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Fluorite Refractors

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#26 Jeff B

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Posted 29 June 2015 - 09:19 PM

 FPL-53 is GOOD, but it can't QUITE match fluorite in some ways (yet FPL53 is better than flurotie in other ways)

 

Please elaborate. 



#27 GShaffer

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Posted 29 June 2015 - 11:01 PM

I have one of the APOGRADE lenses installed in a StellarVue Nighthawk OTA.....it was claimed to be a fluorite lens but who knows? I do know it works quite well and is my favorite small refractor I have owned.....

A green laser pointer will quickly show if the element is fluorite, or fluoro-crown glass.


I am guessing that might be so if one knew what they are looking for and how to interpret what they see.....without that I doubt it :)

 
Sent an email to William Optics. I think you will find that your "Fluorite" objective uses FPL-53...I have yet to hear of a WO scope the actually used CaF2 and I have following this story for about 10 years.
 
Jon


Guess you missed my implication above that I don't really care, I just know it works great :)

 
Greg: 
 
I can't really judge your interest level other than the fact that you made the effort to write a post about it. I can assure you that whatever your interest level is, your objective is FPL-53..
 
Jon


And it still works great lol

#28 Jon Isaacs

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Posted 30 June 2015 - 07:05 AM

 

And it still works great lol

 

:waytogo:

 

I am with you on that one.

 

My 80mm WO Megrez FD has that same objective, they called it Fluorite but it's FPL-53.  If you look at the Partial dispersions.. they are very close.  I am like you, it's a refractor.. has excellent color correction, it's 80mm, it's not going to blow the world away viewing the planets.. 

 

The advantage of FPL-53 over fluorite, it is less fragile, easier to work.  I think in the old days, fluorite objectives were uncoated for that reason.. 

 

As far as whether the Chinese have produced a scope with Fluorite, I don't know where the Burgess Optical and StellarVue "Fluorite" scopes were made but I believe they are actually Fluorite...   

 

Jon



#29 Mark9473

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Posted 30 June 2015 - 07:39 AM

 

Sent an email to William Optics. I think you will find that your "Fluorite" objective uses FPL-53...I have yet to hear of a WO scope the actually used CaF2 and I have following this story for about 10 years.

 

Jon

 

 

WO admitted it was FPL-53 on the WO Yahoo Group shortly after the 80FD was launched. Still a very good objective.


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#30 Element79

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Posted 30 June 2015 - 07:56 AM

The problem with FPL-53 is that it is a glass.  A glass is made by mixing together a whole bunch of different materials and melting them all together.  One melt is not exactly the same as the next.  For this reason the color correction on one telescope will not be the same as on the next 'supposedly identical' telescope.  Quality manufacturers will analyze the diffraction properties of each melt and slightly adjust their design to compensate for the difference so that the final product is as good as it can get.  High volume manufacturers do not have the time or motivation to do this and their final products will all be slightly different.  I have actually seen these differences in side-by-side tests of 'supposedly identical' telescopes, and some do indeed have better color control than the one next to it!

 

CaF2 is a crystal.  Every atom in its matrix is fixed.  Every piece has identical properties.  It is brittle.  It is difficult to work with.  It doesn't weather well.  But each and every piece of it is identical and the color control can be made to a very high standard.  But it's so #### expensive!  I wish I could afford a Takahashi CaF2 instrument but it would take a small miracle to get that purchase past my wife...


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#31 BillP

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Posted 30 June 2015 - 09:18 AM

Bill, you seem to be crossing over true Fluorite crystal (CaF2) and FPL-53, which is merely fluorite enriched glass.

 

For instance, the FSQ-85, FSQ-106 ED's and FSQ-130 are all FPL-53. Takahashi went away from true fluorite for a while and brought out FPL53 replacements or new models: TSA series, TOA series and the New Q's. 

 

Thanks.  I thought the Qs had CaF2.  Will remove them.



#32 LewisM

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Posted 30 June 2015 - 09:23 AM

Only the FSQ-106 and FSQ-106N have fluorite. The FSQ-106ED and FSQ-106EDXIII are all FPL-53 (as is the FSQ-85 and FSQ-130)



#33 Jon Isaacs

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Posted 30 June 2015 - 10:56 AM

The problem with FPL-53 is that it is a glass.  A glass is made by mixing together a whole bunch of different materials and melting them all together.  One melt is not exactly the same as the next.  For this reason the color correction on one telescope will not be the same as on the next 'supposedly identical' telescope.  Quality manufacturers will analyze the diffraction properties of each melt and slightly adjust their design to compensate for the difference so that the final product is as good as it can get.  High volume manufacturers do not have the time or motivation to do this and their final products will all be slightly different.  I have actually seen these differences in side-by-side tests of 'supposedly identical' telescopes, and some do indeed have better color control than the one next to it!

 

CaF2 is a crystal.  Every atom in its matrix is fixed.  Every piece has identical properties.  It is brittle.  It is difficult to work with.  It doesn't weather well.  But each and every piece of it is identical and the color control can be made to a very high standard.  But it's so #### expensive!  I wish I could afford a Takahashi CaF2 instrument but it would take a small miracle to get that purchase past my wife...

 

If one is spending enough to purchase an Astro-Physics refractor, then Roland's attention to detail is legendary and does include measuring the optical properties of each melt and adjusting the curves of the lens accordingly.   But for the scopes I am buying, I suspect the variability is not in the design and the exact mathing of the materials but rather in the execution of the design...

 

And I am OK with that because I do not look to refractors to provide the most resolution, the most perfect color correction, the greatest light gathering..  

 

Jon


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#34 Jeff B

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Posted 30 June 2015 - 12:45 PM

Actually, I believe it's a mistake to assume all CaF2 is created equal.  It's not.

 

A CaF2 blank can have any number of defects including internal stress, and inclusions, to name a couple and others unique to CaF2 like multiple internal crystals, all reasons to reject a blank.

 

Just go to the TEC Yahoo group and look in the file called "glasses".  Very informative as Yuri usually is.

 

Some OEMs swear by it, others swear at it.  I have enjoyed the spirited debates that Yuri and Roland have engaged in concerning its use.

 

I also have to comment concerning the "laser" test.  I do see a difference in "scatter" in some of the comparison photos I've seen but I have to ask, does it really matter?  I mean a laser is just an incredibly intense light source.  What I see in the photos is a very mild side/back scatter that my gut tells me is vanishingly small compared the axial intensity of the beam.   So is there, in the real world, a  real practical advantage (like being actually visible in the eyepiece or even a CCD sensor) in "scatter" of CaF2 compared to optical glass?   

 

Jeff



#35 Peter Besenbruch

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Posted 30 June 2015 - 12:51 PM

And I am OK with that because I do not look to refractors to provide the most resolution, the most perfect color correction, the greatest light gathering..  

 

And that's a problem, because the difference between the high end an the cheap stuff is so small. ;)



#36 dan_h

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Posted 30 June 2015 - 01:02 PM

And I am OK with that because I do not look to refractors to provide the most resolution, the most perfect color correction, the greatest light gathering..

 
And that's a problem, because the difference between the high end an the cheap stuff is so small. ;)


I think you misunderstood the comment.

I too do not look to refractors to provide the most resolution. That needs aperture. And for rich wide field views at low power, resolution is not the point.

I don't look to refractors for the most perfect color correction. A mirror does it better every time.

I don't look to refractors for the greatest light gathering. That again, needs aperture.

Refractors have their place but I agree with Jon.

dan

#37 Peter Besenbruch

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Posted 30 June 2015 - 01:30 PM

I think you misunderstood the comment.


I too do not look to refractors to provide the most resolution. That needs aperture. And for rich wide field views at low power, resolution is not the point.

I don't look to refractors for the most perfect color correction. A mirror does it better every time.

I don't look to refractors for the greatest light gathering. That again, needs aperture.

Refractors have their place but I agree with Jon.

 

No, I think you misunderstood. Jon was speaking about not purchasing the most expensive refractors. The context was a discussion of adjusting each scope run to get the ultimate out of each glass melt. He wasn't looking for that in his refractors, though he didn't say why. The point I was making is that cheaper APOs are so close to their more expensive cousins that the large increase in price isn't worth it.

 

On the other hand, I agree with you about reflectors. If you want to see more, you have two options: A site with darker skies and steadier seeing, and a larger telescope. If you want to go above six inches, the price will tend to dictate using a reflector.



#38 Paul G

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Posted 30 June 2015 - 01:53 PM

Actually, I believe it's a mistake to assume all CaF2 is created equal.  It's not.

 

A CaF2 blank can have any number of defects including internal stress, and inclusions, to name a couple and others unique to CaF2 like multiple internal crystals, all reasons to reject a blank.

 

Just go to the TEC Yahoo group and look in the file called "glasses".  Very informative as Yuri usually is.

 

Some OEMs swear by it, others swear at it.  I have enjoyed the spirited debates that Yuri and Roland have engaged in concerning its use.

 

I also have to comment concerning the "laser" test.  I do see a difference in "scatter" in some of the comparison photos I've seen but I have to ask, does it really matter?  I mean a laser is just an incredibly intense light source.  What I see in the photos is a very mild side/back scatter that my gut tells me is vanishingly small compared the axial intensity of the beam.   So is there, in the real world, a  real practical advantage (like being actually visible in the eyepiece or even a CCD sensor) in "scatter" of CaF2 compared to optical glass?   

 

Jeff

 

Can't speak to photography but for visual I found absolutely no visible benefit of CaF2 in my FS128. My 130 EDF had a little better contrast than the FS. Using a simple ortho with a fluorite doublet scope the light is passing through five other pieces of glass in addition to the fluorite element, kind of dilutes any effect.

 

The real benefit of CaF2 is its transparency deep into the UV, this is why they use it to "print" computer chip components smaller than the wavelength of green light. However, add one piece of glass to the image chain and it becomes completely opaque in the deep UV.


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#39 peleuba

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Posted 30 June 2015 - 02:27 PM

 

 

And that's a problem, because the difference between the high end an the cheap stuff is so small. ;)

 

 

It certainly can be; you generally pay dearly for the last 20% of performance at the eyepiece or the sensor.  But you would be surprised how good some inexpensive imported telescopes are and how poor some high-end samples can be.



#40 Peter Besenbruch

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Posted 30 June 2015 - 04:17 PM

 

And that's a problem, because the difference between the high end an the cheap stuff is so small. ;)

 

It certainly can be; you generally pay dearly for the last 20% of performance at the eyepiece or the sensor.  But you would be surprised how good some inexpensive imported telescopes are and how poor some high-end samples can be.

 

I basically agree, except I'm going to say you pay dearly for that last 5-10%. ;)



#41 Boki

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Posted 30 June 2015 - 04:19 PM

How delicate is a fluorite lens in a doublet refractor from average user's point of view? I read statements that flourite is fragile compared to FPL53, difficult to work with, but as an average user, is there a chance to damage fluorite lens with normal use? Mounting OTA, removing OTA, transporting it on short distances? And if the flourite element is in front of the objective is there special care needed to clean the objective and protect it from contaminants - dust, water, etc?

I would love to get a fluorite doublet in near future, but i don't want to end up with a cracked objective, or one dissolved by water ;-)

 

CS, Boki



#42 Element79

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Posted 30 June 2015 - 04:34 PM

The matching crown element is ALWAYS put in front of the CaF2 element, but it is still somewhat subject to moister in the air.  By the time it gets to the consumer all the risks of it being fragile and hard to work with are essentially removed.  But you will always need to make sure there are no rapid temperature changes when you take your telescope outside into the elements.  A CaF2 telescope makes a WONDERFUL instrument, but my problem with it is that it is so expensive!  All I care about is great visual performance and a FPL-53 doublet or a FPL-51 triplet will get me to where I want to go...



#43 GShaffer

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Posted 30 June 2015 - 04:39 PM

The matching crown element is ALWAYS put in front of the CaF2 element, but it is still somewhat subject to moister in the air.  By the time it gets to the consumer all the risks of it being fragile and hard to work with are essentially removed.  But you will always need to make sure there are no rapid temperature changes when you take your telescope outside into the elements.  A CaF2 telescope makes a WONDERFUL instrument, but my problem with it is that it is so expensive!  All I care about is great visual performance and a FPL-53 doublet or a FPL-51 triplet will get me to where I want to go...


Not ALWAYS.....as I recall there is a very popular TAK or Vixen where it is in the front, I forget which it is and I have the Vixen..lol
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#44 Erik Bakker

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Posted 30 June 2015 - 05:43 PM

The Tak FS and FC-L series have the fluorite as their front element. The FC series and Vixen FL series have the fluorite as their back (focuser) side element. I've had many samples of both over the decades and found them both to be very stable and maintenance free. My oil-spaced triplet on the other hand needed service because of oil-leakage. In my experience, fluorite doublets are more maintenance free. Compared to my old f/12-f/15 Fraunhofer doublets, the f/8-f/8.8 fluorite doublets put up a much better image while being similarly maintenance free.


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#45 LewisM

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Posted 30 June 2015 - 05:52 PM

Yup, asErik said, the Tak FS line and the FC-L's all have front elements of multi-coated CaF2. The FSQ106 and 106N also had front fluorite (I THINK! Not 100% on that).

 

The remainder of Tak and Vixen scopes have rear fluorite behind the crown.



#46 Paul G

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Posted 30 June 2015 - 07:10 PM

My understanding is that coating technology had an affect on location of the fluorite element. In the earlier scopes the fluorite was uncoated because the temps required for coating weren't very friendly to the crystalline element so they protected the uncoated fluorite with another piece of glass. When coating processes became available that were effective on fluorite it freed the designers to move it to the front.



#47 Carl N

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Posted 30 June 2015 - 07:14 PM

Weren't some Pentax scopes Flourite?

#48 BillP

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Posted 30 June 2015 - 09:29 PM

How delicate is a fluorite lens in a doublet refractor from average user's point of view?

 

 

It is so very fragile that Takahashi has managed to put it in production mass market consumer telescopes continuously for what, the past 43 years!  TEC also has used it with no consequence to the consumer.  It is fragile to work with for those that do not have the knowledge, skills, & expertise to work with it, and who want to instill fear in others as a counter-marketing tactic.  If you are worried about getting a fluorite telescope, you should also probably be concerned about getting an oil spaced one as well ... both very very bad :lol:


Edited by BillP, 30 June 2015 - 09:30 PM.

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#49 RichA

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Posted 01 July 2015 - 12:49 AM

 

How delicate is a fluorite lens in a doublet refractor from average user's point of view?

 

 

It is so very fragile that Takahashi has managed to put it in production mass market consumer telescopes continuously for what, the past 43 years!  TEC also has used it with no consequence to the consumer.  It is fragile to work with for those that do not have the knowledge, skills, & expertise to work with it, and who want to instill fear in others as a counter-marketing tactic.  If you are worried about getting a fluorite telescope, you should also probably be concerned about getting an oil spaced one as well ... both very very bad :lol:

 

That fluorite was fragile and thermally sensitive was spread by those using FPL glasses in their telescopes.  I've seen DOZENS of fluorite scopes have never seen a verifiable example of problems, even though some people treat their scopes like c---.  If either issue were an issue, we'd have examples.  I have seen issues (like a Questar 3.5 totally covered with mold) on some other scopes and eyepieces.  The most recent being a 22 Panoptic from Ebay that had mold on it.



#50 Jon Isaacs

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Posted 01 July 2015 - 05:55 AM

That fluorite was fragile and thermally sensitive was spread by those using FPL glasses in their telescopes 

 

 

As a materials person I am skeptical of your claim. I think the fact is that Fluorite is a relatively soft crystalline material that  cleaves whereas glass is harder and being amorphous does not cleave.. A few years ago my 80 mm took a header into the garage floor. The dew shield took the brunt of the hit and the scope survived.. I was glad the objective was FPL-53 rather than Fluorite..

 

Someone mentioned that with FPL-53 there could be small variations in the properties that, for best results, would require characterization of each melt, something Astro-Physics does. In the same way, I would think the mating elements would require characterization as they are also glasses and subject to small variations.

 

Jon


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