There
ARE definitions of an APO telescope. The "original" one, by Ernst Abbe (Zeiss), and a more modern one championed by late Thomas Back. Google it.
"An objective in which the wave aberrations do not exceed 1/4 wave optical path difference (OPD) in the spectral range from C (6563A - red) to F (4861A - blue), while the g wavelength (4358A - violet) is 1/2 wave OPD or better, has three widely spaced zero color crossings and is corrected for coma.
By whichever one you go, it is definitely possible to make an APO using just two lenses. That is, three colors crossing, and correction for spherical at two wavelengths."
What you CANNOT do, is have a
fast, well corrected APO with only 2 lenses.
Here's a 6" APO using only two (extreme) types of glass with FOUR color crossings (no Fluorite needed
).
Note:
is f/14, and one of the crossings is in IR and another in near UV.
But by all definitions this doublet is
better than APO.
I can give you at least half a dozen designs of doublets with 3 color crossings, some of them can be as fast as f/8. And if we reduce aperture, they can be even faster. What 3rd lens allows us is to make lenses less steep/curved (easier to make, keep collimated, and less spherochromatism), and also faster for same amount of color error.
PS you should stop reading what "other people" say in popular groups. There's plenty of proper, peer reviewed information available on the web, as well in old fashioned books (gulp). Look up Vladimir Sacek, Roger Ceragioli, Harrie Rutten & Martin van Venrooij. There you will actually find a "formal" definition of a semi-apo (based on color error). Its roots have also been established by Zeiss.