did try at one point a geared head system like what Andrea describes, hoping to remove enough flex to be able to go past 400mm, but I couldn't, not on a consistent basis.
One issue with the Lightrack is that the entire system is on a camera tripod. If you polar align the system and then try to frame the object, you have to be very careful not to move the tripod even minutely, or you lose PA. You can frame and polar align, but then you lose accurate framing.
The other issue was that the altitude adjustment on their mount which attaches to your camera tripod is very difficult to adjust minutely to the accuracy needed to get perfect polar alignment. This became quite frustrating at long focal lengths like 400mm.
All of this is a different way of saying that the system works well with short focal lengths, but quickly gets cumbersome as you go longer. And unless they're doing something different, you are limited to 100 minutes or so before you have to reset the system and reframe the object (and likely redo polar alignment).
I agree that the Redcat 51 will be a nice fit for this, but you should have not very high expectations of going much higher in focal length.
Manfrotto geared heads are very very good in this albeit rather expensive but you get wat you paid for, me thinks.
It is also true about the casual photographic tripod but an heavy duty one (again, manfrotto amongst others) does not move easily. Best of all is to ditch the photographic tripod altogether and go for surveyor's tripods. In my experience they simply do not move on grass at all once put in place. Other sufaces may fare differently. But I agree that the most difficult thing is to get everything framed the way you may want it across the sky. The Gt81 450mm FL should be doable with the counterbalancing system which makes everything easier. What I find really difficult at those focal lengths is centering the subject by dead reckoning (if you don't have or can't use a proper finder).
