Tak FS series are doublets. Sometimes an FS102 comes up. A bit heavy though for 102mm but its a fluorite doublet and quite well corrected.
Doublets usually have a degree of chromatic aberration typically blue. That shows up as coloured fringes on bright objects visually and blue rings around bright stars in images.
Apparently a highly corrected doublet is possible - Agema doublets.
https://www.agemaoptics.com/
The graph comparing various high end refractors is interesting.
An ED80 is a very common starting scope that performs well, is inexpensive and will even do some decent imaging.
Greg.