We describe an approach to the analysis of the optical system of the solar tower concept, which is designed to provide maximum design information. By inputing all quantities in terms of angles or dimensionless ratios the results remain relatively scale independent. First the power redirected from the heliostat field to the central receiver is computed, then solar images are projected from each heliostat to the central receiver and interception factors are obtained. This information allows us to size the central receiver, to enhance performance by redistributing heliostats in the field, and finally to define the rim angle of the field. The procedure for this step-by-step definition of the solar tower optical system results in optimal heliostat fields, receiver flux distributions and diurnal power curves.