavelengths. Well, there  is still room to have slightly better performance but it is of different order of magnitude than the plot suggest.



#74 BillP

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Posted 11 August 2015 - 10:04 AM

 Wow! I never knew there were so many optical experts on CN! I'm blessed to be here! :jawdrop:

 

I believe the phrase is "armchair optician" :rofl:   ...the posts are fun...and educational...just don't take them too seriously.



#75 Eddgie

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Posted 11 August 2015 - 10:08 AM

 

 

 

Just for record. When the lens is made out of only two types of glasses, the level of color correction is the same being it dublet, triplet, quadruplet,....

The trick with same two ED glasses in triplet, like in TOA, is that such lens has significantly larger lens radii compared to the case with one ED glasd. So you can either make such triplet faster, or if you keep the speed reasonably slow, like in TOA, you can make the lens essentially perfect, with minimal spherochromatism and higher order spherical aberration.

 

 

The trick is there is no trick :)

 

 

Edif300, you have nice web page with lot of interesting information about Takahashi telescopes. One plot (relevant for the discuss topic) caught my attention. It is the one which is supposed to show how better is transmission of CaF2 with respect to traditional cworn BK7

 

http://www.astrosurf...2/FS-curves.jpg

 

If you look at it without too much thinking (or knowledge) you will immediately say, wow, CaF2 is indeed much better than BK7.

 

However, the plot is completely irrelevant in terms of discussion of internal glass scattering. As far as I can say, the plot describes the transmission of 10mm thick plate INCLUDING the reflection on two air-to-glass surfaces. This is dominant source in optical part of spectrum (in the part where the two curves are flat). This air-to-glass reflection is described by formula [(n-1)/(n+1)]^2. The reason why CaF2 transmits more light in this curve is its lower refractive index. From this formula, CaF2 should peak at about 0.94, BK7 at about 0.91. So mostly, the plot show differences between uncoated lenses from CaF2 and BK7. Proper coatings will makes this difference negligible.

 

In order to have a plot that describes the internal light loss, one has to subtract this air-to-glass reflections. Here, the differences are much smaller. OHara gives for their crown (S-BSL7)  internal transmittance for 10mm glass path between 99.5-99.8% for the 390-1200nm wavelengths. Well, there  is still room to have slightly better performance but it is of different order of magnitude than the plot suggest.

 

 

Yes, that was my point earlier.  The difference between a FPL 53 and CaF2 telescope will be so faint that it is impossible to see.

 

Using a third element with ED glass makes it far to easy to surpass what can be done with a CaF2 doublet and with modern coatings, the difference in transmission is meaningless.

 

To suggest that just because a telescope has a Fluorite element it is going to surpass all ED scopes is of course quite a stretch of the reality of physics.  It is all about the design of the system and the mating elements.




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