An annular pack for a cylindrical member for use in a wellbore includes a housing adapted to be mounted about an exterior circumference of the cylindrical member. A detrack-value setting unit is configured to set a detrack value for focusing an objective lens in the medium based on the jitter value. In a preferred aspect of this method, at least one of the blades of a rotationally imbalanced blisk is treated with the chemical etchant for a period of time sufficient to change the chord andor thickness of these blades so that the blisk is rotationally balanced.