Meridian Tracking Problem

Good subs to 20 minutes and longer when well off the meridian but within 10 degrees very poor tracking neccessitates drastically shorter subs even at FLs as short as 135mm.

Above: Mount is shown in the meridian position where tracking quickly goes bad. All images show Canon EF135mm f/2.0L mounted with SBIG ST-8300M and FW8. All screws and bolts are tight and have been checked numerous times. The problem happens EVERYTIME! It is totally repeatable. Onset is very quick but precise angular meridian distance depends, logically enough, on sub length. With 5 minute subs at 135mm it goes unnoticed. At 20 minute sub length it is glaring and long subs must be discarded.

Above: Details on camera mount. ST-8300 and FW8 are held tight against the Losmandy dovetail with a stainless steel 1/4 x 20 screw. Galvanized washers are inserted to prevent any possible rocking from side-to-side. I have had this tracking problem for quite some time, originally with a mounting system that might well have been prone to flex. I have come to the mounting system depicted in increments but nothing has improved the situation. I can not see how this mounting can flex, especially at such a short FL as 135mm.

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