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Hi Mark. The maker recommends these for visual use.
Despite that, I have one on an 8inch f4 and use it for imaging. It's OK for long integration astrophotography, where the atmospheric seeing introduces much more blur than focusing errors. But out of the box it is inadequate for high res imaging (eg planetary), since the average step size on mine (depending on load) is about 40microns, which is about the full focal depth of the optics. I had to reduce the step size electronically to make it usable. For anything above about f5 the standard step size should be OK.
Backlash in the gearbox is about 2-3 steps, which is not too onerous, and the thing is solid and powerful enough to carry quite heavy loads. The hard coating under the bearing surfaces on mine has disintegrated since I use long Barlows with significant side loading - resulting in slightly harsh movement - and the gearbox came loose in the housing (easily fixed), but it is still a pretty good bit of gear and much better than any other focuser I have used. In operation, it is vastly better than hand focusing, but for imaging you may possibly be better off with a stepper motor version, with temperature compensation and repeatable positioning -I will probably upgrade at some stage. Hope this helps.
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