159 replies to this topic

[Alan French]

Many discussions on Cloudy Nights focus on "Which is better?." An occasional topic is how fluor-crown glass, FPL53, compares to synthetic crystal fluorite, CaF2. Fortunately, Vladimir Sacek offers examples at http://www.telescope...po_examples.htm - nicely answering the question.

 

Design #15 is for a FPL53 doublet (ZKN7 and FPL53) and design #16 is for a CaF2 doublet (K5 and CaF2), allowing for an easy comparison. Both are 100mm f/10 instruments.

 

Designs #18 and #19 allow comparisons of triplets, with the first featuring a ZKN7/FPL53/ZKN7 triplet and the second a K7/CaF2/K7 version. Both are 140mm f/7 telescopes.

 

Note, especially, the polychromatic Strehl in the last column for each design.

 

Clear skies, Alan

[M11Mike]

Alan - I'm not all that 'technical' --- but do I read this as #15 (FPL53) is actually superior to #16 (Fluorite) 100mm APO "doublets" ???  

 

Mike

[Element79]

Where Fluorite shines is in the violet and deep reds where it can control CA much better than FPL-53 can.  If your eyes are insensitive to violet and deep red and if you don't take pictures then there will be no gain using Fluorite over FPL-53.

[nicoledoula]

If ZKN7 glass is used as a mating element. 

[gnowellsct]

To my untutored eye FPL 53 does better in most of the apertures and focal lengths than CaF2, but my eye is untutored.  The chart appears to be a little old, there's no FPL 55.    

 

All I can say is that one of the top apo makers, Yuri of TEC, has said he thinks fluorite is the real deal and better than the alternatives, and what he says I would pay attention to.  Another person I pay attention to is Catlin Fus and he seems to have a positive opinion of FPL 55 (he also uses FPL 53 in some scopes).     Given how intensely people feel about their refractors and the competitiveness of the market I don't think he would use FPL 55 if it was seriously at a disadvantage. 

 

This chart doesn't tell me why Yuri is going to go the extra distance for fluorite.  The classic f/7 140 (see number 18) is .97 to .983 (see no. 21) which is "sensibly perfect" at either range.  Number 19, the CaF2 140 mm f/7, is a shoddy .965. (joke)

 

I don't see how anyone would see a dimes worth of difference, but for imaging OK, I'll listen to the pros.  And if Yuri says fluorite is great I believe him.  But it looks to me from the numbers that the issue may not be the glass and the numbers but questions of supply or access to sufficient quantities of glass at a reasonable price.

 

I note that from right to left we're talking about the number of lenses and the mating elements about which I know even less than what I've just written.  I have no idea who is putting what lens where.

[Element79]

Here is a spread sheet provided by Yuri for the choice of ED elements in the new TEC140.  Note how the Strehls are much better for Fluorite verses both FPL-53 and FPL-55 in the reds and violets:

 

               Violet  (g line)     Blue (F line)        Green (e line)      Red (C line)

 

FPL-53          .29                     .80                        .99                     .85

FPL-55          .03                     .90                        .99                     .75

CaF2             .70                     .93                        .99                     .97

 

Fluorite also has far less scatter than a glass would have...

[Alan French]

Alan - I'm not all that 'technical' --- but do I read this as #15 (FPL53) is actually superior to #16 (Fluorite) 100mm APO "doublets" ???  

 

Mike

Both look excellent and the differences are minor enough that I wouldn't award any bragging rights to one of them.

 

Clear skies, Alan

[Yuri]

Many discussions on Cloudy Nights focus on "Which is better?." An occasional topic is how fluor-crown glass, FPL53, compares to synthetic crystal fluorite, CaF2. Fortunately, Vladimir Sacek offers examples at http://www.telescope...po_examples.htm - nicely answering the question.

 

Design #15 is for a FPL53 doublet (ZKN7 and FPL53) and design #16 is for a CaF2 doublet (K5 and CaF2), allowing for an easy comparison. Both are 100mm f/10 instruments.

 

Designs #18 and #19 allow comparisons of triplets, with the first featuring a ZKN7/FPL53/ZKN7 triplet and the second a K7/CaF2/K7 version. Both are 140mm f/7 telescopes.

 

Note, especially, the polychromatic Strehl in the last column for each design.

 

Clear skies, Alan

Alan, I would exclude ZKN7 from the list due to its small scale internal micro-roughness that causes scatter in best case (see picture with laser). In worst case it could ruin production. See second picture, it is a Foucault-gram that shows comparison of interiors Zkn7 vs BK7 of similar sizes.

Attached Thumbnails

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Edited by Yuri, 10 May 2018 - 09:33 PM.

[gnowellsct]

Well the problem here is that the TEC 140s as they have been produced for....a long time, I'd say well over ten years....are world renowned and revered for their excellence.  And now we look at a few violet and red Strehls and say, ah, it should have been fluorite all along? 

 

Gimme back my FS128!

 

I'm just going to guess that this is one of those things that is primarily of interest to imagers who stretch beyond the normal visible spectrum in their quest for detail.  

[Alan French]

Alan, I would exclude ZKN7 from the list due to its small scale internal micro-roughness that causes scatter in best case (see picture with laser). In worst case it could ruin production. See second picture, it is a Foucault-gram that shows comparison of interiors Zkn7 vs BK7 of similar sizes.

Yuri,

 

Thanks. We can look at a variety of designs, but few of us know the reality of the difficulties with some glasses. I've heard similar stories over the years, and sometimes marvel at the persistence of the folks making high end optics.

 

Clear skies, Alan

[Alan French]

Well the problem here is that the TEC 140s as they have been produced for....a long time, I'd say well over ten years....are world renowned and revered for their excellence.  And now we look at a few violet and red Strehls and say, ah, it should have been fluorite all along? 

 

Gimme back my FS128!

 

I'm just going to guess that this is one of those things that is primarily of interest to imagers who stretch beyond the normal visible spectrum in their quest for detail.  

I did not write down the source, and numerous graphs can be found on the web, but I here's a table of wavelength and "relative luminosity" (appearance by eye) on the first page of my refractor notes. This is for the photopic (bright) eye. The sensitivity shifts toward the blue for the scotopic (dark) eye - and our visual acuity takes a huge hit.

 

380nm  0 

420nm  0.4 violet

(456.1 F)

460nm  6 blue

500nm  32 blue-green

540nm  95 green

555nm  100  yellow-green

580nm  87 yellow

620nm  38  orange

(656.3 C)

660nm  6  red

720nm  0.1  deep red

760nm  0   

 

Clear skies, Alan

[gnowellsct]

I did not write down the source, and numerous graphs can be found on the web, but I here's a table of wavelength and "relative luminosity" (appearance by eye) on the first page of my refractor notes. This is for the photopic (bright) eye. The sensitivity shifts toward the blue for the scotopic (dark) eye - and our visual acuity takes a huge hit.

 

380nm  0 

420nm  0.4 violet

(456.1 F)

460nm  6 blue

500nm  32 blue-green

540nm  95 green

555nm  100  yellow-green

580nm  87 yellow

620nm  38  orange

(656.3 C)

660nm  6  red

720nm  0.1  deep red

760nm  0   

 

Clear skies, Alan

Well it looks like violet is of minimal importance for visual observing, but red maybe somewhat.  So the poor strehl performance in violet for naked eye is not necessarily something to get worked up about, unless it matters to a camera.  The red at 660 nm is more significant, but, not huge, but on this end the "bad" glasses do better.  Maybe not as good but better.  So the worst strehl (violet) is concentrated in the least critical area of the spectrum....

 

...unless I'm wrong.  GN

[Alan French]

In spite of an interest, still very much in progress, in how things work, I find actually seeing telescopes under the stars is more instructive. Happy to have attended many conventions and a good number of smaller events over the years.

 

Clear skies, Alan

[Fomalhaut]

An important advantage of fluorite (as an optical material) is the fact that, due to its unique cristalline structure, every batch of its production will deliver identical physical properties, whereas every melted glass (such as all FPLs) provides slight variations of optical specifications (e.g. Abbe numbers).

For this reason, partner glasses can be matched more easily to the constant parameters of each production run of fluorite and optical quality tends to be more consistent.

 

Chris

Edited by Fomalhaut, 11 May 2018 - 06:37 AM.

[Erik Bakker]

I leave good optical design and manufacturing to the experts. And unfortunately, there is no such thing as a free lunch here 

[Alan French]

An important advantage of fluorite (as an optical material) is the fact that, due to its unique cristalline structure, every batch of its production will deliver identical physical properties, whereas every melted glass (such as all FPLs) provides slight variations of optical specifications (e.g. Abbe numbers).

For this reason, partner glasses can be matched more easily to the constant parameters of each production run of fluorite and optical quality tends to be more consistent.

 

Chris

Chris,

 

That is certainly an advantage, but the matching one or, more often, two elements still vary and that has to be accounted for.

 

Its crystalline structure also means it can't be heated to high temperatures safely for traditional coatings.

 

I note that Canon, in discussing synthetic crystal calcium fluoride claims it takes four times as long to work. Now this was in some obvious marketing hype designed to promote their L series lenses - and perhaps justify their high cost - so I'm taking that with a grain of salt. But I have heard a number of claims that fluorite is more labor intensive and requires special techniques.

 

Again, in the end product, I don't think the choice of fluorite, FPL53, or the newer FPL55 should be considered a deal breaker. There are obviously fine examples of lenses using fluorite on the market, and others using FPL53. Since FPL53 is gong away, we'll be seeing FPL55 based designs before long.

 

Clear skies, Alan

[Jeff B]

Thanks for the topic Alan, always good for some spirited discussions.  

 

This to me really does show it's the glass combinations that give the system performance, not a single glass type.  And I really appreciate Yuri's grounding us in the real world.

 

Jeff

 

PS:  At that link, the design I really find interesting is #7.  I have just such a one-off instrument.  I was using it on Jupiter last night and the views were killer.  May be "old school" but it is Ivy league in every visual aspect.

 

Jeff

[Suavi]

Alan, CFF has already switched to FPL55 and most of their refractors are based on this glass, from 105mm all the way up to 230mm f/7.

[vahe]

There was an old post by Roland on tec-ug, here is what Roland said about FPL-53 vs CaF2:
.
"To make a fully color corrected triplet lens with oil space, where color correction extends all the way to 400nm, the choices are threefold. You can use an FPL-53 aspheric oiled triplet design which makes production a bit slow and dependent on highly skilled opticians. You can use an FPL-53 airspace design with all spherical surfaces, which requires that all 6 surfaces be very smooth and accurate. You can use Calcium Fluorite oiled triplet with all spherical surfaces."
.
My knowledge of optics and glasses used in optics remains at a primitive level, that said my interpretation of what Roland is saying is that Fluorite does offer something or an advantage over FPL-53 that allows a fully corrected triplet to be all spherical where as the FPL-53 version needs additional aspherics to match that level of performance.

.

Vahe

[Element79]

When FPL-53 was available you couldn't even get it in a 230mm diameter blank, so I am surprised that CFF can get FPL-55 blanks that large.  I am even more surprised that Yuri can get Fluorite in 250mm blanks!  Those have to cost a fortune by themselves...

[Element79]

There was an old post by Roland on tec-ug, here is what Roland said about FPL-53 vs CaF2:
.
"To make a fully color corrected triplet lens with oil space, where color correction extends all the way to 400nm, the choices are threefold. You can use an FPL-53 aspheric oiled triplet design which makes production a bit slow and dependent on highly skilled opticians. You can use an FPL-53 airspace design with all spherical surfaces, which requires that all 6 surfaces be very smooth and accurate. You can use Calcium Fluorite oiled triplet with all spherical surfaces."
.
My knowledge of optics and glasses used in optics remains at a primitive level, that said my interpretation of what Roland is saying is that Fluorite does offer something or an advantage over FPL-53 that allows a fully corrected triplet to be all spherical where as the FPL-53 version needs additional aspherics to match that level of performance.

.

Vahe

 

I wonder why Yuri still aspherises his oiled triplets when he could get away with an all spherical design?  I am sure there is a reason but since I don't have an advanced degree in optical engineering I will just have to live with the fact that he feels that it's necessary.

[Jeff B]

I wonder why Yuri still aspherises his oiled triplets when he could get away with an all spherical design?  I am sure there is a reason but since I don't have an advanced degree in optical engineering I will just have to live with the fact that he feels that it's necessary.

Are you sure about that?

[Element79]

I am pretty sure that I read somewhere that the TEC oiled triplets were aspherised.   Maybe that was just for the FPL-53 based models.  I'll have to revisit their web page when I get a moment to look again.  Personally I would rather have all spherical surfaces since they can be polished much smoother and more accurately than a aspherised surface could.

[Yuri]

I am pretty sure that I read somewhere that the TEC oiled triplets were aspherised.   Maybe that was just for the FPL-53 based models.  I'll have to revisit their web page when I get a moment to look again.  Personally I would rather have all spherical surfaces since they can be polished much smoother and more accurately than a aspherised surface could.

CaF2 is available up to 400mm. Just imaging - a drop of cold water can make it in pieces...

as per APO140ED - touch-ups - yes, aspherising - no, after all this model has gone...

Edited by Yuri, 11 May 2018 - 10:07 AM.

[Uwe Pilz]

Please keep in mind that fluorite has a crystalline structure. It is not a glass and cannot be polished like glass, at least not in an easy way. You have to live with steps and edges which derive form the crystal structure. This is the reason for oiling fluorite lenses: If you choose an oil with a refractive index similar to that of the fluorite, you smear the crystalline structure which would give you an underperforming image otherwise.

 

So you give up the degree of freedom which the air space is.