For primary mirrors of Newtonians without center marking, the center must be determined "visually"

For Newtonian primary mirrors without a center mark, the center must be determined visually. For this purpose, we usually use the mirror's three mounting brackets. These brackets allow for a good estimate of the center.

For a more precise measurement, the primary mirror must be removed and the center determined by measurement. It is then sufficient to mark it with a permanent marker.

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