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Hi Richard,
Will have to do a similar thing shortly with my 16" primary. It currently has a cross marked in the centre with a permanent marker but I want to accurately place a stick on doe nut.
The method suggested with my collimating tool instructions, which sounds reasonable, is to find a sheet of reasonably stiff paper, place the mirror on it and mark out a disk by running a pencil around the edge of the mirror. If you cannot source paper large enough then tape a couple of sheets together.
Cut out the disk, fold it in half then half again to form a sort of acute triangle then snip off the point, unfold the paper and you have a disk with a square hole in the centre, lay the paper over the mirror, secure it with tape to the mirrors edge in a few places so it cannot move sideways, (obviously any pieces of tape would be adhered to the paper prior to placing it over the mirror so no pressure would be exerted on the mirror surface). You now have accurately defined the centre of the mirror and could use the cutout to place a doe nut or mark a spot.
It goes without saying that if this was attempted the surface and whole adjacent area that the mirror would be placed on should be wiped down with a damp cloth so there is no dust or loose crap in the region.
Hope this helps.
Regards
Trevor
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