In this paper we discuss optical considerations and present design simulation results for a self-tracking (passive) solar concentrator. The self-tracking mechanism uses a reversible in-plane paraffin thermal actuator to couple short-wavelength light into a lightguide at the position of the solar focus. By splitting the solar spectrum using a longpass dichroic faceted reflector for actuation energy, this device adaptively self-tracks and concentrates solar light into a planar waveguide.