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Thanks David & Joshua,
Since the change from 46% to 22% tilt was due to the aperture mask, I'm fairly sure it is an optical problem. CCDInspector has (for me) given variable results depending on the amount of stars in the image.
The camera is bolted to the dovetail plate, square, both across the plate and vertically. The lens was attached and the camera placed on it's back with the lens vertical and the front clamping ring fitted with the screws just touching the lens, ie. no strain on the camera adapter. The lens cannot move anywhere near enough to cause that amount of tilt.
David, I had thought about adding a spacer, taking into account the filter but I think your comment about the IS mechanism might be a better clue. How does one 'power up' said mechanism? That could also possibly explain my T-Point calibration run last night. Last time I used the MX mount was with my camera and a Pentax 300mm Prime and the Sky RMS was 4.3 arc sec. Last night with the Canon, I couldn't get it better than 43.6 arc sec! The scatter diagram was all over the place suggesting something may be loose.
I have a small Surface Plate on order to do as Joshua suggested and accurately check the flange and adapter thickness, spacing etc. but that won't arrive for a week or more, probably at the same time as the next clear sky!
Thanks again,
Charles
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