A universal truth about telescope mounts is that none of them are perfect. While there is a general trend towards higher tracking precision as the cost of the mount increases, it's an absolute certainty that they will have some mechanical errors. Even the largest observatories in the world need to find ways to minimize the effects of such errors.
One common problem results from imperfections in the gears inside telescope mounts that use a "worm" gear system. This problem is called "Periodic Error" because it tends to repeat itself as the drive repeatedly turns along the worm. By modelling the error and then issuing commands to counteract the error, this type of error can be significantly reduced - but never 100% eliminated.
We are fortunate to have many tools available to determine the error and program an automatic correction for it. One of these is called PEMPro, published by Ray Gralak and CCDWare. I met Ray at the AIC2006 conference a few days ago and was impressed by his knowledge and programming talent. An update coming soon will make this software even more powerful.
I spent a couple of hours in the dome tonight, working on obtaining a Periodic Error model that I will be able to use to develop a Periodic Error Correction (PEC) file to improve the RCX tracking. Although the scope is tracking well already with autoguiding (in fact last night I spent a few hours doing spectroscopy with it), there is always room for improvement. This picture shows the PE obtained by running PEMPro across about 3 worm cycles.
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