Originally Posted by San Rensho
Lay the rim on a glass table to see if it lays fairly flat, a mm or 4 is not a big deal, and check the diameter at a few points.
Lay a straightedge on the table first. They can be way out of flat. If you have access to a cabinet saw, that table would be a reliable reference.
/astronomer who has stressed out over mirror cells and glass flex before...