Hi Andy, it does depend on the size of sensor. The larger like a DSLR the more critical it has to be but 1 or 2 mm is not a problem in most cases. The 0.8 Reducer needs 55mm back focus so you will need the sensor to class measurement from ZWO for the camera then add spacers tp make it 55mm to back of 0.8 reducer not the lens inside.
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