There is a definition of apochromatism which was developed more than a 100 years ago. It does not mean the objective will be free of chromatic aberration or even free of false color visually.
"May I point out that this original definition: "bringing 3 wavelengths to a
common focus and be corrected for spherical aberration at two wavelengths" came
about historically because the very first apochromats actually did this -
thus the definition was made to fit the example."
I like this line "Finally, it is not the ED or Fluorite which determines the overall correction, rather it is the mating element." everyone is forgetting about the mating element.
Yeah, people get swept up in that one particular aspect of perfectionism. People might complain about a doublet because it has a tiny residual color error, but forgive a fancy triplet for being so expensive and heavy that you have to give up a bunch of aperture to choose it. Never mind that the aperture lost reduced the detail you can see much more than the doublet's color error did. And the figure is also a huge factor.
I really think that the only people who need to stress about perfect apochromatism are hard-core imaging folks and the few people for whom even the tiniest amount of purple grates on their nerves like an off-tune instrument. Which is a pity for them, because there are a lot of great achromats and imperfectly corrected apos out there.
Classic telescopes are never mentioned nowadays especially with newbies.