We have performed Monte Carlo simulations of hard spheres on a pattern under gravity. We found that a crystal formed at a moderate gravity strength contains essentially no defects while the one formed at a higher gravity strength contains a significant amount of defects. This result suggests the possibility of using gravitational tempering in a colloidal epitaxy to reduce the number of defects in the colloidal crystals. Moreover, we wish to emphasize on the possibility of obtaining a perfect crystal.