A description is given of the design and performance of a quadrant detector offering improvements in sensitivity and ease of construction over earlier devices. The optics are compact, and low-noise high-sensitivity operation is achieved with the use of avalanche photodiode detectors. The device is installed on the SIS spectrograph on the CFH 3.6-m telescope and allows image stabilization to be achieved by a tip-tilt mirror system using guide stars as faint as magnitude 18.5.