... The very demanding goal of 50 milliarcseconds root-mean-square (mas RMS) for the astrometric precision of the Sloan Digital Sky Survey implies that focus and collimation motions of the secondary must be very smooth. The Apache Point Observatory 3.5-m telescope secondary is a hot-gas-fusion borosilicate lightweight (25% of solid) mirror. ... Differential rotation provides tilt motions. ... Also, we made measurements with the indicators sensing the motion of the whiffletrees near actuators 1 and 2. ...