A pair of plano-aspheric lenses can be used to transform a collimated, radially symmetric, Gaussian beam to a radially symmetric flat-top beam. Diffraction of the output beam due to the choice of irradiance profile, as well as the finite aperture of the optics, must be considered if a propagating beam is required. Choosing both lenses to be positive, one can show that the aspheric surfaces are strictly convex, which facilitates fabrication by magnetorheological figuring. A fused silica lens pair is demonstrated, which can be used at any wavelength from 250 to 1550 nm to transform a Gaussian to a flat-top beam. Measurements of both the irradiance profile and phase of the output beam are presented and compared to the ideal design. These optics transform 78% of the total input beam power into the flat-top region of the output beam, which is uniform to better than 5% rms. For applications requiring uniform illumination, this represents a fourfold improvement in power utilization over the Gaussian input. The output wavefront is flat to a quarter wave at 514 nm, resulting in a beam that propagates approximately 0.5 m without significant change in profile.