Abstract : As feasibility study has been conducted to investigate whether compact adaptive optical (AO) systems based on the use of transparent wavefront modulators are viable. The results presented here consider the generic properties of suitable wavefront sensor technologies and establish sufficient conditions for their exploitation. Schemes by means of which these properties might be exploited in a multi-conjugate AO system are then considered. It is shown that the prospects for such systems are good, but further work is required to establish necessary conditions and to optimize the system.