Just a bit of lateral thinking and not strictly digiscoping. But the Pentax Q interchangeable lens camera now has third party adapters for other makes of lenses. Being effectively a compact size sensor with a 5.5x crop factor. This means if you had a 300mm DSLR lens and put it on the Q with the appropriate make adapter you would have a 1750mm equiv lens. Extrapolate that to something like my Sigma 150-500 which you could put on the tripod and fit the camera to it you could get 2750mm with a camera with pretty good high Iso performance. I don't know what the quality would be like but I've ssen some pretty impressive moon pics with it. Just a thought and apologies if similar elsewhere but not allowed to do a search with the single letter "Q".