Thanks to David and Tyler. Will check out flexure as possible cause of image shift while using the autoguider. I have in the past used eyeball guiding with a second scope and I am in the habit of bolting everything together as tight as possible but SCT mirror movement is a possibility as is flexure. In regards to my JPEG binning question; Large says 3888X2592 pixels and Medium says 2816X1880 pixels. What have they done to give a smaller pixel count? That ratio is 0.724. Does JPEG 'chuck' pixels, change bit depth and call it a change in pixel count or what? Again just curious and can't seem to Google an answer. Now with some off and on clouds in S UT I'll spend some time processing a couple of images. Shot M-8 using a Pentax 67 200 mm f4 lens coupled to the EOS 40D at ISO 800. Thanks again, Steve