160 replies to this topic

[gnowellsct]

 

 

 

Whatever all that really means doesn't concern me too much. All I know is that my FC100DC knocked the socks of my excellent SW 120 ED during side by side comparisons while observing Jupiter. Even DSO's are remarkably bright in the smaller fluorite.

That may say something about the compromises made in that particular line to keep the cost down, not the characteristics of the glass itself.  In any case there is more than one type of "ED".   I would also bet, if the 100 mm was doing *so* much better, that the SW probably had/has an issue with its lens cell and collimation.   Mechanical issues have optical effects.  

 

 

GN

[Alan French]

Takahashi state regarding Calcium Fluorite (CAF2) in their instruction manual for their FC100D  series, on page 20; Its very low refractive index makes it the best of materials to use in the manufacture of apochromatic telescopes........ 

 

Now, calcium fluorite crystal can be hard multi-coated for maximum light transmission.....

 

The band pass of fluorite of 1000 to over 100,000 Angstroms eclipses by many magnitudes that of any optical glass. Additionally, the use of multi-coatings further increases light transmission over any ED glass. These features make the fluorite objective the premier photo/visual instruments for deep sky or lunar and planetary applications in their size class.

 

 

Whatever all that really means doesn't concern me too much. All I know is that my FC100DC knocked the socks of my excellent SW 120 ED during side by side comparisons while observing Jupiter. Even DSO's are remarkably bright in the smaller fluorite.

Yes, it's called marketing. Read what Canon says about their L-series lenses sometime.

 

There are extra-low dispersion (ED) glasses very similar to calcium fluoride, which has a Vd of 94.9. Ohara FPL53 with a Vd of 95.0, the newer FPL55, apparently replacing FPL53, with a Vd of 94.8, and Hoya FCD100 coming in at 95.1. Tak is also not the only company using CaF2.

 

Clear skies, Alan

[Cpk133]

CFF is already putting out FPL55.  I appear to be in line for the fall, don't know whether it will be FPL 53 or FPL 55.  

 

I will say that the Dragonfly array has caught me up short on this light scatter thing.  I always thought it was nonsense.  But these guys are using the principle of ultra low (fluorite technology) Canon lenses to find dim extended galaxies that scopes like Keck can't touch.  GN

The nanostructure coatings that cannon developed is what gives their lens the big advantage.  Of course, the lower scatter of fluorite and refractive optics in general play a role as well.  I posted this YouTube vid on another thread somewhere. Around 13 min, the presentation talks about the optics.  https://m.youtube.co...h?v=EUr1mxanYmc

[gnowellsct]

The nanostructure coatings that cannon developed is what gives their lens the big advantage. Of course, the lower scatter of fluorite and refractive optics in general play a role as well. I posted this YouTube vid on another thread somewhere. Around 13 min, the presentation talks about the optics. https://m.youtube.co...h?v=EUr1mxanYmc

Great video. I saw this guy at NEAF but this is a better presentation.

I wonder if these coatings will make their way into amateur class apos at an affordable price and render all our TECS and APs and CFF obsolete. Likely wouldn't matter for visual though.

[gnowellsct]

> But it still doesn't change the interesting item that these Canons have crazy low Strehls like .4 or something like that. I don't know how they can get a good picture of a bird

Photographic lenses are constructed in a very different way, with different goals. A telescopic lens should be diffraction limited on axis, but not in the field. Field curvature ans astigmatism grow fast outside the axis. Photographic lenses are constructed to give a decent image in a relative wide field, and being wide opened (f/ around 2.8) at the samte time.

Normal photographers don't image point sources as we do. Optical errors are far less remarkably at colored areas.

Well thank you. I'm not sure that helps completely but it is something. These are wide field images these guys are taking and they're getting star points right to the edge. But I guess it's very low magnification and ultra fast FR so that must help. A fast achromat is at its best at your lowest power.

[AllanDystrup]

The nanostructure coatings that cannon developed is what gives their lens the big advantage.  Of course, the lower scatter of fluorite and refractive optics in general play a role as well.  I posted this YouTube vid on another thread somewhere. Around 13 min, the presentation talks about the optics.  https://m.youtube.co...h?v=EUr1mxanYmc

Wow, — ground breaking, revolutionary, creative, original... great potential!!

Looking forward to Dragonfly v.3.

 

Thanks for sharing!

 

Allan

[emilslomi]

You are right on the money there, so to speak.  But it still doesn't change the interesting item that these Canons have crazy low Strehls like .4 or something like that.  I don't know how they can get a good picture of a bird or a sports event with that kind of Strehl and was hoping someone would take the opportunity to explain it to me,  so I can say "I read about that once but didn't really understand."

Airy disk at f2.8 is way smaller than the pixel size of premium chips. One would have to look at the scatter of the principal rays around the airy disk, but unless that is abysmal, one would be hard pressed to see a difference in sharpness in prime focus unless one zooms in and compares neighbouring pixels from a high and low Strehl lens. Visually, it is a different matter when the image is magnified umphteen times by the eyepiece.

Cheers, Emil

[Paul G]

You are right on the money there, so to speak.  But it still doesn't change the interesting item that these Canons have crazy low Strehls like .4 or something like that.  I don't know how they can get a good picture of a bird or a sports event with that kind of Strehl and was hoping someone would take the opportunity to explain it to me,  so I can say "I read about that once but didn't really understand."  

Others have responded as to why. I can add that I have used a Canon 400 f2.8L IS and it is a big, beautiful lens that gives gorgeous images. I'd love to own one, anyone wants to give me a birthday present that lens would be at the top of my list, bar none.

[Edward Try]

FPL-53 and FPL-55 have very similar optical properties and will not affect the design drastically. But the thermal parameters are different. Thermal expansion for FPL-55 is less then FPL-53. It makes the polishing process to be more manageable. Also, viscosity of FPL-53 depends on the changing temperature. Its thermal classification is "short" glass. The thermal classification for FPL-55 is "long" glass. The viscosity dependency from the changing temperature is much less. This allows to produce FPL-55 in bigger blanks with limited presses at molding. For FPL-53 molding process is more complicated and limited by time. To make it simple I would compare both glasses to the glue... Fast drying "glue" is FPL-53, you have to work fast and have no room for mistakes otherwise it will adhere incorrectly. You have to work fast to mold, otherwise the blank will be rejected. Longer drying "glue" is FPL-55, it is more forgiving -you have more time to make it correct.   You have more time for molding and make it accurate and precise. Optical properties of Fluorite are still better than both glasses. 

Edited by Edward Try, 13 May 2018 - 04:46 PM.

[gnowellsct]

Others have responded as to why. I can add that I have used a Canon 400 f2.8L IS and it is a big, beautiful lens that gives gorgeous images. I'd love to own one, anyone wants to give me a birthday present that lens would be at the top of my list, bar none.

No one doubts that they are big beautiful lenses that give gorgeous images.  The question is how they do it with baboon butt ugly Strehls.  -G.

[Alan French]

No one doubts that they are big beautiful lenses that give gorgeous images.  The question is how they do it with baboon butt ugly Strehls.  -G.

I think it's simple. They are not using magnifications that test the limits of their aperture. They're also acquiring data they can process.

 

Scout around for information on our eye's Strehl, especially with a large pupil. Also check out visual acuity of the dark adapted eye.

 

Clear skies (some night), Alan

[gnowellsct]

I think it's simple. They are not using magnifications that test the limits of their aperture. They're also acquiring data they can process.

Scout around for information on our eye's Strehl, especially with a large pupil. Also check out visual acuity of the dark adapted eye.

Clear skies (some night), Alan

I can already tell you my eyes' strehls are the pits. In fact these apo glasses we're debating, in the hands of the master craftsmen that make the scopes, are probably orders of magnitude superior to the jellyball eyes we use to look through them.

[LMO]

Two comments:

 

1. An older thread on iceinspace.com.au covered essentially the same topic as posed for this thread -
  Calcium fluoride vs Fluorite vs Fluoro crown
  <http://www.iceinspac...d.php?t=121155>

 

In the iceinspace.com thread, posts by Bratislav provided technical background for a comparison of FPL-53 and CaF2 in refractors, including, in part, the same telescope-optics.net page cited by Alan French in his post starting the present thread.

 

Bratislav began his outline with a simple statement:

 

"Actually, there is no inherent advantage of CaF2 over say FPL53 in astronomical instruments. Fluorite's main advantage (transparency deep into UV spectrum) is completely negated by a practical mating element which is from 'main sequence' and as such will be completely opaque in UV."

 

He then provided the tech details supporting that statement in the same and successive posts.

 

2. Given Roland's choice of FCD100 in the new Stowaway 92, replacing the FPL-53 used in other recent Astro-Physics triplets, vs. Yuri's choice of fluorite in place of FPL-53 in the updated TEC 140, I am taking the liberty of trying to extend this discussion to FCD100 -- 'Comparing FPL-53, FCD100, and CaF2'

 

Is there yet data for FCD100 triplets with 'ideal' mating glass(es) like the data on the Vladimir Sacek telescope-optics.net page cited by Alan French in his original post?  What exactly is the significance of the Astro-Physics claim that the new Stowaway 92 provides 'unprecedented color correction'?  In 'Color correction' specifications provided on A-P's Web sites describing the Stowaway 92 and the GTX 130, the color spectrum spanned by the 0.005% focus-variation criterion is actually somewhat narrower for the Stowaway than for the GTX.

 

    Larry

[Yuri]

Two comments:

 

In the iceinspace.com thread, posts by Bratislav provided technical background for a comparison of FPL-53 and CaF2 in refractors, including, in part, the same telescope-optics.net page cited by Alan French in his post starting the present thread.

 

Bratislav began his outline with a simple statement:

 

"Actually, there is no inherent advantage of CaF2 over say FPL53 in astronomical instruments. Fluorite's main advantage (transparency deep into UV spectrum) is completely negated by a practical mating element which is from 'main sequence' and as such will be completely opaque in UV."

 

I hope Bratislav is not matting FPL-53 with ZKN7...

[bratislav]

Hi Yuri,

 

if you don't like ZKN7 so much, feel free to try KzFSN2 or LaKN14/LaKL21 as a mating element.

 

But I'll remind you about that little known company from Illinois which apparently (very successfully) sold a truckload of APOs based on FPL53/ZKN7 combo  so not all examples of ZKN7 blanks are rotten...

 

Cheers

 

PS I can show you bad examples of BK7 too. No glass is immune (admittedly it is far more likely with less common types)

Edited by bratislav, 16 May 2018 - 02:23 AM.

[bratislav]

I will only add that these discussions, although they crop up quite often, are in my opinion completely useless. 

Theory is one thing, practice completely another. As Yuri has shown, you can end up with a completely useless piece of glass (and I'm sure Roland can add quite a few more, not just of mating glass but main, expensive Fluorocrown too!).

Individual melts differ too (and that is another crucial advantage of CaF2 from manufacturing point of view; Fluorite being a crystal is always the same, so tooling can be reused). 

 

On the other hand, varying melts can be good too (if you make one-off); particularly lucky melt allowed design of a doublet with a FOUR wavelength crossings!

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[bratislav]

Of course, advantages are mostly academic, but bragging rights remain 

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[Yuri]

Hi Yuri,

 

if you don't like ZKN7 so much, feel free to try KzFSN2 or LaKN14/LaKL21 as a mating element.

 

But I'll remind you about that little known company from Illinois which apparently (very successfully) sold a truckload of APOs based on FPL53/ZKN7 combo  so not all examples of ZKN7 blanks are rotten...

 

Cheers

 

PS I can show you bad examples of BK7 too. No glass is immune (admittedly it is far more likely with less common types)

Bratislav, we will never know full story about "truckload of APOs" in terms how many tons of rejected glass left on shelves. How many times the money were borrowed from bank to survive, etc.
That glass has only three grades: 1 so-so, 2 bad, 3 terrible. We have never had  problem with it as being lucky to receive a sample of 3rd category in very beginning of glass choice for APOs.

ps. thanks for other mating glasses for FPL53, they are not on my list.

[Yuri]

On the other hand, varying melts can be good too (if you make one-off); particularly lucky melt allowed design of a doublet with a FOUR wavelength crossings!

- agree here, I am always "playing" with varying melts to get best from design.

BTW. two of your crossings are not in human eye perception range, and not in diffraction range, too - could be an "academic" sample

Edited by Yuri, 16 May 2018 - 08:50 AM.

[Alan French]

Hi Yuri,

 

if you don't like ZKN7 so much, feel free to try KzFSN2 or LaKN14/LaKL21 as a mating element.

 

But I'll remind you about that little known company from Illinois which apparently (very successfully) sold a truckload of APOs based on FPL53/ZKN7 combo  so not all examples of ZKN7 blanks are rotten...

 

Cheers

 

PS I can show you bad examples of BK7 too. No glass is immune (admittedly it is far more likely with less common types)

Glass designations are a bit puzzling.

 

Alternate matches are shown on Vladimir Sacek's diagram 148, including the above suggestion of KzFSN2. There is also a N-KzFS2, close by on the diagram. The latter is in the current Scott catalog, while KzFSN2 is not. The Scott catalog also lacks LaKN14 and LaKL21, but shows LaK14 and LaK21. Are they different glasses (perhaps slightly different) or different designations for the same glass.

 

Of course there are a variety of other glass manufacturers and cross references between different designations.

 

Clear skies, Alan

[Alan French]

Hi Yuri,

 

if you don't like ZKN7 so much, feel free to try KzFSN2 or LaKN14/LaKL21 as a mating element.

 

But I'll remind you about that little known company from Illinois which apparently (very successfully) sold a truckload of APOs based on FPL53/ZKN7 combo  so not all examples of ZKN7 blanks are rotten...

 

Cheers

 

PS I can show you bad examples of BK7 too. No glass is immune (admittedly it is far more likely with less common types)

AP has never made any telescopes using FPL53 with ZKN7.

 

Clear skies, Alan

Edited by Alan French, 16 May 2018 - 02:23 PM.

[MooEy]

AP has never made any telescopes using FPL53 with ZKN7.

Clear skies, Alan

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

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

[MooEy]

And why are we debating that zkn7 can’t be used? At least 1 European scope maker is using it and I bet the Chinese are just laughing as they are reading this. There are at least 3-4 different triplet apos made by the Chinese that is quite likely to use this glass.

[jay.i]

And why are we debating that zkn7 can’t be used? At least 1 European scope maker is using it and I bet the Chinese are just laughing as they are reading this. There are at least 3-4 different triplet apos made by the Chinese that is quite likely to use this glass.

It is apparently very difficult to get a decent batch of it, and even then, that's only so-so. Yuri from TEC provided a rating scale just a few posts up...

[gnowellsct]

And why are we debating that zkn7 can’t be used? At least 1 European scope maker is using it and I bet the Chinese are just laughing as they are reading this. There are at least 3-4 different triplet apos made by the Chinese that is quite likely to use this glass.

That the China firms are using zkn7 (if true, I have no way of knowing) is not exactly a ringing endorsement.  In fact it could be viewed the other way. GN