elements to resolve this system instability the key observation is that if equal but opposite torque is applied to the mirror and counterbalance then there should be no net reaction torque applied back on the flexible spacecraft with dual high gain position channels this is impossible to do in a practical sense because of parameter variations one specific problem is the need to exactly calibrate the command cross feed gain g cf which as mentioned earlier is the ratio of the mirror to counterbalance inertia the effective inertias of the mirror and counterbalance depend on their bending characteristics and are therefore difficult to predict precisely a straightforward solution is to feed the commanded torque t cmd sbsb m with a negative sign to the counterbalance and open the counterbalance control loop thus we have equal and opposite torques applied to the mirror and counterbalance this configuration is shown in fig5 b the open loop counterbalance controller in fig5 b appears to be an ideal solution to the control problem as it eliminates the need to cross feed the commanded mirror angle so that the inertia ratio need not be known accurately and it eliminates the possibility of the counterbalance exciting system flexible body modes unfortunately it has the downfall of all open loop schemes in that it is very sensitive to parameter differences in the plant in fact this configuration is unstable the counterbalance will slowly drift away from its center point due to plant differences between the mirror and the counterbalance obviously a closed loop control channel for the counterbalance is still needed the low bandwidth counterbalance controller shown in fig5 c removes many of.