94 replies to this topic

[LMO]

Again with Roland's permission, I am copying below, in its entirety, another of his posts to the A-P Users' Group.  It adds significantly to the topic of this thread, reinforcing and extending comments in his earlier post, quoted above.

 

The new post underscores his earlier comments with the observations that FPL53 "has absolutely no optical advantages over the more modern glasses of Ohara FPL55 and Hoya FCD100," adding later that it "is getting to be an obsolete glass."

 

There could hardly be more definitive and informed answers to the question posed by the OP.

 

 

   Larry

----------------------
From: "Roland Christen via groups.io" <chris1011=aol.com@groups.io>
Subject: Re: [ap-ug] Glass availability
Date: January 7, 2021 at 7:05:02 PM PST
To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>
Reply-To: main@ap-ug.groups.io

 

FPL53 is very expensive because it is difficult to make and is subject to de-vitrification if not processed in a very tight range of temperatures. It is quite difficult to polish due to its softness and is easily broken due to its fragility. It is getting to be an obsolete glass and only available in pressings up to 6" diameter and strip up to 5.5 to 5.8" width at enormous cost. It has absolutely no optical advantages over the more modern glasses of Ohara FPL55 and Hoya FCD100. In fact, there exists a slightly better combination of 3 elements to almost perfect color error with the Hoya glass versus the Ohara FPL55.

 

Russian OK4 glass is a distant second in suitability for color correction because it has a lower Vd value (Vd=92 vs Vd=95 for FPL53, FPL55 and FCD100). OK4 is closer in characteristics to Ohara's obsolete FPL52.

 

Schott has their own versions in the form of FK54, which is equivalent to the obsolete FPL52 and the newer FK56 which is close to Ohara FPL55 and Hoya FCD100. Personally I would never buy any of the Schott glasses due to some very bad experiences in the past where they shipped very poorly annealed glass with horrendous internal homogeneity and refused to admit to it, even after having been shown the test data.

 

For me, both Ohara and now Hoya have produced nothing but top quality material and after processing a thousand blanks had only one bad piece in a 175mm blank from Ohara.

 

Today, the more modern super ED glasses have harder surfaces, are easier to polish to a high level and much less brittle and subject to damage during fabrication. Optically they are truly super glasses. I don't know why anyone in their right mind would make lenses out of FPL53 anymore when there are superior glasses available. Could anyone tell if FPL53 is in a lens versus FPL55 or FCD100 or FK56? No way, the difference cannot really be measured. Any slight difference is totally swamped by the mating elements, because the choice of mate is the thing that determines the final color correction.

 

This is not the final analysis of available glasses because there are now some lower cost ED glasses made in China that may come close in performance to the Japanese glasses. However, so far they remain in secondary position (not proven quality yet).

 

As far as availability, we have no problem at this time for the lenses that we want to make. But in order to get the glass made, we do have to commit to a large quantity. Getting single pieces of ED glass is problematic and hugely expensive.

 

Rolando
----------------------

[RedzoneMN]

LZOS lenses have a good reputation for color correction despite using OK4. 

Edited by RedzoneMN, 09 January 2021 - 05:23 PM.

[pao]

LZOS lenses have a good reputation for color correction despite using OK4. 

+1

Absolut zero color with Lzos lens. Zero at all. Impossible to detect any chromatic aberration if the lens is intact.

[junomike]

Again with Roland's permission, I am copying below, in its entirety, another of his posts to the A-P Users' Group.  It adds significantly to the topic of this thread, reinforcing and extending comments in his earlier post, quoted above.

 

The new post underscores his earlier comments with the observations that FPL53 "has absolutely no optical advantages over the more modern glasses of Ohara FPL55 and Hoya FCD100," adding later that it "is getting to be an obsolete glass."

 

There could hardly be more definitive and informed answers to the question posed by the OP.

 

 

   Larry

----------------------
From: "Roland Christen via groups.io" <chris1011=aol.com@groups.io>
Subject: Re: [ap-ug] Glass availability
Date: January 7, 2021 at 7:05:02 PM PST
To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>
Reply-To: main@ap-ug.groups.io

 

FPL53 is very expensive because it is difficult to make and is subject to de-vitrification if not processed in a very tight range of temperatures. It is quite difficult to polish due to its softness and is easily broken due to its fragility. It is getting to be an obsolete glass and only available in pressings up to 6" diameter and strip up to 5.5 to 5.8" width at enormous cost. It has absolutely no optical advantages over the more modern glasses of Ohara FPL55 and Hoya FCD100. In fact, there exists a slightly better combination of 3 elements to almost perfect color error with the Hoya glass versus the Ohara FPL55.

 

Russian OK4 glass is a distant second in suitability for color correction because it has a lower Vd value (Vd=92 vs Vd=95 for FPL53, FPL55 and FCD100). OK4 is closer in characteristics to Ohara's obsolete FPL52.

 

Schott has their own versions in the form of FK54, which is equivalent to the obsolete FPL52 and the newer FK56 which is close to Ohara FPL55 and Hoya FCD100. Personally I would never buy any of the Schott glasses due to some very bad experiences in the past where they shipped very poorly annealed glass with horrendous internal homogeneity and refused to admit to it, even after having been shown the test data.

 

For me, both Ohara and now Hoya have produced nothing but top quality material and after processing a thousand blanks had only one bad piece in a 175mm blank from Ohara.

 

Today, the more modern super ED glasses have harder surfaces, are easier to polish to a high level and much less brittle and subject to damage during fabrication. Optically they are truly super glasses. I don't know why anyone in their right mind would make lenses out of FPL53 anymore when there are superior glasses available. Could anyone tell if FPL53 is in a lens versus FPL55 or FCD100 or FK56? No way, the difference cannot really be measured. Any slight difference is totally swamped by the mating elements, because the choice of mate is the thing that determines the final color correction.

 

This is not the final analysis of available glasses because there are now some lower cost ED glasses made in China that may come close in performance to the Japanese glasses. However, so far they remain in secondary position (not proven quality yet).

 

As far as availability, we have no problem at this time for the lenses that we want to make. But in order to get the glass made, we do have to commit to a large quantity. Getting single pieces of ED glass is problematic and hugely expensive.

 

Rolando
----------------------

Good to know however my eye(s)  (and only my eyes) will have the final verdict on whether the replacements are worse, equal, or better than FPL-53

[LMO]

Copied below, again with permission, is Roland's reply in the A-P Users' Group regarding OK4 glass. 

 

My own impression is that his comments, as usual, add constructive information to the discussion, and I appreciate his willingness to have them added to CN.

 

   Larry

----------------------
From: "Roland Christen via groups.io" <chris1011=aol.com@groups.io>
Subject: Re: [ap-ug] Glass availability
Date: January 10, 2021 at 9:32:28 AM PST
To: "main@ap-ug.groups.io" <main@ap-ug.groups.io>
Reply-To: main@ap-ug.groups.io

 

As I should have known, posting anything that ends up on CN is bound to bring out a few disagreements on any issue. In this case whether the Russian OK4 glass is on the same par as the most modern Japanese ED glass.

 

What i said is true that OK4 is closer to Ohara FPL52 (now obsolete) so is not as suitable from an optician's standpoint. It has both a lower Vd value and a lower PgF value than the modern glasses. the added complication is that the manufacturer of this glass will not sell it to their competitors.

 

Can high color correction be achieved with OK4? Of course, but the downside is that there are fewer mating elements available to achieve that. Color correction depends on what mating glass is used, but that's not the only criteria. For any given aperture and focal length the lower the Vd and PgF values, the worse will be the sphero-chromatism (over and under corrections at the ends of the spectrum). Sphero-chromatism is also part of color correction. To control that you either increase the focal length, or decrease the aperture or add significant internal air spaces.

 

With lower Vd/PgF, the optical designer is forced to use more expensive mating elements, increase the airspaces and use deeper internal curves. Longer airspaces will affect thermal stability, deeper curves will make the lenses more prone to de-centering. The lens may also require aspherization if designed with a short focal length. So there is always a tradeoff, and if there is a choice, the designer will always want to minimize all the variables that affect performance.

 

One can indeed make a perfectly color corrected lens (visually) with any of the available ED glasses, even the cheapest lowest performance glasses. I can certainly design a perfect lens using the lowly FPL51, whether triplet or even doublet, within certain limitations. In fact, one of the first ED lenses was introduced by John Gregory in an S&T article June 1987, that used Ohara's first ED glass FK01 (now FPL51) paired with Schott SK16 high index crown glass to yield an 8" F15 doublet apochromat that was for all intents and purposes perfectly color corrected. The tradeoff was that it needed to be F15 because of the low Vd value of the glass. It also required some fancy 5th order aspherization because of the high internal curvatures. If he could have used the modern FPL55/FCD100 glasses, his labors would have been much reduced, and the lens could have been as fast as F12 with even better color correction.

 

Roland Christen
----------------------

[RedzoneMN]

 

With lower Vd/PgF, the optical designer is forced to use more expensive mating elements, increase the airspaces and use deeper internal curves. Longer airspaces will affect thermal stability, deeper curves will make the lenses more prone to de-centering. The lens may also require aspherization if designed with a short focal length. So there is always a tradeoff, and if there is a choice, the designer will always want to minimize all the variables that affect performance.

 

One can indeed make a perfectly color corrected lens (visually) with any of the available ED glasses, even the cheapest lowest performance glasses. I can certainly design a perfect lens using the lowly FPL51, whether triplet or even doublet, within certain limitations. In fact, one of the first ED lenses was introduced by John Gregory in an S&T article June 1987, that used Ohara's first ED glass FK01 (now FPL51) paired with Schott SK16 high index crown glass to yield an 8" F15 doublet apochromat that was for all intents and purposes perfectly color corrected. The tradeoff was that it needed to be F15 because of the low Vd value of the glass. It also required some fancy 5th order aspherization because of the high internal curvatures. If he could have used the modern FPL55/FCD100 glasses, his labors would have been much reduced, and the lens could have been as fast as F12 with even better color correction.

 

Roland Christen
----------------------

I wish more telescope makers would engage in such discussion. I would rather not have to take a course in optics to understand some of the nuances.

 

Reading between the lines, if a doublet using using FPL51 needed to be F15 to be perfectly color corrected, then the APM doublet on my wish list may need an extender.

 

It is eye opening that even with "modern" FPL 55 glasses, a focal length of F12 would be par for the course if good color correction is required. Don't think anybody makes an 150mm F12 doublet. Oh well......

 

"One can indeed make a perfectly color corrected lens (visually) with any of the available ED glasses, even the cheapest lowest performance glasses " is the bombshell here for visual only observers. I should take a second look at the Evostar 150, particularly after the S&T report from Feb 2020.

 

ADDENDUM: D&G optical makes 6 inch F12 scopes!

Edited by RedzoneMN, 10 January 2021 - 06:15 PM.

[pao]

Again, visually speaking, try to look through a Lzos lens and let me know how many aberrations, of any type, you are able to detect. And how much of a singular aberration, if you can really see one, you can really detect. I think that the answer is: zero. In my experience is zero. So, if this is the result with the “obsolete” OK4 glass (and believe me, this IS the result visually), any other discussion about OK4 glass and other “obsolete” glasses is simply marketing.

[Spikey131]

Again, visually speaking, try to look through a Lzos lens and let me know how many aberrations, of any type, you are able to detect. And how much of a singular aberration, if you can really see one, you can really detect. I think that the answer is: zero. In my experience is zero. So, if this is the result with the “obsolete” OK4 glass (and believe me, this IS the result visually), any other discussion about OK4 glass and other “obsolete” glasses is simply marketing.

You are right.

 

The point is that the newer glasses make it easier on the optician to make a better optical systems.

 

Making a quality observing tool has to do with more than the raw materials employed.  It is not only about the glass type use, but the skill and knowledge of the builder.  Skilled opticians have made good scopes with glasses that are not as good as what is available today.  No doubt poor opticians can make mediocre scopes with the new glasses.

 

When we astronomers are looking for quality optics, we need to rely more upon the reputation of the maker of those optics than some limited personal knowledge of materials science.

[junomike]

I agree with Roland.  I had a APM 130 F6 (LZOS) and now have an AT130 F6 (FPL-53).  Mating elements aside the AT is IMO significantly better at CA control.

[Alan French]

[In Part]

 

It is eye opening that even with "modern" FPL 55 glasses, a focal length of F12 would be par for the course if good color correction is required. Don't think anybody makes an 150mm F12 doublet. Oh well......

 

 

Please note that the discussion was about John Gregory's 8-inch doublet, so Roland's comment, "If he could have used the modern FPL55/FCD100 glasses, his labors would have been much reduced, and the lens could have been as fast as F12 with even better color correction." applies to a 200mm aperture doublet. By reducing the aperture to 150mm, a somewhat faster design would do as well.

 

Clear skies, Alan 

[peleuba]

Again, visually speaking, try to look through a Lzos lens and let me know how many aberrations, of any type, you are able to detect. And how much of a singular aberration, if you can really see one, you can really detect. I think that the answer is: zero. In my experience is zero. So, if this is the result with the “obsolete” OK4 glass (and believe me, this IS the result visually), any other discussion about OK4 glass and other “obsolete” glasses is simply marketing.

 

 

I think "obsolete" is the wrong term.   OK4 is melted and used and manufactured into a lens by only a single company LZOS (Lytkarino Optical Glass Factory).  There is a lot of upside to owning the entire vertical and horizontal integration.  Basically - LZOS controls every aspect of manufacturing from the integration of the raw materials, to the melt, to the final figuring and certification of a lens.  Yes, their are other ED glasses that when combined with mating elements can achieve higher levels of correction, but you'll not find a lens marketed to amateur astronomers where a single entity controls all aspects of production.

 

Any air spaced LZOS lens at any of the popular focal lengths and apertures is a terrific lens.  

[Badger1992]

Every lens design will have some compromises depending on what properties the optician is trying to optimize.  While the raw materials (glasses) are an important choice, it's the execution of the design that makes a superior scope.  The quality of the glass, figuring of the lenses (which is more laborious in air spaced triplets), coatings, lens cell construct, collimization, etc. are all important to deliver an optimal image.  I think we all get a little too hung up on glass, and I'm as guilty as the next guy.  It is fun to discuss and learn though!

[sector99]

This thread is a testament to Max Planck's famous quote "Science advanced one funeral at a time".

 

The science of optical materials theory (OMT) changed for the better with the findings of Mercado/Robb (Lockheed Missiles and Space) in the early 1980s. Indeed, their 816 glass list was further updated to an even more advance listing (permitting crown-flint doublet matchs) in 2002 wherein Mercado/Robb's essential work to rid OMT from the burden of non-linear mathematics which resulted eg in extreme dependence on single elements, crippling design flexibility.

 

Zeiss immediately adopted the new 2002 243 commonly available optical materials listing which included CaF2/MgF2 as well as fused silica. Thorium, lead and other harmful radioactive materials were then relegated to the extremes of designer choices–thereby resolving a long-standing debate regarding environmentally damaging optics. The science only needed 243 optical materials, as the late Walter Bessenmatter (Then Chief designer at Zeiss) frequently commented.

 

All advanced designs released since 2002 as well as recently added materials thus have superior performance (in some cases 2x lateral resolutions), less environmental damage and use commonly available and in many cases less expensive materials. This is due to a crucial Mercado/Robb finding that earlier work (H. A. Buchdahl) permitted OMT dispersion dynamics to operate on LINEAR MATHEMATICS–making it more predictable.

 

All this means that mating lists now exist allowing 3 & 4 colors to be brought to focus with only two listed materials. It's clear today that leaders such as Takahashi et al have availed themselves of this new advance while others yet struggle with what is now a dated, less flexible system of optical design.

[TareqPhoto]

Someone linked me to this thread/topic, and i didn't read it yet but only few posts in this last page, now my question is, should i really worry or care much about having FPL-55 scope? I ordered a scope last year but it is a new model and it will be available by end of this month so i should expect it to be shipped for me next month, it is a triplet with FPL-55, at first i thought FPL-55 is better than FPL-53 because of number, but later i read more and i see that FPL-53 is very very slightly better.

 

The other question or issue i feel or think about is, what about Fluorite itself, so if i have a scope with a Fluorite even if it is one lens/optic, doesn't that means it should be the best? i even asked before about having a nice design doublet with Fluorite and another nice design triplet with FPL-53, i still feel that the answer isn't clear or straightforward, it is like I love a triplet and i love a Fluorite, i can't choose one separately. 

[Jeff B]

"..should i really worry or care much about having FPL-55 scope?"

 

If the truth be known, my CFF 160 with FPL-55 keeps me up at night.

[TareqPhoto]

"..should i really worry or care much about having FPL-55 scope?"

 

If the truth be known, my CFF 160 with FPL-55 keeps me up at night.

Is that CFF 160 with FPL-55 for example outperform TEC 160 either with FPL53 or Fluorite or even say Esprit 150mm which already has FPL-53 or Explorer Scientific 165mm with FPL-53 so it is only 5mm aperture more?

[Jeff B]

Is that CFF 160 with FPL-55 for example outperform TEC 160 either with FPL53 or Fluorite or even say Esprit 150mm which already has FPL-53 or Explorer Scientific 165mm with FPL-53 so it is only 5mm aperture more?

Of course not.  Nor will the others outperform it.

 

Now, mine is the F6.5 version so it does have a tiny bit of color error at the very ends of the visible spectrum in the blue and red which can be seen at very high powers on Vega, Sirius, and Venus.  However, the other 96% of the visible spectrum is excellent.  The F7.9 version should be color free for visual use.

 

https://www.cloudyni...f65-evaluation/

 

https://www.cloudyni...-ed-evaluation/

 

Remember, all refractors have color error.  I have learned that for visual use excellent optical corrections for spherical, coma and astigmatism are far more important than very small color errors at the very ends of the visible spectrum.  

 

Jeff

[TareqPhoto]

Of course not.  Nor will the others outperform it.

 

Now, mine is the F6.5 version so it does have a tiny bit of color error at the very ends of the visible spectrum in the blue and red which can be seen at very high powers on Vega, Sirius, and Venus.  However, the other 96% of the visible spectrum is excellent.  The F7.9 version should be color free for visual use.

 

https://www.cloudyni...f65-evaluation/

 

https://www.cloudyni...-ed-evaluation/

 

Remember, all refractors have color error.  I have learned that for visual use excellent optical corrections for spherical, coma and astigmatism are far more important than very small color errors at the very ends of the visible spectrum.  

 

Jeff

For me it is about imaging, astrophotography, not about visual, for visual for now i am very happy to use my Mak 180, color errors FREE, or even if i have to then my 8" [200mm] F/5, i have 2x and 5x Powermate, so visual isn't my issue, but for photography, dunno hw much that FPL-55 can do vs. FPL-53 for example, and it is 90mm aperture scope and i am planning to use a reducer to make it faster.

[Jeff B]

If this is the scope you are thinking about:

 

https://www.teleskop...ularauszug.html

 

I imagine you will be extremely happy with it.  Simply scaling the apertures would give a longitudinal color error similar to a 160mm F11, which is basically perfect.  Spherochromatism will dominate but even it should be very well controlled with the smaller 90mm aperture F6 airspaced design. 

 

Enjoy!

 

Jeff

[TareqPhoto]

If this is the scope you are thinking about:

 

https://www.teleskop...ularauszug.html

 

I imagine you will be extremely happy with it.  Simply scaling the apertures would give a longitudinal color error similar to a 160mm F11, which is basically perfect.  Spherochromatism will dominate but even it should be very well controlled with the smaller 90mm aperture F6 airspaced design. 

 

Enjoy!

 

Jeff

Thinking about? I already ordered it last year and i am still waiting as you can see it will be due at the end of this month.

 

Good to hear that, if i am using ST80 or a Canon lens since years then this APO triplet will be a nice jump or improvement for me no doubt, i think that FPL-55 it won't bother me and i am sure i won't have so issues with colors, processing these days getting better and better anyway, i even think about buying 0.9x reducer CC for my Newt so then i can use that for RGB [or OSC] free CA and use this APO triplet with either Ha only or tighter Lum filter.