It ended up with a medium format lens, the Pentax 645 300 mm ED f/4 lens. First I tried the Canon 300 mm L f/4. It was sharp but too heavy vignetting, that's the reason I switch over to the medium format lens.
One test shot from light polluted area (Bortle class 9):
http://www.astrofriend.eu/astronomy/...n-cluster.html
It works fine with USB-focuser motor too. Control it from a Raspberry computer. I also bought a new mount, the HEQ5 mount, save some kilograms with that one.
In october when we get a dark sky agian I have to find a good battery to the equipment, maybe a 20Ah LiFePO4 battery, have no experience from that kind of battery yet. It must last for 10 hours at least. The equipment take 1.5 Amp at 12 volt.
/Lars