Michael, you misinterpreted the graph. It's about the red correction for an f/6 scope, NOT the Vixen fluorite.
In the text Roger Vine explains why some scopes do poor in the red end of the spectrum. NOT the FL80 S however, that's why he likes it so much. Other quality f/8 wish fluorite doublets like the Tak FS/FS also do well in the red. Not ED doublets as he explains. Others have also found the fluorites to be well corrected in red. The TSA and DL do even better, but are very different scopes.
Here is a similar graph across the spectrum of a fluorite doublet.
A quote from the same review you mentioned sums up Roger Vine's opinion nicely:
"The FL80 is, in my view, another example of where fluorite matters. In this case the magic mineral provides a level of correction for chromatic aberration that I have never seen in an ED doublet of similar spec’ (though I’m willing to stand corrected if one turns up). Make no mistake about it, the difference is quite subtle, but turn up the magnification on a double star or planet (especially Mars) and the FL80 performs noticeably better than a fast ED doublet, even an excellent one like the TV-76 or TV-85."
Hope this clarifies the matter.
Thanks for chiming in here. No, I fully understood what he was saying. The reason it caught my eye is because he was, as you point out, talking about how well the Vixen does in this regard, BUT how poorly other, well, I'll just quote:
"The following graph, of optical quality, in the guise of Strehl ratio, at different wavelengths for a fast APO, explains why such scopes can be poor on Mars. To put those Strehl figures into context, 0.95 is excellent, 0.80 barely acceptable. So, in the red (and even yellow to some extent) your expensive, beautifully figured APO lens turns into a piece of bottle glass!"
Now, HERE is where I certainly may have misinterpreted ...Are both of these not "Fast APO's"?
Then, there is this graph from telescopes.net for the TSA:
Looks to me like the red spectrum is the worst of the bunch...but maybe it is all relative, and its still great compared with other scopes?
In his review of the FC-100DL:
he shows another graph in comparison of the FOA-60...looks as though the DL and TSA are pretty similar in the red spectrum...or am I reading that incorrectly? And again, for visual...does it matter?
Edited by pb12, 20 August 2018 - 03:13 PM.