Story continues.
Not having succeeded (enough) with my BDT, I then went over to the dark side and spent $$$ on 2nd hand gear. I really really really wanted my backyard Hubble telescope photos. And I wanted them NOW.
The only affordable set on Trademe then was an EQ5 Goto with retrofitted GotoNova handcontroller (now iOptron) and an 8" newton. NZD 1000. The owner had used it for visual and that had worked for him. I took it home, got familiar with the scope tech and the weight... 8kg was the scope, I remember, all included (tube rings, finder, focuser).
On an EQ5... 'nough said.
Or not?
Well. My camera, mounted piggy back onto the scope, certainly gained exposure time before star trailing. 2 minutes at ~18-55mm was no problem. But the motor and the stability of the EQ5 didn't appreciate being ridden by an 8kg ~1m long tube with the majority of the weight being far away from the tripod, and reasonably heavier at the very top end of the tube.
So the 20 secs exposures I took in prime focus = camera mounted onto the focuser, all showed dips in very regular intervals. As if jumping from one little tooth to the next was heavy duty for the little wheels inside. Broke my heart.
The mount's load capacity for visual must have been something between 9 and maybe 12kg. Half that weight in a scope would have been proper astrophotography standards. I did know that from reading threads on iceinspace. But I felt that wouldn't apply to me. Like with cancer from smoking. It only ever happens to others.