The Series 9480H from Gurley Precision Instruments, Troy, NY, is a robust, metrology-grade optical encoder with SS construction and multiple read heads. The encoder is designed for very precise positioning of rotary tables on machine tools, coordinate measuring machines, dividing apparatus, antennas, and test equipment. It generates up to 7,200,000 counts/rev. (after your 4X quadrature decode), for a resolution of 0.00005° or 0.18 arc seconds. Accuracy is ±2.5 arc seconds.
Metrology-Grade Angle Encoder from Gurley
Gurley Precision Instruments