Gemini AAF Bracket for SLR Lenses/V Dovetail

This is a bracket designed to hold a Gemini astronomical autofocus motor at 90 degrees for use belt-driving focus on an SLR lens. The focuser should mount a spur gear that will drive a closed timing belt (400mm GT2 belt in my case), which goes around the lens's focus ring. For stability and alignment, the bracket wraps the sides of a Vixen dovetail, but this can be cut off quite easily if undesired for your application.

The specific design here is intended for use with a Canon 300mm f/2.8 L IS USM lens at minimum clearance; it should be usable with other lenses if needed, as very few lenses are going to be wider than the 300/2.8. There is a small amount of adjustability in/out to tension and detension the belt; measure the needed belt diameter and order appropriately.

Side-mounting was chosen because I have a top-mounted guidescope, and I did not want to put the lens up on a riser block to minimize torque on the mount.

Print with the focuser support plate pointed upward and the focuser mounting plate horizontal. Use supports on build plate only to provide support to the focuser mounting plate during printing.

UPDATE 9/20/2025:

I printed my first one of these in PLA.  It bent under tension during the summer.  Recommend printing in a more heat-tolerant filament - I used PC.