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Mike, just to clarify, the image shift that occurs with a SCT is different from image wiggle in a Newt. What happens visually is that the image will move across the FOV and often out of the FOV. as already stated the mirror moves when doing coarse focus and it is this movement of the mirrror that causes the image to shift. This is very frustrating as you need to move the scope in RA and DEC to recentre the image. My scope has considerable image shift now after 12 months, so much so that in a 26mm EP it will shift from centre to almost out of the FOV. The pattern is a little consistent and this means I can account for it most of the time. The only time this becomes a real problem is under high magnification for planetary work when going from the EP to a powermate insertion. The coarse focus must be changed and the image will shift out of the field of view and so focus must be done incrementally interspersed with moving the scope so that the image stays in view.
However once the coarse focus is achieved the micro focusor works perfectly to get sharp focus. You don't get any image wiggle or movement. The reason for this is that the MF is screwed to the visual back of the scope and only moves in and out like a Newt Focusor. It does not have any relationship with the Mirror which must move to obtain coarse focus. The motors of the MF are very smooth and do not vibrate. The amount of travel that the MF has amounts to around 15mm at most from close in to full out. Unfortunately it has limited applications in relation to Prime focus photography. Mounting a camera on the MF will prevent the camera from passing through the forks and will result in either damage to scope or camera. So when imaging in prime focus the issue of image shift rears its ugly head yet again.
Hope that this explained image shift better.
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