The control of normal displacements of the telescope mirror surface being under the actions of the mass forces and support reactions, is considered basing on the reaction assignment. The method for determination of the mirror surface point displacements and its root-mean-square deviation from the preset surface is suggested. The method gives an opportunity to minimize the support reactions, as well. The method was applied for calculation of the homogeneous elastic ring plate with constant thickness taking into account its shear. Three variants of the plate fixation were investigated. The problem was solved in dimensionless form without shear and for three values of shear.