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My C11 suffers from mirror shift a bit, it usually gets progressively worse as you move up in mirror size... however adding a crayford fixed the problem completely.. rough focus with the primary mirror, compose subject, perform critical focus via crayford, and fire away...
The williams optics SCT crayford is around $250 if i recall correctly, and the GSO model is closer to $150.... they are a good place to start.
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