Quote:
Originally Posted by gts055
Fpl 51 or Fpl 53 means nothing. Its the mating elements used in the design that give maximum correction to the parameters sought. The only advantage of using FPL53 over FPL51 is that a shorter f ratio may be achieved in using FPL53 glass. The same color correction can be achieved using FPL51 at an albeit longer focal length. Mark
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In a hypothetical debate I would agree with you, but in this case we know exactly the parameters of the scope in question. I'm sure everyone has seen what is capable from an F7.5 FPL-53 doublet like the orion ED80, and it is very plausible that the CA is present here because you have a slightly faster focal ratio of F7 combined with a glass that has a significantly reduced abbe number, FPL-51.
The pictures posted above reminded me alot of pictures I have seen taken with the WO ZenithStar 80 II f/6.8 ED Doublet, which also uses FPL-51.