IIRC, the distances for flatteners have +/-2,5mm tolerance. For dedicated
reducers (larger scopes), it is +/-1mm. Based on my experience, the
distance is most critical with large chips as - when you move out of the
tolerance zone - star images start to detoriate off axis first. The
smaller the chip, the lesser the need for exact spacing, but off course it
is recommended to stay inside tolerance zone. Anyway, with just 1mm
additional light travel, you are within the required zone for flattener.
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