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What's wrong with FCD100 and FDL55?

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#26 Vla

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Posted 03 September 2018 - 09:53 PM

That is closer than I thought. I find Ohara giving identical P(g,F) and no P(F,e), only P(F',e), 0.5072 for FPL53 and 0.5069 for FPL55. For a perfect (i.e zero secondary spectrum) match for FPL53 that would produce about 1/20 wave p-v of the blue-red defocus wih FPL55 and a 6 inch f/8 doublet. Worst case scenario, with the matching glass on the opposite side by about as much, it would double to 1/10 wave. Guess, enough for a perfectionist to call it a "small step down". But could have been something else, not performance related.



#27 LMO

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Posted 03 September 2018 - 11:18 PM

That is closer than I thought. I find Ohara giving identical P(g,F) and no P(F,e), only P(F',e), 0.5072 for FPL53 and 0.5069 for FPL55. For a perfect (i.e zero secondary spectrum) match for FPL53 that would produce about 1/20 wave p-v of the blue-red defocus wih FPL55 and a 6 inch f/8 doublet. Worst case scenario, with the matching glass on the opposite side by about as much, it would double to 1/10 wave. Guess, enough for a perfectionist to call it a "small step down". But could have been something else, not performance related.

You are right that the Ohara data sheets do not give explicit PF,e values for FPL53 and FPL55.  I calculated those from partial dispersions that are given:
     PF,e = (nF – ne) / (nF – nC),
     with (nF – ne), in turn, calculated as
     (nF – ne) = (nF - nC) - (ne - nC)

 

Such calculations are subject to errors of precision owing to limitations in the significant figures provided by the Ohara data sheets, but, reassuringly, the Pg,F values calculated similarly from partial dispersions that are in the data sheets:
     Pg,F =  (ng - nF)/(nF - nC),
agree with Pg,F values given directly in the sheets.

 

My aim was to provide a collection of data that could be used just as you have done to get some idea of how 'bad' FCD100 and FPL55 might be as replacements for FPL53.  As you note, FPL55 doesn't seem all that bad. 

 

In the quick hunt I did, I wasn't successful in identifying potential mating types to be used with FCD100 to provide very low secondary spectrum error in both visible and violet regions.  But one has to suppose that Roland did succeed, at least for the new 'small aperture' air-spaced Stowaway 92.

 

And it certainly seems reasonable to guess that Yuri's decision against FPL55 for the TEC 140 might well have been based on other issues than correction of secondary spectrum error.

 

Thanks very much for the thoughtful replies to the thread -

 

    Larry



#28 Astrojedi

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Posted 04 September 2018 - 02:16 AM

Very useful thread. I see no harm in being an informed customer.



#29 Fomalhaut

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Posted 04 September 2018 - 03:08 AM

Very useful thread. I see no harm in being an informed customer.


No harm for the customer, I agree...

#30 Vla

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Posted 05 September 2018 - 09:51 AM

In the quick hunt I did, I wasn't successful in identifying potential mating types to be used with FCD100 to provide very low secondary spectrum error in both visible and violet regions.  But one has to suppose that Roland did succeed, at least for the new 'small aperture' air-spaced Stowaway 92.

Since it is also quite similar to FPL53, a near perfect match is Schott N-ZK7. A doublet at 150mm f/8 just makes it as a "true apo" by the WF error criterion, and the triplet can go down to about f/7. Two different glasses that nearly offset each other's deviations vs. perfect match are also viable option. As an example, an "odd couple", Schott BK7 and Chinese (CDGM) H-K5 do quite well with the Hoya's glass. I didn't check, but they should also have the photopic poly-Strehl >0.95, although the more representative, mesopic poly-Strehl could be a bit short of 0.95.

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#31 jrbarnett

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Posted 05 September 2018 - 03:24 PM

Haven't FCD100 designs been in volume, mid-market production for quite some time (at least 2 years)?  I'm thinking of the JOC triplets using that glass type (Maxvision, Explore Scientific, etc.).

 

How do people like the FCD100 Explore Scientific triplets?

 

- Jim 



#32 daveCollins

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Posted 05 September 2018 - 04:50 PM

 denis0007dl

 

FCD100 series are essentially colour free, and produce excellent image. I had 100mm and 127mm FCD100 models.

Excellent optics, and what Mike says, $400 price difference is defintely worth upgrade.


Edited by daveCollins, 05 September 2018 - 04:53 PM.


#33 glend

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Posted 05 September 2018 - 05:02 PM

Regarding a 150mm f8 "just making it" as a true APO, i believe you will get a fair amount of argument with that statement. The Skywatcher ED150 (with it's unknown objective) has recently demonstrated that trying to build cheap at that size brings a number of problems. However, the APM 152 seems to be a better proposition, using FPL51 and Lanthanum. The 150mm triplets now hitting the market at a slightly higher price will no doubt cause imagers to opt for a "truer" APO.


Edited by glend, 05 September 2018 - 05:02 PM.


#34 Vla

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Posted 05 September 2018 - 08:12 PM

Regarding a 150mm f8 "just making it" as a true APO, i believe you will get a fair amount of argument with that statement. The Skywatcher ED150 (with it's unknown objective) has recently demonstrated that trying to build cheap at that size brings a number of problems. However, the APM 152 seems to be a better proposition, using FPL51 and Lanthanum. The 150mm triplets now hitting the market at a slightly higher price will no doubt cause imagers to opt for a "truer" APO.

I don't see what kind of argument there could be about something self-evident. I said it just makes it as a "true apo" according to the wavefront error criterion (less than 1/4 wave p-v in F and C, and less than 1/2 wave in g and r), and the raytrace shows that to be the case. Also, don't see why is a doublet with FPL53-like glass and the Schott matching glass "cheap", and FPL51 with the Chinese glass is not. As for the APM 152 being a "better proposition" that is not supported by the Strehl-across-the-wavelengths graph supplied by APM. It drops quickly toward both, blue and red end, having about 0.6 Strehl in the F-line, and little over 0.2 in the C. That translates into nearly 0.4 waves p-v in the F, and over 0.6 waves in the C. Nowhere close to the 1/4 wave limit for a "true apo", let alone the violet g-line at nearly 2.5 waves. Here's what it looks like in the raytrace, for comparison. I'm pretty sure it falls short of the 0.95 photopic poly-Strehl too.

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#35 Jeff B

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Posted 07 September 2018 - 11:00 AM

John and Alan, you guys forgot about the mating element in the Newtonian....the secondary mirror and its "secondary spectrum".  winky.gif Tisk, tisk.  

 

Everybody else.  Nice discussions.  

 

I just got a CFF 160 F6.5 with good old...err, new FPL-55.  It is a very good, sharp optic!  And, yes, of course, it does show spherochromatism in the out of focus images, which are very different from each other inside and outside of focus.  After all it is a very fast large aperture oiled triplet so there will be junk outside of focus.

 

But I don't view the moon, planets or anything else for that matter, out of focus.  In focus it excels!

 

And another thing.  Tables showing the fall off in violet are interesting but to me the LA, OPD and other plots can give me clues as to why it's falling off.   And another, nother thing, I'm sure the tables were made assuming spherical surfaces.  Applying an aspheric of some kind to one surface can help balance out the higher order spherochromatism with the primary spherochromatism giving tighter OPD plots at the extremes in color.   

 

Jeff


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#36 LMO

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Posted 10 September 2018 - 01:18 AM

Below is an updated version of the table I originally posted to start this thread.  It differs only in the addition of Schott FK56, which Roland has been quoted as grouping with FPL53, FPL55, and FCD100 as ED glasses for which "there is no real difference in color correction...[all being] capable of extremely high correction if you choose the correct matching glass, and you have at least one airspace as a variable."  See post #210 in
<https://www.cloudyni...actors/page-9>.

 

Curiously, as was the case for FPL55, the value of PF,e calculated from data published by Schott would put FK56 at a somewhat different location in a plot of Vd vs PF,e than is shown in Sucek's Fig.148:
<https://www.telescop..._refractor.htm>

 

Suggestions of possible explanations for these discrepancies would be welcome.

 

     Larry

 

 

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#37 X3782

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Posted 10 September 2018 - 02:18 AM

You are right that the Ohara data sheets do not give explicit PF,e values for FPL53 and FPL55.  I calculated those from partial dispersions that are given:
     PF,e = (nF – ne) / (nF – nC),
     with (nF – ne), in turn, calculated as
     (nF – ne) = (nF - nC) - (ne - nC)

 

Such calculations are subject to errors of precision owing to limitations in the significant figures provided by the Ohara data sheets, but, reassuringly, the Pg,F values calculated similarly from partial dispersions that are in the data sheets:
     Pg,F =  (ng - nF)/(nF - nC),
agree with Pg,F values given directly in the sheets.

 

My aim was to provide a collection of data that could be used just as you have done to get some idea of how 'bad' FCD100 and FPL55 might be as replacements for FPL53.  As you note, FPL55 doesn't seem all that bad. 

 

In the quick hunt I did, I wasn't successful in identifying potential mating types to be used with FCD100 to provide very low secondary spectrum error in both visible and violet regions.  But one has to suppose that Roland did succeed, at least for the new 'small aperture' air-spaced Stowaway 92.

 

And it certainly seems reasonable to guess that Yuri's decision against FPL55 for the TEC 140 might well have been based on other issues than correction of secondary spectrum error.

 

Thanks very much for the thoughtful replies to the thread -

 

    Larry

 

FPL53 is quite soft and hard to polish, very easily scratched and a bit sensitive to degradation (fogging) in a humid atmosphere when uncoated, I believe it is very slightly hygroscopic. Touching its bare surface with your fingers can be catastrophic requiring a repolish, it demands a protective coating applied immediately after polishing to preserve the surface quality. All these technical issues contribute to its high price. FPL55 is an "almost replacement" product with superior abrasion resistance (387 vs 451) and weathering resistance (class 2 vs 3-). FPL53 is rated by the manufacturer to show fading when exposed to a chamber at 50 degrees C and 85% humidity for 24 hours when inspected at 1500 luxes. In terms of stability of the new FPL55 glass, this is enhanced to "no fading is observed", though it is still not good enough to merit a class 1. In practice FPL55 is indeed a bit easier to handle and polish; price is reduced. For this reason, volume users for non-astronomical applications are likely to prefer FPL55, which would make the glass manufacturer deemphasize the older (more expensive, more difficult to handle) product, once the lot-to-lot differences can be controlled and the users gain confidence about the new glass.

 

The relative partial dispersion of FPL55 is slightly lower than that of FPL53, which makes it slightly less desirable from the purely optical point of view; that in theory makes it match with glass having smaller Abbe number differentials, so that spherical aberration etc. may be more difficult to correct over all wavelengths, but in practice the difference is so very small. I would suppose things will shift to FPL55 or FCD100 whether one wants to or not.


Edited by X3782, 10 September 2018 - 03:24 AM.

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#38 Riccardo_italy

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Posted 10 September 2018 - 09:21 AM

Why Schott glass (N-FK56) is not used by any telescope producer?

 

Schott ED glass is used in the sport optics market (just think at Zeiss binoculars) and Schott is very well known by the general public.



#39 SandyHouTex

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Posted 10 September 2018 - 09:35 AM

FPL53 is quite soft and hard to polish, very easily scratched and a bit sensitive to degradation (fogging) in a humid atmosphere when uncoated, I believe it is very slightly hygroscopic. Touching its bare surface with your fingers can be catastrophic requiring a repolish, it demands a protective coating applied immediately after polishing to preserve the surface quality. All these technical issues contribute to its high price. FPL55 is an "almost replacement" product with superior abrasion resistance (387 vs 451) and weathering resistance (class 2 vs 3-). FPL53 is rated by the manufacturer to show fading when exposed to a chamber at 50 degrees C and 85% humidity for 24 hours when inspected at 1500 luxes. In terms of stability of the new FPL55 glass, this is enhanced to "no fading is observed", though it is still not good enough to merit a class 1. In practice FPL55 is indeed a bit easier to handle and polish; price is reduced. For this reason, volume users for non-astronomical applications are likely to prefer FPL55, which would make the glass manufacturer deemphasize the older (more expensive, more difficult to handle) product, once the lot-to-lot differences can be controlled and the users gain confidence about the new glass.

 

The relative partial dispersion of FPL55 is slightly lower than that of FPL53, which makes it slightly less desirable from the purely optical point of view; that in theory makes it match with glass having smaller Abbe number differentials, so that spherical aberration etc. may be more difficult to correct over all wavelengths, but in practice the difference is so very small. I would suppose things will shift to FPL55 or FCD100 whether one wants to or not.

I believe you are confusing FPL-53 with Fluorite.  FPL-53 is actually a glass.  What you describe sounds like Fluorite.  And uncoated Fluorite was used by Tak for many years in a Steinheil configuration (positive element in back).  Most manufacturers went to FPL-53 because of the fragility of Fluorite.



#40 Riccardo_italy

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Posted 10 September 2018 - 09:47 AM

Italian language, a bit outdated but maybe useful (some ED are missing from LMO's table).

 

Vetri più usati e loro coefficente Abbe:

    Fluorite (CaF2)...............................1,43388....95,19
    Vetro Ohara S-FPL 53....................1,43875....94,99
    Vetro Schott N-FK56......................1,43425...94,95
    Vetro LZOS OK4............................1,44734...92,05
    Vetro Hikari E-FKH2.......................1,456.......91,32
    Vetro Hoya FCD10.........................1,4565.....90,27
    Vetro Schott N-FK51A....................1,48656...84,47
    Vetro Hoya FCD 1..........................1,498.......81,63
    Vetro CDGM H-FK61......................1,497.......81,61
    Vetro Ohara S-FPL 51....................1,497.......81,54


Edited by Riccardo_italy, 10 September 2018 - 09:47 AM.


#41 LMO

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Posted 10 September 2018 - 04:00 PM

Italian language, a bit outdated but maybe useful (some ED are missing from LMO's table).

 

Vetri più usati e loro coefficente Abbe:

    Fluorite (CaF2)...............................1,43388....95,19
    Vetro Ohara S-FPL 53....................1,43875....94,99
    Vetro Schott N-FK56......................1,43425...94,95
    Vetro LZOS OK4............................1,44734...92,05
    Vetro Hikari E-FKH2.......................1,456.......91,32
    Vetro Hoya FCD10.........................1,4565.....90,27
    Vetro Schott N-FK51A....................1,48656...84,47
    Vetro Hoya FCD 1..........................1,498.......81,63
    Vetro CDGM H-FK61......................1,497.......81,61
    Vetro Ohara S-FPL 51....................1,497.......81,54

Riccardo -

 

Thank you for those additional nd and Vd values.  Small differences in values from ones in my table could be from rounding errors or from differences of refractive-index measurements used in published data.  Temperature can apparently affect measured values significantly.  As mentioned in the notes with my table, I found considerable variation among values for CaF2, for example Abbe number Vd values ranging from 94.96 to 95.31.  The value used in the table, 95.23, is from a 'Schott LITHOTEC-CAF2' source.

 

    Larry

 

Edited for appearance only; content unchanged


Edited by LMO, 10 September 2018 - 08:04 PM.


#42 Vla

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Posted 10 September 2018 - 04:20 PM

Curiously, as was the case for FPL55, the value of PF,e calculated from data published by Schott would put FK56 at a somewhat different location in a plot of Vd vs PF,e than is shown in Sucek's Fig.148:

<https://www.telescop..._refractor.htm>

FPL53 I probably added (it was years ago) and it was either error in calculation or placement (although I get a bit lower value for Pf,e: 0.45356). But FK56 is where Atmos has it on its RPD diagram. Data on it I can only find in older sources, seems to be replaced by FK58 (Pfe=0.45418). It may be that Schott never got to produce FK56 to a needed consistent quality.

 

N-KZFS2 is a good match for CaF2, even better than Hikari's short flint (J-KZFH4) which Takahashi used for its FC100DF. But when it comes to the triplets, there are single glasses that are good matches to the ED glass, and there are pairs of glasses that neither is a good match alone, but they are as a pair (offsetting each others deviations from a good match).



#43 LMO

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Posted 10 September 2018 - 06:05 PM

 

Curiously, as was the case for FPL55, the value of PF,e calculated from data published by Schott would put FK56 at a somewhat different location in a plot of Vd vs PF,e than is shown in Sucek's Fig.148:

<https://www.telescop..._refractor.htm>

 

FPL53 I probably added (it was years ago) and it was either error in calculation or placement (although I get a bit lower value for Pf,e: 0.45356). But FK56 is where Atmos has it on its RPD diagram. Data on it I can only find in older sources, seems to be replaced by FK58 (Pfe=0.45418). It may be that Schott never got to produce FK56 to a needed consistent quality.

 

N-KZFS2 is a good match for CaF2, even better than Hikari's short flint (J-KZFH4) which Takahashi used for its FC100DF. But when it comes to the triplets, there are single glasses that are good matches to the ED glass, and there are pairs of glasses that neither is a good match alone, but they are as a pair (offsetting each others deviations from a good match).

Vla -

 

Thank you for the clarifications in your reply.  I also did not see FK56 in recent Schott data sets, only FK58, which has a significantly lower value of Vd (90.90).  The data I used for FK56 was, as you suggest, from a likely older edition of the Schott catalog:
<https://www.physics....ic_catalog.pdf>

 

I added FK56 to the table mainly as a result of Roland's quoted inclusion of it with FPL53, FPL55, and FCD100.  If Schott is no longer producing it, it is certainly 'out of the running' for current production scopes.

 

And, as an added note, if you are the Vladimir Sacek who produced the very impressive text at telescope-optics.net, please add my very big thanks to what must be a large collection of thanks from others.  It is a real treasure of information and insight that I have barely begun to digest.

 

    Larry



#44 X3782

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Posted 10 September 2018 - 06:24 PM

I believe you are confusing FPL-53 with Fluorite.  FPL-53 is actually a glass.  What you describe sounds like Fluorite.  And uncoated Fluorite was used by Tak for many years in a Steinheil configuration (positive element in back).  Most manufacturers went to FPL-53 because of the fragility of Fluorite.

 

No confusion, compare datasheet of FPL-53 with FPL-55, under "abrasion" and "weathering resistance". "Fluoride glass" is very slightly hygroscopic even though it is stabilized, it contains some local crytalline structure of metal fluorides which cannot be avoided, so it is way more fragile than e.g. N-BK7. FPL-55 is a bit more stable and harder than the older FPL-53 which is the major selling point. All these soft glasses are hard to polish, though not as bad as CaF2.

 

Takahashi (or anyone else) never used an uncoated CaF2 lens for an astronomical telescope which would not be stable, you can buy one from the optics suppliers and after a while in a humid place, you will start to see degradation. The earliest ones used single(?) coatings, this was before it was widely exported to US. These very earliest lenses are famous for now being fogged, even these coatings had too many pinholes that allowed the passage of moisture. But lenses coated after the mid 1980's are quite stable after decades.

 

"Fluorite" makes me think of the natural mineral, "synthetic fluorite" is what Canon produces. Many people in the industry simply call it CaF2, calcium fluoride. To be distinguished from magnesium fluoride or "fluoride glass".


Edited by X3782, 10 September 2018 - 07:33 PM.


#45 X3782

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Posted 10 September 2018 - 07:42 PM

Riccardo -

 

Thank you for those additional nd and Vd values.  Small differences in values from ones in my table could be from rounding errors or from differences of refractive-index measurements used in published data.  Temperature can apparently affect measured values significantly.  As mentioned in the notes with my table, I found considerable variation among values for CaF2, for example Abbe number Vd values ranging from 94.96 to 95.31.  The value used in the table, 95.23, is from a 'Schott LITHOTEC-CAF2' source.

 

    Larry

 

Refractive indexes of CaF2, FPL-53, FPL-55 etc. are highly temperature-dependent, at the level of 6-7 ppm per degree C. At low temperature the crystal or glass contracts and the density increases and thus the refractive index increases (to rough approximation). CaF2 having a cubic fluorite structure has some natural birefringence, i.e., the refractive index varies depending on the relationship between the crystal orientation and light polarization. The Abbe numbers of glasses and crystals vary from lot to lot and position to position within the furnace, though the manufacturer adjusts mixtures to try and equalize the values to the catalog numbers. The values tabulated in the literature are for reference only - if you anneal the material, the Abbe numbers and refractive index shift away from the standard ones; if you apply mechanical pressure, it changes (stress-induced birefringence). Sometimes the manufacturer changes the glass content or manufacturing process (e.g., to conform to environmental regulations or improve transmission in the blue or red), in this case you will see slight shifts too.

 

If you want to be perfect, the refractive index at various wavelengths have to be measured after annealing.


Edited by X3782, 10 September 2018 - 08:04 PM.


#46 Jeff B

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Posted 10 September 2018 - 10:30 PM

Why Schott glass (N-FK56) is not used by any telescope producer?

 

Schott ED glass is used in the sport optics market (just think at Zeiss binoculars) and Schott is very well known by the general public.

Well, I stopped someone on the street the other day and asked him if he was familiar with Schott glass.  He said he was and referred me to this.

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#47 R_Huntzberry

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Posted 11 September 2018 - 01:09 AM

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#48 Vla

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Posted 11 September 2018 - 02:04 PM


And, as an added note, if you are the Vladimir Sacek who produced the very impressive text at telescope-optics.net, please add my very big thanks to what must be a large collection of thanks from others.  It is a real treasure of information and insight that I have barely begun to digest.

 

    Larry

Yup, it's me. It's all my fault '). Thank you very much, makes it worthwhile if it's so. And talking about the new Stowaway, for which AP says will have the Strehl of 0.90 and above over the entire visible spectrum, I tried to see what it would look like. In terms of wavefront error it means that all wavelengths should be below 0.052 wave RMS. In his short history of AP lenses, Thomas Back wrote that Roland triplets (later generation) evolved from FPL53 and ZKN7 to FPL53 and two crowns, which further reduced spherochromatism. Now it is FCD100, but since it's nearly identical to FPL53, it gives nearly identical output w/ZKN7. At left is such a triplet, which does have a noticeable higher order SA residual and 0.068 wave RMS (better than the "diffraction limited" 0.0745 for 0.80 Strehl, and well within the "true apo" criterion), but still well short of the needed 0.051 wave RMS. The two red lines are at 0.054 wave, just short. On the right, ZKN7 is replaced by two CDGM's crowns, which tightened up the violet to 0.058 wave, and brought the two red lines below 0.052, down to 0.048. With a bit of tweaking (slightly more of undercorrection in the optimized line), all three could be brought to about 0.052 wave RMS. Almost there...

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#49 LMO

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Posted 11 September 2018 - 03:41 PM

... And talking about the new Stowaway, for which AP says will have the Strehl of 0.90 and above over the entire visible spectrum, I tried to see what it would look like.

...

 Almost there...

Vla -

 

Thanks very much for taking the time to model some possibilities aiming at what Roland seems to have achieved with FCD100 in the new Stowaway.  He has pretty publicly rejected N-ZK7 for practical use, though models with it and ED glasses similar to FPL53  do offer benchmarks.  It seems very rare that he reveals the mating type actually used in an A-P scope, the only exception I've seen being specification of 'ultra-clear BK7' with 'premium' FPL53 for the A-P 175 f/8 EDF.

 

If other CN contributors know of the mating types actually used in other A-P scopes, that would be of interest.  (Based on comments from Roland  about the problems associated with  Z-NK7, it's not clear that he ever used it as suggested in Thomas Back's 'Brief History of Astro-Physics Lenses.')

 

   Larry



#50 Vla

Vla

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Posted 12 September 2018 - 11:47 AM

I can add so much that CDGM's H-ZK1 is near identical to Ohara's S-BAL22, and H-K9L to good old BK7, Hoya's BSC7 and Ohara's S-BSL7.




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