Example of the multi-wavelength gradient descent algorithm not finding the correct solution. The scalar ZWFS is used with wavelengths 600 and 1000 nm and the mask consists of a dot with 5π/2 phase shift and 2 λ/D dot diameter at 600 nm. (a) Input wavefront of 168 nm RMS. (b) Residual error after reconstruction of 38 nm RMS. (c) Calculated gradients at convergence at each wavelength and the overall gradient obtained by averaging.