Active optical system approaches offer the possibility of designing large, spaceborne optical systems that weigh considerably less than conventional passive designs. They offer the further possibility of active compensation for on-orbit thermal and mechanical disturbances. However, a key enabling technology is the ability to accurately and rapidly sense optical system misalignments and figure errors. This paper describes the sample point interferometer (SPI), a measurement technique that can meet the sensing requirements of active optical systems. By monitoring the optical path lengths of an array of pencil beams projected to a corresponding array of retroreflectors affixed to the mirror, the SPI can provide simultaneous measurements of rigid body alignment and figure. The SPI uses semiconductor laser diodes as a light source for two-wavelength interferometry, and provides alignment and figure measurements that are repeatable, have a large dynamic range, and can be acquired at high bandwidths.