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."
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>
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.