thanks for the valuable inputs.
About 8 years ago I have purchased the cheap Chineese rich field 6" f5 SkyWatcher refractor. First thing to be replaced was the diagonal, and the cheap EPs included. The useless focuser has been upgraded with Baader to get it into focus.
Now, the refractor works well with the Astro Professional 5mm (ED) EP without noticable color fringes at 150x magnification. M13 is nicely resolved, and the Ring Nebula shows the donut with contrast. The Televue Zoom can ramp up the magnification to 180x, but without showing much more detail added.
I'd say, what matters with the cheap refractors is, which EP suits best. This was my expensive lesson learned.
Regarding the refractors for the astrophotography, any fast (f#) triplet above 4" is expensive. The are still lots of discontinued fast telelenses on the market, like the Pentax 4/400mm for the 7cmx6cm format. Using convertor rings, they can be mated with any DSLR. I am using the Leica APO 3.4/180mm, and going to use the Carl Zeiss Sonnar 4/300mm. But my objective is the rich field astrophotography.
If I could afford it, I'd take one of those large Leica APO-Telyts,
Leica R 1:5,6/800 mm Apo-Telyt-R ROM 11842 + 11845 - SH photo GmbH
Leica R 1:2,8/400 mm Apo-Telyt-R 11842 + 11843 - SH photo GmbH
with 142mm lenses.