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The point of the offset is to ensure even illumination of the field of view in the eyepiece. This is necessary when you are trying to minimise the size of the diagonal. However, with the GSO dobs, the secondary is usually over generous in size, so that anything beyond the "partial offset" which happens automatically with a collimating eyepiece is probably unnecessary. Note that the offset doesn't do anything for or against the quality of the image in the centre of the field.
Geoff
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