I have now been using a Telescope Drive Master (TDM) on my Vixen GPD mount for a few months and the results are very impressive. I now have a mount that has a RA "periodic" error of +/-1arcsec. In addition, I built a converter to allow me to have the RA encoder do double duty to provide a signal for the TDM for periodic error correction (<2.5arcsec p-p) and also a signal for my Argo Navis for position information (20,000 tic/rev). To match the RA resolution, I picked up a surplus encoder off ebay and mounted that on the DEC axis to get the same resolution - thanks to Bob Grant for helping me with that challenge! With this combined capability I can quickly polar align to with a few arcmin using the Argo Navis (four star fit), locate objects no hassle to within the same accuracy and do unguided exposures to between 5-10min using the Stellacam3. Hope to show some of you this when I get a chance to attend a star party or two this year (certainly the AstroFest at Bryce!). I've loaded a few images of the kit in my gallery for those interested. Cheers David