Hello,
There is a built in correction associated with the focal length, hence the visual doublets coming in at F9+. The faster focal length is good for astrophotography, so its about trade offs. TS-Optics normally review well, Bresser is Explore Scientific partner.
The link below is from another IIS user, and is a great link for understanding the F/L trade offs for CA on doublets.
Steve
https://imgur.com/nEjoLHd