Anselmo Pérez Serrada wrote ... Perhaps the mirror could be made somewhat larger with an X marked from corner to corner, thus allowing us to find the center of the light spot even if partly blocked. ...
A design on the mirror surface will not be imaged onto the ceiling. Usually you will want to choose the mirror small enough so that its angular diameter as seen from the dial surface (ceiling) is significantly less than the angular diameter of the sun (0.5 degree). In that case, it doesn't make much difference if part of the mirror's surface is in shadow (or if part is X-ed out). --Art Carlson
