We propose an innovative low-cost mission capable of detecting potentially habitable planets around a sample of solartype stars near the sun. The finding of rocky planets in temperate orbits among our immediate stellar neighbors will be a signature discovery. Our mission will deliver relative measurements of stellar position and motion at sub-micro arcsecond precision. These data, in turn, will reveal the presence of orbiting exoplanets. For the case of our primary targets Alpha Centauri A and B, objects below one Earth mass will be accessible when the end-of-mission astrometric precision requirement of 0.4 micro arcsecond is achieved. TOLIMAN will directly reveal the presence of sub-earth mass planets and constrain it orbit and mass This paper describes the optical and mechanical architecture of the mission, and first order instrument design. We also explain the instrument stability requirements imposed by the diffractive pupil post-processing calibration limitations. Our design baseline is a stable two-mirror telescope that images the field directly on CCD camera minimizing the number of reflections and optical components.