Some have wondered what encoders do for a mount, especially what do the very expensive encoders on AP mounts do to make tracking more precise. Besides eliminating the periodic error of the mount, they also provide real time update of exactly where the axes are pointing, and they do this at a rate of more than 780 per second.
We test every mount in-house before they are sent out so that we know they will meet our PE specs. We do this with a special version of PEMPro that measures the mount axis rotation with a precision lab encoder. These tests take time, approximately 2 hours just for the test sequence. We typically do 5 worm cycles, create PE curves, load them into the CPX controllers and test again with compensation on. In the case of the encoder mounts we also measure encoder tracking performance.
The attached image shows three tracking tests of the same mount (a production unit which will be shipped to a customer). Top is with encoders off and the raw periodic error of the mount. Middle is Encoders off with PEM turned on. The third one is tracking with the encoders turned on.
Compare the values of tracking accuracy with each setup.