160 replies to this topic

[gnowellsct]

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

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

[gnowellsct]

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.

Judging from Alan's Post #13 even if your eyes *are* sensitive to violet and deep red it's going to be a minuscule element of the final image put together by your brain.  Cameras though, I'm sure can use filters and such to concentrate on one side of the spectrum or another, and that could lead to different imaging results.

[jay.i]

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

Catalin's post in the vendor forums says it'll be FPL-55. Interestingly enough, it's apparently more expensive than the past model with FPL-53, despite FPL-55 being "not as good" as FPL-53, and reportedly cheaper, which is one of the reasons some manufacturers are moving to FPL-55. At least, I thought that was the case. I don't know if FPL-53 prices have just gone up that much and 55 is still cheaper.

[Jeff B]

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.

Interesting though Uwe:  Give up a degree of freedom to make things work in the real world.  An interesting metaphor.  

[Fomalhaut]

Interesting though Uwe:  Give up a degree of freedom to make things work in the real world.  An interesting metaphor.  

As far as I have caught things, Zeiss oiled their APQs (in the manner earlier suggested by Wolfgang Busch) because they considered this easier to accomplish than to have to finepolish Fluorite lenses (which art was not really mastered by them).

And OTOH, Takahashi made air-spaced doublets and triplets because they, with the little help of their friends at Canon-Optron having been the first to introduce artificially grown Fluorite to astro-optics, had actually learnt to master this art, thus gaining one degree of freedom...

 

(By the way: My personal 25 years old FCT100/640 Fluorite triplet shows perfect smoothness of optics, be it scrutinized with a flashlight from the front or in the star-test from the other end...  

Edited by Fomalhaut, 11 May 2018 - 12:01 PM.

[peleuba]

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.

 

I am almost certain that TEC triplets are NOT aspherized.  I've owned a 160ED F/8, 160FL F/7 and a 110 F/5.6   All were spherical.  I am pretty sure the 140ED is spherical, too.  

 

The TEC110FL, really needed to be aspherized to realize optimal performance at such a fast focal ratio.  It was not a great high-power performer either on my bench or under the stars.  The 160's were very good however.

Edited by peleuba, 11 May 2018 - 12:39 PM.

[peleuba]

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.

 

I have seen TEC's Fluorite blanks being polished when Yuri gave me a tour.  And, it appeared to me that it was the exact same polishers being used - just much slower polishing action then you would think.  Perhaps different tooling/pitch/grit, but the same polisher just at a slower speed.  It reminded me of old world optical shop.  Very charming yet high tech at the same time.

[Paul G]

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

Those look like 400mm f/2.8L IS II USM lenses which have both fluorite and what Canon calls UD glass (probably FPL-53), so it takes advantage of both, 16 elements in 12 groups.
 
 

Judging from Alan's Post #13 even if your eyes *are* sensitive to violet and deep red it's going to be a minuscule element of the final image put together by your brain.  Cameras though, I'm sure can use filters and such to concentrate on one side of the spectrum or another, and that could lead to different imaging results.

Brownie points for spelling minuscule correctly.

[LMO]

At least the following three questions are raised by the interesting discussion in this thread, and in previous threads touching on the same issues:

 

1. Are there references comparing the shapes of the Strehl vs. wavelength plots for various designs such as those on the telescope-optics.net page cited by Alan?  Do some designs result in graphs with sharp, high Strehl peaks but rapid fall-off at either side, while other designs provide relatively flat, high plateaus, with fall-off mostly at (visible-) spectrum ends?

 

2. As implied in an early post by gnowellsct, why was FPL-53, rather than fluorite, used in the original design of the very highly regarded TEC 140 f7 triplet?   (Comments elsewhere by Yuri suggest that the change to fluorite was prompted largely by anticipated reduced availability of FPL-53 and fluorite's significant advantages over the suggested FPL-55 substitute.  Both Yuri and Roland Christen have also commented on the difficulties of obtaining satisfactory batches of FPL-53's 'ideal' mating glass ZKN7.)

 

3. Now we have Roland Christen's (Astro-Physics) announced move from FPL-53 to FCD100 in the new Stowaway 92 f6.65, "for the polychromatic Strehl ratio to be above 90% for the entire visual spectrum with a peak value of 99.8% at the visual peak... [and] unprecedented color correction."
<http://www.astro-phy...92f665/92f665>.
Are there published parameter targets for designs using FCD100 like the parameter lists for other glass choices on the telescope-optics.net page?

 

My thanks to Alan, Yuri, and 'vahe' for their informative contributions to the thread -

 

    Larry

[gnowellsct]

Those look like 400mm f/2.8L IS II USM lenses which have both fluorite and what Canon calls UD glass (probably FPL-53), so it takes advantage of both, 16 elements in 12 groups.
 
 

Brownie points for spelling minuscule correctly.

 

In another thread someone said these Canon lenses have crazy low Strehls.  But apparently it doesn't matter.

[peleuba]

2. As implied in an early post by gnowellsct, why was FPL-53, rather than fluorite, used in the original design of the very highly regarded TEC 140 f7 triplet?  

 

Probably cost (relative to Fluorite) and ease of production.

Edited by peleuba, 11 May 2018 - 01:19 PM.

[SchrödingersCat]

Catalin's post in the vendor forums says it'll be FPL-55. Interestingly enough, it's apparently more expensive than the past model with FPL-53, despite FPL-55 being "not as good" as FPL-53, and reportedly cheaper, which is one of the reasons some manufacturers are moving to FPL-55. At least, I thought that was the case. I don't know if FPL-53 prices have just gone up that much and 55 is still cheaper.

Catalin told me that he sometimes finds blanks at very low prices and instead pocketing the money he has passed the savings on to the customer.

[Alan French]

Ohara keeps a data sheet of melt frequency and price for its optical glass. 

 

Prices are all relative to S-BSL7. The current data sheet lists S-FPL53 as 18 times the price of BSL7 and S-FPL55 as 15.5 times the price of BSL7.

 

Clear skies, Alan

[Alan French]

Catalin's post in the vendor forums says it'll be FPL-55. Interestingly enough, it's apparently more expensive than the past model with FPL-53, despite FPL-55 being "not as good" as FPL-53, and reportedly cheaper, which is one of the reasons some manufacturers are moving to FPL-55. At least, I thought that was the case. I don't know if FPL-53 prices have just gone up that much and 55 is still cheaper.

I understand FPL53 will not continue to be available.

 

No one has provided any real evidence that FPL55 is "not as good" as FPL53.

 

If look at Vladimir's figure 148 at http://www.telescope...o_refractor.htm you'll see FPL55 not far from FPL53, and some matching glass candidates over on the right.

 

Clear skies, Alan

[jay.i]

I understand FPL53 will not continue to be available.

 

No one has provided any real evidence that FPL55 is "not as good" as FPL53.

 

If look at Vladimir's figure 148 at http://www.telescope...o_refractor.htm you'll see FPL55 not far from FPL53, and some matching glass candidates over on the right.

 

Clear skies, Alan

I put "not as good" in quotes because it's not necessarily true, but is portrayed that way. I was referencing the supposed Strehl ratios (posted earlier in this thread) for the TEC140 lenses based on FPL-53, FPL-55, and CaF2. I have no doubt it is still a great super ED glass.

[Element79]

_{"Supposed Strehl ratios"?  I didn't make it up...}

 

_{Here's a link to the info - scroll down the page:}

 

_{https://groups.io/g/...091,ct=1&jump=1}

[Alan French]

_{"Supposed Strehl ratios"?  I didn't make it up...}

 

_{Here's a link to the info - scroll down the page:}

 

_{https://groups.io/g/...091,ct=1&jump=1}

There's a very basic problem here. We only have numbers. Unlike the references I provided, we have almost no design data. We know one is a TEC design with fluorite and carefully selected crown elements. We know the others use FPL53 and FPL55, but we don't know if the designs simply use the elements that were selected for fluorite, or if proper mating glasses selected for FPL53 and FPL55.

 

Without specific design information - and it's understandable we won't get that for a manufacturers product - the numbers alone are simply not sufficient evidence for the claimed superiority of fluorite. From the relative partial dispersion diagrams in the reference, it's obvious there are a number of candidates to match any of three possible choices for an ED element. (And, of course, other glass makers offer more choices for the ED element.)

 

From reports in the field, and personal experience, there are different ways to make apochromats that perform well.

 

Clear skies, Alan

[jay.i]

_{"Supposed Strehl ratios"?  I didn't make it up...}

 

_{Here's a link to the info - scroll down the page:}

 

_{https://groups.io/g/...091,ct=1&jump=1}

I didn't mean to insinuate that you made it up, I just didn't have the source for the information. Thanks for linking it! Bookmarking that for sure.

[Alan French]

Vladimir Sacek's "Notes on Amateur Telescope Optics" (http://www.telescope-optics.net/) is a valuable and detailed resource. It contains some instructive examples of apochromats and an extensive discussion of how the glasses are selected. Smith, Berry, and Ceragioli's "Telescopes, Eyepieces, and Astrographs: Design, Analysis, and Performance of Modern Astronomical Optics" contains further examples, and nice comparisons of several choices in focal ratio. From these we can get some knowledge of the apochromats on today's market.

 

But we don't know much about what goes on "behind the scenes." There are a number of companies making optical glass and a wide variety of glasses available from each. The number of extra-low dispersion glasses, generally the choice for today's apochromats, is limited, with a larger but still limited pool of potential matches for each. Synthetic calcium fluoride, CaF2, or fluorite is also an extra-low dispersion material available for making apochromats. 

 

From this thread we know that Yuri has had problems with ZKN7 and does not recommend its use. It's not the first cautionary tale I've heard about an optical glass choice. Experience likely places limits on the choices a manufacturer is willing to make. (Maybe someone else got a good batch of ZNK7 on another day, or maybe it's simply not a good choice.)

 

We also have little idea about the availability of the necessary glasses. Ohara provides a hint in their frequency of melt data, with FPL53 listed as 4, or "low," only exceeded by 5, "lowest." (FPL55 is shown as 3, "medium.") How these translate into lead times is something only the folks using the glasses know. Availability may place further limits on the choices made.

 

So all the examples available to us may not translate into a viable product. There are also other choices that could be made for which we have no examples. 

 

Clear skies, Alan

Edited by Alan French, 12 May 2018 - 07:32 AM.

[gnowellsct]

I have the thought that if'n y'all can't see the CA introduced by the corrector on a c8....and by the way, it is totally absent from the insults hurled at scts every day on CN for as long as the forum has existed, so don't go inventing it now...that y'all ain't got a snowball's chance in h*ll of detecting, visually, the difference between fpl 53 and fpl 55 (for example).

It's like a drag race where one car takes 3.21 seconds and another is at 3.22, the second one is slower but the only folks that know are the ones operating the measuring equipment.

GN

[Paul G]

In another thread someone said these Canon lenses have crazy low Strehls.  But apparently it doesn't matter.

FWIW, those 400 f2.8 L II's are $10K each so that large 24 lens array has a quarter of a million dollars of Canon glass.

[gnowellsct]

FWIW, those 400 f2.8 L II's are $10K each so that large 24 lens array has a quarter of a million dollars of Canon glass.

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."  

[Uwe Pilz]

>  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.

[mikeDnight]

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.

[Suavi]

 

 

The band pass of fluorite of 1000 to over 100,000 Angstroms eclipses by many magnitudes that of any optical glass.

Not sure how much it matters when calcium fluorite is sandwiched in between two glasses that will block most of those additional wavelengths, and then there is a field corrector or a reducer that has a few more types of glass plus a cover glass on the sensor itself...besides, if we had a triplet passing 1000 to 100,000 angstroms, it would certainly not be capable of effectively bringing to focusing such wide spectrum of wavelengths.