Quote:
Originally Posted by casstony
FPL51 is still a very useful glass type and I doubt it will be disappearing anytime soon. FPL53 allows the same performance as FPL51 (both with apropriate mating elements) at a shorter focal length. A lot of factors contribute to making a good refractor including cost and availability of ED glass types and mating elements, care taken with grinding and polishing and stability of the lens cell.
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Vic Maris at Stellarvue has said many times that the mating element is just as important as the "headline" element of FPL-53, FPL-51 or whatever. It's also dependent on how well put together they are. I've seen this myself with my SV110ED which has less colour error than a William Optics 80ED of the same focal ratio and both using FPL-51. All other things being equal, the Stellarvue should show more colour since it has a larger aperture, but it does not. Hence all other things are not equal, and one of those things is the additional effort Stellarvue puts in to get the lens "as good as it can be" in Vic's own words.
In terms of getting rid of FPL-51 stocks, I've read that the price of Japanese "FPL" glass has increased significantly and therefore it's only getting more and more likely that the cheaper scopes will be using FCD1 or FK61 or other Chinese equivalents, which may or may not match FPL for colour correction, homogeneity or any other characteristic you care to mention.