An apparatus for assisting in establishing a correction for correcting heterotropia or heterophoria by testing a prescription for correcting the heterotropia or heterophoria before the prescription is actually realized, the apparatus, comprising a simulation device with - an input interface for receiving diagnostic data (d) indicating whether heterotropia or heterophoria is present, classification data (c) indicating which kind of heterotropia or heterophoria is present, magnitude data (m) indicating the magnitude of the heterotropia or heterophoria and prescription data (p) indicating a prism or prisms suitable for correcting the heterotropia or heterophoria, - a generator unit for generating a right simulation image for the right eye and a left simulation image for the left eye where the right simulation image and the left simulation image each represent the same object or scene looked at by the right eye under a right viewing angle and the left eye under left viewing angle, respectively, and where the right viewing angle and the left viewing angle are calculated based on the diagnostic data (d), the classification data (c), the magnitude data (m) and the prescription data (p) such that the right viewing angle and/or the left viewing angle correspond to viewing angles which would be achieved by applying the prism or prisms indicated by the prescription data (p), - a viewing device with separate displays (49A, 49B) or display sections for the right eye and the left eye of the patient for displaying the simulation image for the right eye and the simulation image for the left eye, respectively, and - a means (51A, 51B) which allows for correcting refraction of the patient's eye, said means (51A, 51B) that allows for correcting refraction of the patient's eye being a trial frame and/or comprising at least one wave front manipulator. - an input interface for receiving diagnostic data (d) indicating whether heterotropia or heterophoria is present, classification data (c) indicating which kind of heterotropia or heterophoria is present, magnitude data (m) indicating the magnitude of the heterotropia or heterophoria and prescription data (p) indicating a prism or prisms suitable for correcting the heterotropia or heterophoria, - a generator unit for generating a right simulation image for the right eye and a left simulation image for the left eye where the right simulation image and the left simulation image each represent the same object or scene looked at by the right eye under a right viewing angle and the left eye under left viewing angle, respectively, and where the right viewing angle and the left viewing angle are calculated based on the diagnostic data (d), the classification data (c), the magnitude data (m) and the prescription data (p) such that the right viewing angle and/or the left viewing angle correspond to viewing angles which would be achieved by applying the prism or prisms indicated by the prescription data (p), - a viewing device with separate displays (49A, 49B) or display sections for the right eye and the left eye of the patient for displaying the simulation image for the right eye and the simulation image for the left eye, respectively, and - a means (51A, 51B) which allows for correcting refraction of the patient's eye, said means (51A, 51B) that allows for correcting refraction of the patient's eye being a trial frame and/or comprising at least one wave front manipulator.