  Image computation is a fundamental tool for performance assessment of
astronomical instrumentation, usually implemented by Fourier transform
techniques. We review the numerical implementation, evaluating a direct
implementation of the discrete Fourier transform (DFT) algorithm, compared with
fast Fourier transform (FFT) tools. Simulations show that the precision is
quite comparable, but in the case investigated the computing performance is
considerably higher for DFT than FFT. The application to image simulation for
the mission Gaia and for Extremely Large Telescopes is discussed.
