159 replies to this topic

[Alan French]

I think he did, or someone else thought he did.

http://www.csun.edu/...n/tmb/tmb1.html

The AP lenses with FPL53 did not use ZKN7. Because of issues with glass quality, AP had stopped using ZKN7 even before the adoption of FPL53.

 

The design of the FPL53 triplet used more reliable and easily obtained glasses, and provided better color correction that it would have had using ZKN7.

 

Clear skies, Alan

[gnowellsct]

The AP lenses with FPL53 did not use ZKN7. Because of issues with glass quality, AP had stopped using ZKN7 even before the adoption of FPL53.

 

The design of the FPL53 triplet used more reliable and easily obtained glasses, and provided better color correction that it would have had using ZKN7.

 

Clear skies, Alan

I don't know squat about it but in the link it says:  "The first EDT design used ZKN-7 crown glass and FPL-52 ED glass. The last runs of the smaller EDT models used the same crown glass but FPL-53 ED glass. They were even better corrected for monochromatic and polychromatic aberrations." 

 

So, if two conditions are met:

 

1.  The Back essay is accurate and

2.  The phrase "same crown glass" in the second sentence means ZKN-7, which syntactically appears to be the case, than FPL-53 for at least "the last runs of the smaller EDT models" mated FPL-53 with ZKN-7.  

 

I am simply reading the text here.  I wouldn't know ZKN from my buddy Zeke.

 

Greg N

 

p.s. actually I don't know anyone named Zeke.  

[Alan French]

It's simple. If Tom Back claimed AP used ZKN7 with FPL53, he was mistaken.

 

Clear skies, Alan

[Yuri]

It's simple. If Tom Back claimed AP used ZKN7 with FPL53, he was mistaken.

 

Clear skies, Alan

To make it even simpler - here is Roland's post related to the glass in question:

 

"In the past, one could get essentially perfect color correction in an
oiled lens using ZKN7 and FPL52 or FPL53 as the center element. ZKN7 is
not in good supply as is BK7, neither is it always delivered with good
internal homogeneity. Therefore to use this material is always a gamble,
and one can lose an entire production run because the glass is no good"

Edited by Yuri, 17 May 2018 - 06:42 PM.

[Suavi]

I honestly find such discussions quite amusing, although this is not to say that I am not learning from them.

 

I would leave designing and crafting of the lenses to master opticians. Each of them has own special techniques and preferences and I really trust master opticians who dedicated their lives to making telescopes way more than speculations and requests for an academic proof. In the end, ultimate test is at the eyepiece/camera. Academic discussions, while educational to the point, really easily lose focus and eventually lead nowhere meaningful.

[Alan French]

For an academic discussion, visit http://www.telescope-optics.net/

 

Also note Vladimir's references on his first page.

 

In print, I'd add "Telescopes, Eyepieces, Astrographs: Design, Analysis and Performance of Modern Astronomical Optics," by Smith, Ceragioli, and Berry.

 

Clear skies, Alan

[old_enough]

Common sense tells us that the actual design is more important than just a single element of it. I am sure there are a lot of apos without fluorite which will deliver equal or even superior quality than fluorite apos.

When I was looking for a (bino) telescope, I read loads of user reports. There was a consistent - if slight - bias towards instruments with fluorite lenses. Not only in the reports from owners but also from attendies of star parties who observed through a lot of telescopes in those nights.

When I recently compared my Borg 71FL binoscope with an APM 70mm ED Apo binocular I saw a bit more contrast and definition in my instrument when using the same low mag eyepieces. The difference became more obvious with higher magnifications. I can not say how much of this stems from lens material or from 2 lens vs. 3 lens objective or from erecting prism vs. erecting mirrors.

I guess because of the higher cost of fluorite it will be used in better instruments more often than in mediocre designs. So choosing fluorite might be an easy way to avoid disappointments.

Best,
Sebastian

[garret]

 

I can not say how much of this stems from lens material or from 2 lens vs. 3 lens objective or from erecting prism vs. erecting mirrors.

Erecting mirrors achieve better contrast then a complex erecting prism; the APM 45 degree BT uses 2 complex Schmidt and Rhomboid prism in each tube.

Erecting prism have long glass-path, this causes scatter.

Erecting mirrors have zero false pupils.

If the mirrors of a erecting mirror system have high reflective coating they absorb less light then a complex erecting prism.

Spherical and other aberrations are way better to control with a erecting mirror system.

....

....

 

Garrett

[Alan French]

Why do you think Canon uses so much of fluorite in their DSLR lenses for over 40 years?

Why not always ED glass?

 

Maybe because for specific optical properties, its BETTER then any other glass?

The reality is that the optical properties of fluorite and fluor-crowns are essentially identical.

 

You can see this in figure 148 at http://www.telescope...o_refractor.htm. Yes, the Pfe values are a bit different (as are FPL53 and FPL55), but that just means the matching glass choices might differ.

 

The often cited advantages, lower scatter, consistent indices, and so on, are essentially irrelevant because fluorite is used with normal glasses - lots of normal glass elements in the case of camera lenses.

 

There are fine examples of telescopes using fluorite and there are equally fine instruments using fluor-crowns.

 

Clear skies, Alan

[Sockrateez]

The reality is that the optical properties of fluorite and fluor-crowns are essentially identical.

 

You can see this in figure 148 at http://www.telescope...o_refractor.htm. Yes, the Pfe values are a bit different (as are FPL53 and FPL55), but that just means the matching glass choices might differ.

 

The often cited advantages, lower scatter, consistent indices, and so on, are essentially irrelevant because fluorite is used with normal glasses - lots of normal glass elements in the case of camera lenses.

 

There are fine examples of telescopes using fluorite and there are equally fine instruments using fluor-crowns.

 

Clear skies, Alan

So why do suppose they go through all the trouble of growing the crystal at all?  Why not just use glass?  Serious question, I do not know anything  about making optical glass/crystals, let alone combining them.

[Alan French]

So why do suppose they go through all the trouble of growing the crystal at all?  Why not just use glass?  Serious question, I do not know anything  about making optical glass/crystals, let alone combining them.

The unusual optical properties of fluorite (low dispersion yet well off the Abbe line) were recognized by Abbe around 1870. The development of artificial synthetic fluorite crystals seems to date back at least to the end of World War II.

 

See http://pubs.rsc.org/...df/df9490500294

 

Historically, calcium fluoride predated fluor-crown glasses with similar characteristics. Probably simply an established technology with many continued uses. I doubt the telescope market is large enough to drive the production of either fluorite or fluor-crowns.

 

Clear skies, Alan

[gnowellsct]

The unusual optical properties of fluorite (low dispersion yet well off the Abbe line) were recognized by Abbe around 1870. The development of artificial synthetic fluorite crystals seems to date back at least to the end of World War II.

 

See http://pubs.rsc.org/...df/df9490500294

 

Historically, calcium fluoride predated fluor-crown glasses with similar characteristics. Probably simply an established technology with many continued uses. I doubt the telescope market is large enough to drive the production of either fluorite or fluor-crowns.

 

Clear skies, Alan

But the *camera* market could....taken expansively to mean not just cameras but TV and movie type cameras as well, that's a pretty big industry.  Not as big as the automobile market or the oil industry but pretty big nonetheless.  I think the *astronomy* market in the eyes of the big lens makers is just a minor backwater in the photography market.  Greg N

[Sockrateez]

But there has to be a reason to go though the  trouble of creating the synthetic.  It must have some property, performance criteria, cost implications; something.

[MooEy]

Too many reasons I can think of, I’ll just list a bunch.

1. They already have existing production facilities, if they switch to ED glass, what is going to happen to all these plant, equipment and people?

2. They don’t want to rely on external suppliers, they want to maintain control over the production of the glass.

3. They don’t want to redesign all the lenses.

4. Their marketing dept has already convinced everyone that their fluorite is superior to ED, why would they change that?

[Sockrateez]

Too many reasons I can think of, I’ll just list a bunch.

1. They already have existing production facilities, if they switch to ED glass, what is going to happen to all these plant, equipment and people?

2. They don’t want to rely on external suppliers, they want to maintain control over the production of the glass.

3. They don’t want to redesign all the lenses.

4. Their marketing dept has already convinced everyone that their fluorite is superior to ED, why would they change that?

Your timing is backwards.  It presupposes the crystals existence.

[Alan French]

Your timing is backwards.  It presupposes the crystals existence.

Fluorite's unique optical properties and the restriction of quality naturally occurring fluorite to very small pieces lead to the development of the ability to grow large calcium fluoride crystals. Glasses with similar properties did not exist then, so fluorite predates fluor-crown glasses. When fluor-crown glasses became available. the technology needed to produce quality fluorite already existed and was in use.

 

Clear skies, Alan

[Suavi]

Canon 100-400mm  zoom lens type II, Strehl 89 to 90% : http://interferometr...sm-ii-tele.html

 

Not bad for zoom lens with anti vibration control and 21 elements including one made of single crystalline fluorite because of its unique properties...

1 fluorite element in between 20 'just glass' elements - Im sure such nice Strehl is coming purely from one fluorite element. Just imagine how the overall performance of such complex lens would have suffered if that fluorite was replaced with a quality FPL glass 

[Jon Isaacs]

But there has to be a reason to go though the  trouble of creating the synthetic.  It must have some property, performance criteria, cost implications; something.

 

A few I can think of:  Easier to work, more durable, coating ED glasses was easier.  I believe that older Fluorite lenses are uncoated.

 

Jon

[Sockrateez]

Fluorite's unique optical properties and the restriction of quality naturally occurring fluorite to very small pieces lead to the development of the ability to grow large calcium fluoride crystals. Glasses with similar properties did not exist then, so fluorite predates fluor-crown glasses. When fluor-crown glasses became available. the technology needed to produce quality fluorite already existed and was in use.

 

Clear skies

 

Fluorite's unique optical properties and the restriction of quality naturally occurring fluorite to very small pieces lead to the development of the ability to grow large calcium fluoride crystals. Glasses with similar properties did not exist then, so fluorite predates fluor-crown glasses. When fluor-crown glasses became available. the technology needed to produce quality fluorite already existed and was in use.

 

Clear skies, Alan

 

Predating ED makes sense.  Appreciated.

 

Wonder why they keep using it.  Seems like it is more difficult to work with from what I have read.

 

In any case, I do appreciate the feedback.

[Sockrateez]

A few I can think of:  Easier to work, more durable, coating synthetic was easier.  I believe that older Fluorite lenses are uncoated.

 

Jon

Hey Jon,

 

Just saw your post.  I thought the opposite was suppose to be true: harder to work with, it was fragile, and harder to coat (which was why it was not originally coated).  

 

Thank you for the response.

[MooEy]

The word synthetic is confusing, it can either mean synthetic fluorite or synthetic fluoro-crown glass.

I believe Calcium Fluorite was discovered more than 100 years ago, but only small pieces were ever being used for microscopes. Short flint has been used for over hundred years. The ED glass and larger fluorite only came somewhere in the 80s.

Then again, I could be completely wrong.

Edited by MooEy, 19 May 2018 - 04:55 PM.

[Jon Isaacs]

Hey Jon,

 

Just saw your post.  I thought the opposite was suppose to be true: harder to work with, it was fragile, and harder to coat (which was why it was not originally coated).  

 

Thank you for the response.

 

The word synthetic is confusing, it can either mean synthetic fluorite or synthetic fluoro-crown glass.

Good catch, thanks

 

 I should not have used the word synthetic, ED glasses like FPL-53 iare easier to work, more durable, less trouble to coat.  

 

I edited my post.

 

Jon

[Alan French]

I suspect other markets drive the production of fluorite, and that telescope objectives are a very minor niche with little impact.

 

See the Wiki article on fluorite under the "Optics" header for a bit more.

 

Clear skies, Alan

[Jon Isaacs]

I suspect other markets drive the production of fluorite, and that telescope objectives are a very minor niche with little impact.

 

See the Wiki article on fluorite under the "Optics" header for a bit more.

 

Clear skies, Alan

 

I think Telescope objectives play essentially no role in determining the market for ED glasses and Fluorite.  My thinking is that ED glasses are driven by the camera lens market.  Those big Canon lenses that are at every televised sporting event, they must contain one of more ED elements.  Quality longer focal length camera lenses use ED or Fluorite.  Those are huge markets and we are just scavengers, harvesting what is developed for other markets.

 

Jon

[Sockrateez]

So why do fluorite crystal lenses continue to exist and be used? 

 

It appears to be older technology that is more difficult to work with?

 

What does it provide that newer easier to work with ED glasses do not?  If the answer is nothing, than why is it still around?

Edited by JCAZ, 19 May 2018 - 05:16 PM.