Home and PEC Sensor
The home sensor provides a repeatable alt-azimuth zero position for remote operation. When the mount is powered up, the motor control searches for the leading edge (zero point) of the homing masks. A mask is rigidly attached to the polar axis and declination axes of the mount. When the motors located the zero point, the telescope will have a specific orientation, usually this is on the meridian (RA) and on the celestial equator (DEC).
With the local time and longitude of the observing site known, you know where the telescope is pointed in the sky, i.e. the RA and Declination in the sky. This is all done with no operator assistance. This gives one remote capability.
The PEC sensor simply records each rotation of the worm. There is a small slit in the disk on the end the the worm shaft. When this slit passes the sensor, the worm has made one rotation. Knowing the measured period of the worm, a periodic error correction can be correctly applied for each rotation of the worm. You always know where the worm is in its cycle.
There is no need to calculate the worm period (prone to timing errors) and then use this to determine the worm phase. A PEC mask allows one to measure the worm phase directly. Even if you lose power, the PEC mask correctly records the worm period, once power is restored. One always knows where the worm is in its cycle.