Hello group, I recently got myself a new controller and motors (Boxdeorfer MTS3-SDI and Astromeccanica AM-V's) to replace my dead Vixen DD-1 and have been fussing about with drift measurements and PEC training. I'm using a webcam at f36 on my 180mm MCT so can really see errors in polar alignment and periodic error in my GP-DX mount and have been intrigued by the amount of R.A. drift due to poor alignment, not as much as drift in Dec but still very significant. I was wondering if anyone is aware of the derivation of R.A. and Dec drift rates for a particular location and define altitude and azimuth error on polar alignment? There's lots of qualitative discussion, mostly to do with the "drift polar aligment" procedure and field rotation but I'm interested in a detailed reduction of the problem ie for a particular star, assuming perfect sideral tracking rate, what R.A. and Dec drift should be expected for a defined polar alignment error. Calculating in reverse would be nice as well ie from an observed R.A. and Dec drift rate for a star, what polar alt-az alignment error exists? It would be kind of nice to be able to adjust these at the mount wouldn't it :) If I could get hold of the math then I might modify my mount to accomodate this. Cheers David