An instrument developed to accurately measure small rotations (twist about an optical axis) uses a Faraday rotator to modulate the polarization of a HeNe laser beam. Twist sensors located in the beam path consist of a calcite polarizer (analyzer), a silicon detector and associated electronics. The sensors measure relative changes of the sensor analyzer and the null position of the modulated source beam. Reference measurements of the laser intensity and beam modulation are made to avoid accurately characterizing the modulation amplitude. This instrument has a sensitivity of 0.2 microradians at 5 Hz bandwidths over a 17.5 milliradians measuring range. This is accomplished by controlling the beam wander, reducing the laser divergence, and utilizing synchronous detection.