We present the open and closed-loop results obtained with a set of proportional and integral SISO controllers designed for a multivariable large-scale system once its dynamics has been decoupled. This plant corresponds to the model of the Gran Telescopio Canarias's primary mirror. By means of a decoupling procedure consisting of a change of basis and based on the system eigenvalues multiplicity, the MIMO plant, whose inputs and outputs are strongly interconnected among them, is converted into a set of SISO independent subsystems. We also show how robust the closed-loop decoupled system is when including in the model uncertainties in the dynamics.