94 replies to this topic

[LMO]

It is hard to argue with the frequently expressed observation that, regardless of the glass types employed in a refractor, it is the final result that matters -- measurable performance combined with field reports by experienced users.  Nonetheless, recent choices of replacements for FPL53 remain of interest to me, so prompt the question of this thread.

 

Roland and Yuri have noted the high costs and difficulties of obtaining high quality batches of FPL53, and manufacturers of high-end and mid-range good scopes are moving from FPL53 to alternative choices for the ED element in their most recent refractors:

 

• Roland has chosen FCD100 for the newly announced A-P Stowaway 92 and says the scope will offer "unprecedented color correction."
• CFF appears to be using FPL55 for most of its current refractor offerings.
• Kevin LeGore, 'skyward_eyes,' the Sky-Watcher USA Specialist, has said that "I think over time you will see the move to FPL-55."
• Yuri has announced that fluorite (CaF2) will be used in new TEC 140 models, noting in response to questions I'd posed in an earlier thread that, as replacements for FPL53,
a) "...using [FPL55] will be a small step down in performance mostly in violet part of spectrum (imaging), but still could perfect for visual use."
and
b) "I have not considered Hoya glass, because of dimensional limitation, also when switching to new substrate I wanted to get away from multi-melts.  So, the FCD-100 could be a good choice for smaller objectives, comparing to CaF2 it could be easy coated in case of air-spaced design."

 

Yuri's comments have been the most direct and helpful but do suggest that FPL55 could be 'perfect' for scopes intended primarily for visual observation, and that FCD100 could be a 'good choice' for smaller apertures (perhaps Stowaway-sized ones?).  Roland's prediction of 'unprecedented color correction' with use of FCD100 in the new Stowaway is a pretty strong endorsement of that glass.

 

So, what, exactly, are the disadvantages of FPL55 and FCD100 compared to FPL53?

 

Difficulty of finding mating-glasses for good color correction does not seem to be the answer.  In another, related thread, Alan French outlined the guidelines for choices of mating types to correct secondary spectrum color error <https://www.cloudyni...pes-for-fcd100/>, and those guidelines have been set out very clearly by Vladimir Sacek and Roland Christen.  Briefly, the aim is to select pairings with as small as possible a difference in relative partial dispersion values (PF,e for correction over the visible spectrum and Pg,F for correction in the violet range) along with as large as possible difference in the Abbe number, Vd.
Sacek 9.2 <https://www.telescop..._refractor.htm>
Sacek 9.3 <http://www.telescope...t_achromat.htm>
Christen <https://www.astromar...e=1#post142448>

 

Alan noted that the values of the relevant parameters are very similar for FCD100 and FPL53, and the same appears to be true for FPL55, so mating choices for FPL53, or closely similar mating glasses, should be appropriate for the newer ED glasses, at least with respect to secondary spectrum error.  To document the similarities of the new ED glasses to FPL53 with respect to these parameters, I've assembled a table (see below) using manufacturers' data available on the Web.  Values for CaF2 are included for comparisons, but are subject to uncertainties (see notes with Table).  (Curiously, the value I found for PF,e of FPL55 is nearer FPL53's value than suggested by their relative positions in Fig.148 of Sacek's 9.2 page.)  Also included in the table are two additional frequently cited parameters -- refractive index for the d line, nd, and Abbe number Ve.  Finally, the table includes data for mating glasses frequently suggested, or actually specified as in use for some production scopes, plus values for some others that seem mating candidates, at least for visual use, if I've understood the guidelines correctly.

 

Two further comments;

 

1. Based on the relative partial dispersion values PF,e and Pg,F, I did not find obvious mating candidates for FCD100 or CaF2 promising low secondary spectrum error in both visible and violet regions.

 

2. FPL55 is reported to be less expensive than FPL53, which seems an advantage.  This, along with Kevin LeGore's comment quoted above, invites a speculation relevant to the extensive (and impassioned!) discussions in other threads about the ED glass used in the recently introduced Sky-Watcher Evostar ED150mm f8 doublet, presumed not to be FPL53 as specified for the highly regarded Evostar 'ProED' doublets -- 80mm, 100mm, and 120mm.  Could FPL55 be the unspecified glass in the new 150ED design?   If so, it might well offer quite good color correction despite a low price point.  If I have the history right, introduction of the Synta 80ED model by Orion was initially greeted with skepticism that its low price could be coupled with near-apo color correction.

 

Comments are very welcome from those much more informed on these matters than I am, particularly possible reasons for the apparent low regard for FCD100 and FPL55 compared to FPL53 and corrections to entries in the table.

 

     Larry

 

[Jon_Doh]

Don't know what the final answer will be when it all shakes out regarding what glass manufacturer's will choose to replace FPL-53.  I'm just glad that both of my refractors have it.

[db2005]

Reading glass specifications can be intriguing indeed, but experience has taught me that other less tangible qualities of workmanship put into a telescope mean more than the mere choice of a particular type of ED glass. My Vixen SD81S and my old Orion 80ED reportedly both use FPL-53 glass, but the Vixen has noticeably less CA than the 80ED (I'd say, the difference is about 70% less CA visually, so the difference is very evident). To boot, it produces noticeably sharper and contrasty images at high magnification.

 

Quality of the optical figure and polish, the choice of mating element for the ED element, quality of baffling, even something as low-tech as the choice of a good, smooth focuser (!), etc. all contribute to the optical quality and the "experience" at the eyepiece end. I do find it remarkable how little the Japanese makers Vixen and Takahashi seem to feel the need to "hype" their choice of glasses. Until recently, Vixen didn't even disclose what ED glass they were using. However, they still sell their expensive scopes to refractor lovers who just "know" by experience that they are likely to get a better optic when buying high-end stuff. You pay more for the workmanship going into the lens surfaces than you pay for the glass they are made of.

 

Personally, I've stopped caring too much about the advertised glass types being used, as long as feel I can trust the maker to make an excellent lens. But, I'd love to see Vixen reintroduce their FL80S fluorite APO. Oh, maybe I am a sucker for some lens materials, after all . And Japanese optics, too.

 

Clear Skies,

Daniel

Edited by db2005, 29 August 2018 - 10:39 AM.

[peleuba]

Great post!  I have an Excel file similar to yours.   Two comments of my own.

 

(1)  You will see more designs based on Hoya FCD100/FPL55 once AP starts to deliver the new Stowaway.  The rebranders (and their Chinese OEMs) don't want to be the first to use a relatively unknown (to the masses) type of ED glass as it would not sell.  To wit, FPL53 has become pure marketing hype and, as you know, means little to the overall correction of a given telescope.  I personally cannot wait to get my hands on the Stowaway from A-P. 

 

 

(2)  I am guessing that Yuri went to all Fluorite for the abnormal dispersion element in the TEC lineup because its what he is most familiar with - and - TEC does not have to depend upon (variable) melt data for an ED element that differs from melt data that was used in the design.  Melt data slightly varies between each "melt" (or run) of ED glass production.  Fluorite does not have "melt data" per se and is routinely consistent from run to run. So, with ED glass,  the design always has to be tweaked slightly once the melt data is known for the particular batch of glass you are using.  I have seen TEC's operations and the polishers are slow and seem perfect for Fluorite.  

Edited by peleuba, 29 August 2018 - 01:56 PM.

[LMO]

Daniel -

 

You are right, the importance of 'execution' in manufacture of a scope should not be ignored, whatever might be 'theoretically' possible based on the glass choices.  But my impression is that even the first step -- getting good glass -- can be problematic.  Not all 'FP53' is created equal, and comments from both Roland and Yuri suggest that the increasing costs of obtaining high-grade batches of it have prompted moves to alternatives.

 

Improvements in automated polishing and designs not calling for aspherizing, might be reducing the quality edge once afforded by hand figuring.  And, without real knowledge of the facts, one might speculate that highly automated production using a relatively affordable high grade (e.g., uniform) batch of some 'lesser' ED glass, such as FPL51, might *on average* yield better scopes than would similar production using mid-grade ('mediocre') batches of FPL53.

 

   Larry

[LMO]

peleuba -

 

Thanks for the kind words.

 

Both your comments seem on-target.

 

1. The marketing hesitancy to 'lead' with a glass not yet widely appreciated could well account, at least in part, for Sky-Watcher's new reticence about the ED glass used in its new refractors.  Conversely, some kudos are owed Roland for being up-front about his choice of FCD100 for the new Stowaway.  It's also notable that Astronomics has announced FCD100 as the ED glass to be used in the new AT92 f5.5, based on an original Thomas Back AT-TMB92SS f5.5 design that used FPL53, now updated by Rogher Ceragioli.

 

2. Yuri's own comments indicate that you are right that the consistency of CaF2 figured importantly in his choice of it.  Your added description of TEC's slow (careful) polishing adds to the arguments that likely made fluorite a good choice for him.

 

But I still wonder about his claim that FPL55 would be a "small step down [from FPL53] in performance mostly in violet part of spectrum  (imaging), but still could perfect for visual use. But I prefer going a step up, so, we decide to go with Fluorite instead..."  <https://www.cloudyni...n7-era/page-2> 

 

To the extent that my (admittedly relatively naive) checks of glass parameters on manufacturer's Web sites can be trusted, FPL55, combined with BK7, promises *better* correction of secondary color error in the visible range than the FPL53/BK7 combination (PF,e values) and at least as good correction in the violet range (Pg,F values).  At least based on those parameters, Yuri's characterization of FPL55 as a 'step down' is confusing.  Also, as I noted (with some hesitation), I didn't find mating types for fluorite promising low secondary spectrum error in both visible and violet ranges.

 

I am also inclined to ask, again hesitantly, why Takahashi, famed for its fluorite-based FS and FC doublets, chose to use FPL glasses for its most highly color corrected FSA and TOA triplets.  I'm sure this question has been asked before and likely answered, but I don't have references for the answers.

 

     Larry

[Nippon]

The Vixen ED Japan made scopes have been around for quite some time. I bought an ED103s in 2013. I had no idea about the glass type other than it was an ED. When the scope arrived there was a Vixen catalog packed with the scope and on the description page for the ED doublets it identified the glass as FPL53. When they did the minor redesign (changed the baffeling in the draw tube for full frame DSLRs) They turned the Vixen name 90 degrees on the dew hood, changed the decal on the tube to gray from blue and renamed them "SD." Why at that point they decided to make a point of disclosing the glass type I don't know other than responding to what other makers were doing. I based my choice of the ED103s on my experience of Vixen refractors I'd had in the past and it did not disappoint.

[db2005]

Daniel -

 

You are right, the importance of 'execution' in manufacture of a scope should not be ignored, whatever might be 'theoretically' possible based on the glass choices.  But my impression is that even the first step -- getting good glass -- can be problematic.  Not all 'FP53' is created equal, and comments from both Roland and Yuri suggest that the increasing costs of obtaining high-grade batches of it have prompted moves to alternatives.

 

Improvements in automated polishing and designs not calling for aspherizing, might be reducing the quality edge once afforded by hand figuring.  And, without real knowledge of the facts, one might speculate that highly automated production using a relatively affordable high grade (e.g., uniform) batch of some 'lesser' ED glass, such as FPL51, might *on average* yield better scopes than would similar production using mid-grade ('mediocre') batches of FPL53.

 

   Larry

I wonder if the difficulties in acquiring the right glasses is going to prompt a larger-scale revival of true fluorite scopes. The Vixen FL55SS has returned recently, and I wouldn't be too surprised if they reintroduced the FL80S too.

 

After all, as the Chinese telescope makers up their game with better and better optics, the premium makers probably feel the need to up their game as well. Equipment-wise this is a great time for amateur astronomy: We are spoiled with so many excellent choices, and decent equipment is almost cheap as chips.

[Riccardo_italy]

To the extent that my (admittedly relatively naive) checks of glass parameters on manufacturer's Web sites can be trusted, FPL55, combined with BK7, promises *better* correction of secondary color error in the visible range than the FPL53/BK7 combination (PF,e values) and at least as good correction in the violet range (Pg,F values).

BK7 is a good mating element for FPL53, not expensive and easy to work with. But there are alternative mating elements that provide higher color correction (some lanthanum glasses I think).

[Bennevis]

I think price has a lot to do with it. I was under the impression that FCD100 is cheaper than FPL55 offering more or less the same parameters, therefore it seems a likely candidate for big visual scopes. I'm given to understand that the biggest buyers of ED glass are the large Chinese optical manufacturing complexes and they probably source Ohara and CDGM glass at a cheaper unit price hence the profusion of FPL53 and FCD100 scopes appearing on the market.

[peleuba]

I am also inclined to ask, again hesitantly, why Takahashi, famed for its fluorite-based FS and FC doublets, chose to use FPL glasses for its most highly color corrected FSA and TOA triplets.  I'm sure this question has been asked before and likely answered, but I don't have references for the answers.

 

 

 

Roland, at this point, is selling everything he makes and there is still a very lo

 

Totally agree with the FPL55 - BK7 combination.  Not much of a "step down" unless Yuri is referring to the omnipresent melt consistency battle that is fought with ED glass except those batches at the uber  extreme highest quality (there are several different grades of a given flavor of ED glass).  And, yes, in optical design software it looks like a winning combination.  To this end, I really think its got to do with familiarity with Fluorite.  TEC has been very successful with Fluorite designs in production.  Yuri likes working with it; Roland not so much.  Also using Fluorite takes that one cantankerous variable (melts) out of the equation.

 

The TOA is a dual ED design.  Meaning that each triplet contains an ED element sandwiching a single piece of BK7 (I think its BK7).  If it was dual Fluorite elements, that would be an incredibly expensive telescope.   Economics probably drove the TAK decision.  At the time the TOA came out, both AP and TEC were using a single element of FPL53.  Perhaps TAK subscribed to the "mo' is betta" mindset.      In either case, the TOA 130  is THE most color free  refractor that I have personally seen.  To my eyes its slightly better then the APM-TMB 130 F/9.

Edited by peleuba, 30 August 2018 - 11:29 AM.

[peleuba]

BK7 is a good mating element for FPL53, not expensive and easy to work with. But there are alternative mating elements that provide higher color correction (some lanthanum glasses I think).

 

I had always heard that BK7 is the preferred mating element because its readily available in very high quality.

Edited by peleuba, 30 August 2018 - 11:10 AM.

[Jon Isaacs]

I had always heard that BK7 is the preferred mating element because its readily available in very high quality.

 

The actual reason is this:

 

It is well known that GSO Dobsonians use mirrors made from BK7 and since they are perfectly corrected chromatically, using BK7 in a refractor has to be a good thing. 

 

 

Sorry for the silliness but I just couldn't help myself.

 

Humor break over, now back to your regularly scheduled programming.

 

Jon

[Alan French]

The actual reason is this:

 

It is well known that GSO Dobsonians use mirrors made from BK7 and since they are perfectly corrected chromatically, using BK7 in a refractor has to be a good thing. 

 

 

Sorry for the silliness but I just couldn't help myself.

 

Humor break over, now back to your regularly scheduled programming.

 

Jon

I have a 10-inch from 1950 that has absolutely no secondary color, and it uses Pyrex. Lots of similar scopes around. 

 

(Of course Pyrex is no longer widely available.)  

 

Clear skies, Alan

[JKAstro]

Well as I'm sure many people would like to point out, as a consumer I know just enough about glass types to be dangerous.  But the idea that a vendor would be afraid to advertise a scope having FPL-55 when its direct competitor uses FPL-51 strikes me as improbable and a marketing mistake if it ever were to have happened. 

 

For those vendors who do not specify the glass, might be best not to speculate so that people don't get the idea that it is something it may not be.  Let the eyepiece do the talking, as has been requested.

[LMO]

BK7 is a good mating element for FPL53, not expensive and easy to work with. But there are alternative mating elements that provide higher color correction (some lanthanum glasses I think).

I may indeed have missed some good mating-type candidates.  Scans through manufacturers' Excel files and data sheets, along with inspections of Vladimir's plots of Abbe number vs PF,e or Pg,F are a little like an Easter-egg hunt.

 

However, there are at least two lanthanum glasses included in the table -- Hoya LACL60 and Schott LAK7.  Further, one speculation is that, if there were a significantly better mating type for FPL53 than BK7, Roland might have been expected to use that alternative for the A-P 178 f8 Starfire triplet, which had one of the best specified color correction among all A-P refractors.  In the A-P description of the 178 f8, he states

 

"The design is topnotch using the best glasses available polished to a high degree of perfection. ... The glass is ultra-clear BK7 crown mated to Ohara FPL53 premium ED glass. The result is magic."

 

FCD100 is relatively new, and it is interesting that Roland has chosen it for the new Stowaway.  To my knowledge, he has not indicated the mating type to be used with it.

 

    Larry

[Jon Isaacs]

I have a 10-inch from 1950 that has absolutely no secondary color, and it uses Pyrex. Lots of similar scopes around. 

 

(Of course Pyrex is no longer widely available.)  

 

Clear skies, Alan

 

Alan:

 

Very true.  But even refractor people know that Pyrex is not very good optically whereas BK7 is a high quality optical glass.   

 

Back once again to the regular programming.

 

Jon

[Element79]

This what I think is an interesting post that Yuri made in the TEC section of the Yahoo Groups when he was considering the design of the new TEC140 last year and the calculated Strehl values for different materials:

 

                   g line                F line               e line                 C line

                  (violet)         (blue-green)     (green-yellow)    (deep red)

 

FPL-53        0.29                 0.80                  0.99                  0.85

FPL-55        0.03                 0.90                  0.99                  0.75

CaF2           0.70                 0.93                  0.99                  0.97

 

FPL-55 had a Strehl of 0.03 in the violet whereas CaF2 had a Strehl of 0.70!  That pretty much tells us why designer are not flocking to FPL-55 far new designs.  I would love to see this calculation made withFCD100 but I don't personally have the experience or software to do the math...

[Suavi]

This what I think is an interesting post that Yuri made in the TEC section of the Yahoo Groups when he was considering the design of the new TEC140 last year and the calculated Strehl values for different materials:

 

                   g line                F line               e line                 C line

                  (violet)         (blue-green)     (green-yellow)    (deep red)

 

FPL-53        0.29                 0.80                  0.99                  0.85

FPL-55        0.03                 0.90                  0.99                  0.75

CaF2           0.70                 0.93                  0.99                  0.97

 

FPL-55 had a Strehl of 0.03 in the violet whereas CaF2 had a Strehl of 0.70!  That pretty much tells us why designer are not flocking to FPL-55 far new designs.  I would love to see this calculation made withFCD100 but I don't personally have the experience or software to do the math...

Since human eye sensitivity to light around g line is 30-50 times lower than in green light (might be even less for those of us who have been around for several decades), a telescope with FPL-55 might yield sharper images at eyepiece than a similarly executed telescope with FPL-53, because according to the table, FPL-55 performs better around F line and the same around e line where human eye is significantly more sensitive.

[Element79]

Since human eye sensitivity to light around g line is 30-50 times lower than in green light (might be even less for those of us who have been around for several decades), a telescope with FPL-55 might yield sharper images at eyepiece than a similarly executed telescope with FPL-53, because according to the table, FPL-55 performs better around F line and the same around e line where human eye is significantly more sensitive.

 

An excellent point, but in the deep red, where the human eye is also sensitive, the FPL-55 fared worse than FPL-53 did.  CaF2, however, beat both of them across the board, which is probably why Yuri went that route.  It's just too bad that CaF2 is so dang expensive!!!

[Suavi]

An excellent point, but in the deep red, where the human eye is also sensitive, the FPL-55 fared worse than FPL-53 did.  CaF2, however, beat both of them across the board, which is probably why Yuri went that route.  It's just too bad that CaF2 is so dang expensive!!!

Human eye sensitivity at C line is about 10 times less than in green, so perhaps this is easily offset by the same difference at F line (in favour of FPL55) where human eye is more sensitive to light.

 

To illustrate my point, I added to the attached table a row that includes human sensitivity to particular wavelengths and made perhaps a non-scientific weighting of the Strehls against human sensitivity to a given wavelength (based on this article).

 

It seems that at the wavelengths where it matters the most, FPL-55 could be superior to FPL-53 at eyepiece. Or it is not. But we shouldn't just look at simple optical data without taking into the account the detector (human eye) as our conclusions may deviate from the reality. It would have been similar with CCD/CMOS as their sensitivity also varies with wavelength, plus on top of that anyone who is serious about imaging uses filters that block unwanted light.

 

Interestingly, another telescope manufacturer states that it is possible to manufacture 140mm f6.5 FPL-55 triplet with better than 1/2 lambda PV error in the violet wavelength range (around g line).

 

I totally agree, CaF2 is optically superior to glass. But I find it hard it to accept that engineers at Ohara designed a new optical glass that is meant to replace FPL 53 and that this new glass (FPL55) is optically inferior to its predecessor and can't be used to make superb telescopes.

Attached Thumbnails

• 

Edited by Suavi, 31 August 2018 - 10:25 PM.

[Vla]

We shouldn't use photopic (daylight) eye sensitivity when evaluating telescopic image. Under night sky, eye shifts toward scotopic sensitivity, and generally is in its mesopic mode. Conventionally, that means further decrease in sensitivity on the red end, and significant increase in the blue/violet. But this model is just a paper model. Some experiments showed that mesopic vision has increased sensitivity on both ends, but significantly more in the blue violet (https://www.telescop...al_response.htm , Fig. 249).

 

As for what Yuri has said, that's very likely in the context of matching glasses that he had on disposal. Fpl53 and 55 have somewhat different dispersive properties, and won't give identical result with any given matching glass.

[LMO]

Element79, on 31 Aug 2018 - 1:11 PM, said:

 

This what I think is an interesting post that Yuri made in the TEC section of the Yahoo Groups when he was considering the design of the new TEC140 last year and the calculated Strehl values for different materials:

 

                   g line                F line               e line                 C line

                  (violet)         (blue-green)     (green-yellow)    (deep red)

 

FPL-53        0.29                 0.80                  0.99                  0.85

FPL-55        0.03                 0.90                  0.99                  0.75

CaF2           0.70                 0.93                  0.99                  0.97

 

FPL-55 had a Strehl of 0.03 in the violet whereas CaF2 had a Strehl of 0.70!  That pretty much tells us why designer are not flocking to FPL-55 far new designs.  I would love to see this calculation made withFCD100 but I don't personally have the experience or software to do the math...

----------------------

 

An extended version of Yuri's table is part of an early thread on another site in which Yuri discussed his move from FPL53 to CaF2 for further production of the TEC 140 f7 triplet.  The thread is here:
'fluorite':
<https://groups.io/g/...91,ct=1&jump=1>

 

Yuri's extended table is on p.2 of that thread.  It was cited in one of the previous CN threads comparing FPL53 and CaF2 for use in highly corrected scopes.  There is much of interest in that thread, including discussion of the relatively low visual sensitivity at the red and violet ends of the visible spectrum, quality problems with mating-type candidate ZKN7 ('N-ZK7' in the Schott catalog), and more -
'Comparing FPL-53 and CaF2':
<https://www.cloudyni.../#entry8570811>

 

However, the question I asked in initiating the present thread was what, if any, downsides there were to using FCD100 or FPL55 as replacements for FPL53, not the relative merits of CaF2 compared to any of those glasses.

 

Some additional comments:

 

1. As others have noted, Yuri's table does not specify the optical design (e.g., oil-, or air-spacing?) or the mating types used in most of the entries compared?  Sacek's examples in the telescope-optics.net reference are quite specific on those points.

 

It might be assumed that all the models in Yuri's table involved oil spacing, since that is what was used for the original FPL53-based TEC 140.  It would be interesting to compare those results with air-spaced designs.  Mating types appear to be designated in two cases -- the no longer favored ZK7 in one, and a reportedly now discontinued OC6 in another.

 

2. Often asked :  If CaF2 in tested designs for the TEC 140 yields results so much better than those with FPL53, why was FPL53 chosen for the original, highly praised TEC 140 series?

 

3. In his replies quoted in my OP for this thread, Yuri's comments on FPL55 and FCD100 were admirably moderate.  He noted that FPL55 "still could [be] perfect for visual use" and that "FCD-100 could be a good choice for smaller objectives."

 

4. Yuri did not include FCD100 in his design tests owing to issues of 'dimensional limitations' and 'multi-melts.'  Roland's description of the new Stowaway 92, an air-spaced triplet using FCD100, includes the specification that its design "was chosen 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."  Color correction specification for the scope is "Less than +/- 0.005% focus variation from 656 nm to 410 nm (c to h wavelengths)."  These specifications seem to cover the 436-656nm spectral range in Yuri's table, with excellent performance across it.

5. Pal Gyulai, optical designer for CFF Telescopes, argues that longitudinal color error can be essentially eliminated with modern glasses (such as FPL53), that spherochromatism now constitutes the 'main problem' to be addressed, and that solutions of both are possible with moderate focal-ratio triplets: <http://cfftelescopes...ptical-design/>

 

Most current CFF models are now based on FPL55.  Presumably the 'Super Planetary' versions of those also employ FPL55, and CFF has found it possible to achieve a very high degree of correction for those as it did for versions using FPL53.

 

Maybe the answer to my question "What's wrong with FCD100 and FPL55" [as replacements for FPL53]?" is 'Not much.'

 

My thanks for the several thoughtful replies -

 

      Larry

 

Edited 03 Sept. 2018 to repair accidental mixing of quote from Element79 with my text.

Edited by LMO, 03 September 2018 - 04:01 PM.

[LMO]

We shouldn't use photopic (daylight) eye sensitivity when evaluating telescopic image. Under night sky, eye shifts toward scotopic sensitivity, and generally is in its mesopic mode. Conventionally, that means further decrease in sensitivity on the red end, and significant increase in the blue/violet. But this model is just a paper model. Some experiments showed that mesopic vision has increased sensitivity on both ends, but significantly more in the blue violet (https://www.telescop...al_response.htm , Fig. 249).

 

As for what Yuri has said, that's very likely in the context of matching glasses that he had on disposal. Fpl53 and 55 have somewhat different dispersive properties, and won't give identical result with any given matching glass.

Your idea that mating types involved in Yuri's table of models were ones he had available is very interesting and relevant to the question of whether mating types were optimized for each 'ED' element in the table.

 

The relative partial dispersions I found for FPL53 and FPL55 were very close:

glass  PFe  Pg,F
FPL53  0.4545 0.5340
FPL55  0.4544 0.5340

 

The Pg,F values for the two glasses given on Ohara data sheets are identical to the number of decimal places listed.  The very similar values suggest that the mating type, BK7, often used with FPL53, might also be a reasonable choice for use with FPL55, yielding close, if not identical results.

 

Values for the relative partial dispersions of FCD100 were a bit different from those of the two Ohara glasses.  But Roland's statements about performance of the new Stowaway 92, suggest that he has identified a very good mating type to be used with FCD100.

 

    Larry

[Element79]

TEC buys their glass and Fluorite in specific batches for the upcoming runs of their various telescopes.  They do not stock glass and would certainly not compromise a future design's performance just to "use up existing stock" even if they did have some in stock.  Other than a higher performance level, one of the reasons that they went with Fluorite is that it, being a crystal, never deviates from its measured properties whereas a glass will vary from melt to melt, and what I understand the ED glasses vary hugely.  This limits the amount of checking the dispersion properties of a shipment of glass and having to vary the design accordingly.