http://www.usa.canon.com/cusa/semiconductor/products/encoder_optolectronic_components/laser_rotary_encoder/x_1m?pageKeyCode=writeReviewPage
X-1M
3333349
Overview
Canon X-1M is the technology leading Laser Rotary Encoder. It is ultra high resolution at 225,000 pulse per revolution with accumulated error of astonishing 1 arc-sec or less. Entire encoder design is focused around utmost precision and accuracy.