On-axis Absolute Encoders vs. Motor-based Encoders.

Paramount's equipped with on-axis absolute encoders know where they are at all times.  Paramount's with incremental encoders (or motor-based encoders) know only how far they have moved since they were initialized using a nightly process called homing.

The crucial additional difference is that the on-axis absolute encoders are fitted directly to the mount axes themselves, bypassing the worm drives.  Paramount's with incremental encoders are fitted to the worms instead, which means that with incremental encoders alone, periodic errors in the worm drive show up in the tracking.  These mounts must be calibrated and corrected using a process called periodic error correction (PEC), which measures and records the repeatable gear train errors and removes them.  (TheSky features advanced, multi-harmonic PE detection and PEC calibration that minimizes periodic error in mounts with incremental encoders.)

For mounts with incremental encoders, homing re-zeros the encoders when precise sensors on each of the axes are reached. From then on, the incremental encoders are in effect delivering absolute readings.

However, the crucial additional difference remains, namely that the incremental encoder system is behind the worm drive, and periodic error correction is still required to get the best out of the RA tracking.

High-resolution on-axis absolute encoder technology is significantly more expensive than incremental encoders.