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My understanding is, as there could be a slight variation in the actual centre of view between with and without the coma corrector, if it's collminated with it in then the visual centre (path of light) will be correct, while ever it is being used. Which is really the point of collmination, making sure the light is directed to a single point. A coma corrector added at a later date has the potential to shift that point if its optical centre is different. Well at least that how I read it 'some where'. Happy to go with a popular consensus on this one though.
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