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Fluorit Apos

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#1 Lucullus

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Posted 20 October 2019 - 04:26 PM

When reading descriptions of Fluorit Apos I often read that there is one Fluorit lens element in the doublet or triplet in question. Are there Fluorit doublet Apos with two Fluorit lens elements on the market? Or Fluorit triplet Apos with three Fluorit lenses?



#2 Sky Muse

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Posted 20 October 2019 - 04:33 PM

As far as I know, you only get one fluorite element, whether within a doublet or triplet.  The others will be lesser, and of glass.  Fluorite is not a glass, by the way.  It's crystalline, a crystal.  The fluorite element of my doublet is at the front, and as the "crown"...

 

fluorite doublet2.jpg


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#3 Justin Fuller

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Posted 20 October 2019 - 04:53 PM

No, a fluorite element has to be paired with other lenses of a different index to make a semi-apochromatic or apochromatic telescope. Fluorite isn't THE best lens on it's own, it always needs to be paired with other lens types to make an apochromatic design that also has good correction for other optical aberrations as well as chromatic aberration. I hope this helps.

Edited by Justin Fuller, 20 October 2019 - 05:00 PM.

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#4 Spikey131

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Posted 20 October 2019 - 06:54 PM

I have not heard of a scope to two fluorite elements.

 

The Takahashi TOA line are triplets with two ED elements, but not fluorite.

 

I think none of the Canon lenses have more than one fluorite element.


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#5 Eric H

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Posted 20 October 2019 - 08:50 PM

The Original Tak FSQ-106 was a quadruplet with 2x fluorite lenses, 1 in the front group, the other in the rear group.


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#6 Raginar

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Posted 21 October 2019 - 01:01 AM

What Eric said. My 106n has two fluorite lenses.
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#7 Alan French

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Posted 21 October 2019 - 07:38 AM

The Original Tak FSQ-106 was a quadruplet with 2x fluorite lenses, 1 in the front group, the other in the rear group.

A different approach than the standard doublet or triplet, a Petzval with two widely spaced doublets. 

 

See the discussion part way down this:  https://www.telescop....htm#aberration

 

Clear skies, Alan



#8 Spikey131

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Posted 21 October 2019 - 09:25 AM

Yes, I was thinking of a single group.

 

The older TV Petzvals at one time had fluorite in the rear group only, with ED in the front.

 

The Canon telephoto lenses combine ED elements with fluorite, but never more than one.

 

Interesting that Takahashi uses it in both groups.



#9 25585

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Posted 21 October 2019 - 09:36 AM

Mating glass is important. Takahashi say their new FC100DZ has better mating glass than their DC/F/L models, which DZ is superceding.


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#10 macdonjh

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Posted 21 October 2019 - 10:52 AM

 of a different index to make a semi-apochromatic or apochromatic telescope

There's the key.  Getting lots of wave lengths to focus at a point requires more than one refractive index.  

 

It would be fun to see a ray tracing of a doublet or triplet where all elements have the same refractive index.  


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#11 Bomber Bob

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Posted 21 October 2019 - 11:00 AM

Mating glass is important. Takahashi say their new FC100DZ has better mating glass than their DC/F/L models, which DZ is superceding.

It does seem to me that the Big Strides now are in the mating elements -- maybe that's a result of so many glass types available...


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#12 Alan French

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Posted 21 October 2019 - 11:10 AM

It does seem to me that the Big Strides now are in the mating elements -- maybe that's a result of so many glass types available...

 

The difficulty seems to be that many glass types are in catalogs, but not so many are available in the size, quality, and quantity required. 

 

Clear skies, Alan


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#13 Astrojensen

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Posted 21 October 2019 - 11:42 AM

The difficulty seems to be that many glass types are in catalogs, but not so many are available in the size, quality, and quantity required. 

 

Clear skies, Alan

And many that are, are much too expensive. 

 

We amateur astronomers are a difficult lot. We want perfection, but cringe at paying for it (I am no different myself). 

 

 

Clear skies!
Thomas, Denmark


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#14 Sky Muse

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Posted 21 October 2019 - 07:10 PM

No, a fluorite element has to be paired with other lenses of a different index to make a semi-apochromatic or apochromatic telescope. Fluorite isn't THE best lens on it's own, it always needs to be paired with other lens types to make an apochromatic design that also has good correction for other optical aberrations as well as chromatic aberration. I hope this helps.

No one's talking about a standalone element.  You have to have at least a doublet for an apochromatic system.  I'll take the fluorite element to mate with whatever, rather than a glass element, as I think the fluorite's best.



#15 Alan French

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Posted 21 October 2019 - 09:01 PM

No one's talking about a standalone element.  You have to have at least a doublet for an apochromatic system.  I'll take the fluorite element to mate with whatever, rather than a glass element, as I think the fluorite's best.

There are some design examples using CaF2 and fluor-crowns at  https://www.telescop...po_examples.htm

 

Clear skies, Alan


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#16 Sky Muse

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Posted 21 October 2019 - 09:26 PM

There are some design examples using CaF2 and fluor-crowns at  https://www.telescop...po_examples.htm

 

Clear skies, Alan

I don't see a 102mm f/8 fluorite-doublet in that.

 

Incidentally, which one do you think is the best of the lot, of the doublets only?



#17 n2068dd

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Posted 22 October 2019 - 08:34 AM

A optical design method is daily upgrading. Recently, some fluorite doublet system is nearly equal to the vintage fluorite triplets.

For example,  Takahashi FOA-60 or FC100-DZ. Though, any ED doublet can't match with that level of correction.

Mr.Yuyama Takahashi optic designer who designed all of Takahashi refractors say, 'It still have the slight advantages on fluorite. It's certain.' Takahashi never disclose the mated element on FCZ or FOA. he say ' it's not used before on doublet optic.' indeed, mated lens is the KEY. and he can controls the high order spherical curve on doublets. It's the another KEY which any other company can't catch up with. CFF or Agama maybe the follower, though very less production like rare treasures.

 

Regards

Hiromu


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#18 Alan French

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Posted 22 October 2019 - 01:32 PM

I don't see a 102mm f/8 fluorite-doublet in that.

 

Incidentally, which one do you think is the best of the lot, of the doublets only?

My point was that both CaF2 and fluor-crowns can be used to create good designs. 

 

I know what companies I'd look at, but have certainly not seen enough doublets under the night sky to have an informed opinion.

 

Clear skies, Alan



#19 Alan French

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Posted 22 October 2019 - 01:34 PM

A optical design method is daily upgrading. Recently, some fluorite doublet system is nearly equal to the vintage fluorite triplets.

For example,  Takahashi FOA-60 or FC100-DZ. Though, any ED doublet can't match with that level of correction.

Mr.Yuyama Takahashi optic designer who designed all of Takahashi refractors say, 'It still have the slight advantages on fluorite. It's certain.' Takahashi never disclose the mated element on FCZ or FOA. he say ' it's not used before on doublet optic.' indeed, mated lens is the KEY. and he can controls the high order spherical curve on doublets. It's the another KEY which any other company can't catch up with. CFF or Agama maybe the follower, though very less production like rare treasures.

 

Regards

Hiromu

 

I think most experienced designers have a good idea what everyone else is doing. 

 

Clear skies, Alan


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#20 Sky Muse

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Posted 22 October 2019 - 03:36 PM

My point was that both CaF2 and fluor-crowns can be used to create good designs. 

 

I know what companies I'd look at, but have certainly not seen enough doublets under the night sky to have an informed opinion.

 

Clear skies, Alan

In any event, scant chromatic-aberration would be the least of my worries.  I would be far more concerned as to light-scattering, and the degree of polish.



#21 Justin Fuller

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Posted 22 October 2019 - 03:52 PM

No one's talking about a standalone element. You have to have at least a doublet for an apochromatic system. I'll take the fluorite element to mate with whatever, rather than a glass element, as I think the fluorite's best.


I wasn't talking about a standalone element either. From what I gathered, the OP was asking why there aren't doublets with two fluorite elements or triplets with three fluorite elements, since fluorite is so highly prized. Lenses have to complement each other was my point.
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#22 Alan French

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Posted 22 October 2019 - 04:09 PM

In any event, scant chromatic-aberration would be the least of my worries.  I would be far more concerned as to light-scattering, and the degree of polish.

I guess we must be considering different manufacturers. Neither concern makes a niggle on my radar.

 

Clear skies, Alan 



#23 gnowellsct

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Posted 22 October 2019 - 04:15 PM

A optical design method is daily upgrading. Recently, some fluorite doublet system is nearly equal to the vintage fluorite triplets.

For example,  Takahashi FOA-60 or FC100-DZ. Though, any ED doublet can't match with that level of correction.

Mr.Yuyama Takahashi optic designer who designed all of Takahashi refractors say, 'It still have the slight advantages on fluorite. It's certain.' Takahashi never disclose the mated element on FCZ or FOA. he say ' it's not used before on doublet optic.' indeed, mated lens is the KEY. and he can controls the high order spherical curve on doublets. It's the another KEY which any other company can't catch up with. CFF or Agama maybe the follower, though very less production like rare treasures.

 

Regards

Hiromu

CFF makes triplets as far as I know.  At least my CFF is a triplet.  I don't remember seeing doublets on their web site.  But Agema is deep into doublets.   I don't know how many scopes they are turning out.  CFF is turning out enough that there are people here and there who have them.  

 

Greg N



#24 gnowellsct

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Posted 22 October 2019 - 04:16 PM

In any event, scant chromatic-aberration would be the least of my worries.  I would be far more concerned as to light-scattering, and the degree of polish.

CFF has, of course, a high degree of polish.  :)



#25 Sky Muse

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Posted 22 October 2019 - 04:28 PM

CFF has, of course, a high degree of polish.  smile.gif

Yes, I've heard that.  Do they make doublets?

 

I do hope that there will come a time when triplets and quadruplets go the way of the Dodo.  I would like to see affordable doublets exhibiting no aberrations whatsoever, even per an imager's requirements.  It's been a long time since the achromat was developed, nigh 300 years; yes, our 60mm f/11 spyglasses are that ancient.  Likewise, primaries of Newtonians with a figure, a curve, exotic enough to eliminate all aberrations associated therewith, and to enable 8" f/2 instruments with secondaries the size of a quarter, or smaller even; as well.

 

Yes, pipe-dreams, I know.

 

I guess we must be considering different manufacturers. Neither concern makes a niggle on my radar.

 

Clear skies, Alan 

Yes, I already knew.  Can't have bloated stars, et al.


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