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Hi,
I have a 3.5" version on the CDK20 in Ark, Mark has one also on his Tak106. The backfocus usage is more like 40mm (at least with the adapter for the CDK) and the travel is about 30mm. They are fairly hefty units and do work as advertised. However the interface for the camera is non-standard and needs a custom adapter, which is nothing new really in the imaging department. You need a robofocus to motorize it and the backlash is pretty invisible. Still some flex possible with heavier loads but I don't think it affects my imaging train at f/6.8. The robofocus attached in the standard way sticks out and can get in the way of rotating FWs/OAGs. I mounted mine backwards (robofocus motor pointing towards the scope) in a non-standard way and that solved that problem. There are a lot of screws in this thing (100+ IIRC) holding the hinges in a sort of interference fit, I am sure there is a lot of messing around to get the right recipe for the focuser to work properly (no play and doesn't jam), not for the faint at heart DIY.
EB
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