Solar thermal power plants have a large potential, particularly in the sun belt countries around the world. The objectives of this study are to identify the heliostat fields for solar tower plants with the lowest specific energy cost and to investigate the impact of field size on the energy obtained. The design of heliostat fields is based on a radial staggered layout, which ensures no mechanical touching and less blocking losses. The field is optimised with a multi-objective optimiser (MOO) based on two objectives (specific energy cost versus investment cost). The resulting Pareto curve shows all the optimum solutions, from small fields with small investment cost to large and costly fields. Results show that with the method chosen, solutions with lower specific costs compared to published test cases can be achieved. Examples of fields along the Pareto curve are shown and the relationships between the decision variables and the energy concentrated are discussed.