  This technical note aims at optimizing the concentration ratio of
multi-faceted focusing heliostats implemented into a solar tower power plant.
The ideal shape of a heliostat located off-axis in the field is known to be the
local section of a fictitious parabolo\"id whose parameters are varying
continuously with the Sun angular position. We describe an optimization
procedure applicable to those heliostats. The flux densities formed at the
solar receiver and the achievable concentrating ratios are computed using an
improved convolution algorithm. It is shown that the optimized heliostat shape
can produce typical concentration gains of approximately 10%, even when the
heliostats reflect the Sun under large incidence angles.
