  Planned instruments utilising multi-object adaptive optics systems on the
forthcoming extremely large telescopes require large numbers of high order
deformable mirrors. These devices are a significant cost driver, particularly
if specifications regarding the number of faulty actuators are stringent. Here,
we investigate the effect on adaptive optics performance that such faulty
actuators have, and draw conclusions about how far faulty actuator
specifications (and hence cost) can be relaxed without having a significant
effect on adaptive optics performance. We also provide performance estimates
using a map of faulty actuators from an existing deformable mirror. We
investigate the effect of faulty actuators using an end-to-end Monte Carlo
adaptive optics simulation code. We find that for actuators stuck at a fixed
height above the deformable mirror surface, between $1--2\%$ of actuators can
be faulty before significant performance degradation occurs. For actuators that
a coupled to nearest neighbours, up to about $5\%$, can be faulty before \ao
performance begins to be affected.
