[raymo]

With achromatic refractors it becomes harder to control CA as aperture increases, but
f ratio is kept the same; hence my 102mm f/5 has more CA than my 80mm
f/5 does, and a 120mm f/5 has more still. Its a matter of compromise
really, more aperture is undoubtedly better, and can be used satisfactorily
on objects that are not too bright, and you can reduce CA on brighter objects
by stopping the scope down to say 70 or 80mm. CA increases dramatically with magnification, so when using higher powers you can reduce CA by using for instance a 20mm + 2x barlow, rather than a 10mm, as the barlow
doubles the f ratio to 10.
F/10 refractors obviously show less CA, but it is by no means eliminated.
You have to go out to f/15 and beyond for it to become negligible.
I've run out of time, so somebody else can answer more of your questions.
raymo