[Kevin Sevcik]

What everyone else said, but potentiometer or absolute encoder instead of just an encoder, unless you can guarantee the shooter will always be at the same angle. Most encoders are incremental, so their zero position is wherever they are when your robot code boots up. Or reboots, if there's a serious brownout.

We drove a potentiometer via a three bar linkage before, when our frisbee platform pivoted around a simple hinge and was driven with a leadscrew. One arm attached to the potentiometer shaft, then a linkage from there to the shooter platform.