AP Mount Encoder Tracking Tests

Hello All,

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.

Rolando

Roland

Attached Image:

Test of the 1100 encoder mount showing Encoder data points at 1 second intervals and subsequent PEMPro analysis of the data.

Rolando

Attached Image:

Hi, Rolando. Impressive! Wonderful performance and great data reduction and reporting format. We had done some satellite characterizations that required rock-solid stability (absolute) for months and years. Our test-set had to fit on a 5x10 Newport table, collect data continuously and be able to self-recover from hiccoughs, without blowing the initial set points. Required drift-limit of the test article(s) was 0.1 arc-sec. Our test set had a noise floor of a few mili-arc-sec, (long term!)... fine for the job. Anyway, I'm STILL trying to strobe the Crab Pulsar! Your Fourier Analysis (Especially the Power Spectrum) is what I will use to try to sift the Crab's rotational rate from the anticipated gross noise. Your drive characterizations have the same feel. Like the Neutron Star is a giant gear as big as a big city... spinning at around 30 Hz! Tom

Do you have to have the mount and scope set up and tracking in order
to use PemPro?
Or can this be done with the mount just setting on a tripod and running?
Augie