Last night I used my guide-scope and Orion guider (with PhD software) to plot periodic error. The main scope was idle. PhD was set to "Disable Guide Output" and a guide star was selected near the equator. Then "unguided guiding" was started. Below is a link to the output graph produced by PhD. It shows the overall RA periodic error and an abrupt irregularity in the worm that come around every rotation. Good polar alignment is necessary to use a guider for this, and the image-scale of the guide-scope must be known. Conclusion: The Byers gear on my Schaefer mount is OK but definitely not great. Peak-to-peak PE is 16 arc-sec. Here's the graph and a couple pix of the 12.5 inch astrograph. Just click on the thumbnail to see the graph: http://www.pbase.com/paullind/equipment Paul Lind -- See message header for info on list archives or unsubscribing, and please send personal replies to the author, not the list.