This design included the electronics and opto-electronics for an Autocollimator known as THEAC. In this design a modulated infrared LED emitted light through a mask and was projected out toward a mirror whose perpendicularity to the optical axis was to be established. The bundle of rays from the LED had a square cross section and upon reflection from the mirror returned along the optical axis striking a beamsplitter so the rays were redirected to the silicon quadrant detector. The object in this case was to center that square cross section of light on the quadrant cell. To accomplish the design, the LED was modulated at 3 kHz so that is light could be distinguished from ambient lights. Then the signal from the silicon detector was passed through a set of four preamplifiers, one for each quadrant. It was then synchronously demodulated and collected in four integrators. Simply stated, the four integrator outputs were then grouped into sums and differences, and normalized, so as to produce orthogonal horizontal and vertical components. The resulting outputs were passed through a set of selectable low pass filters, and made available to the user via front panel indicators and connectors.