Turn the dec shaft around in the housing, and make sure you identify any lock screws on the side of the ring nut. On my mount there was exactly one lock screw.

Loosen all lock screws properly (but don't remove any from the ring nut!!) before attempting to unscrew the ring nut. The lock screws are accessed as shown in the picture at left. Failure to loosen up all the lock screws before unscrewing the ring nut may destroy the DEC shaft threads, so be careful with this step.