A method of assembling an opto-mechanical system for a head-mounted device which opto-mechanical system is to be at least partially housed within an optics arm of the head-mounted device, the optics arm being provided for securing the head-mounted device to a head of a user, the method comprising: joining an optics display to a first open end of an optics housing , wherein the optics display is configured to be positioned in front of an eye of a user and to display images to the user that are projected onto the optics display through the optics housing; joining a display module to the optics housing so that the optics housing connects the display module and the optics display together and positioning at least a portion of the display module within a second open end of the optics housing, the display module being configured to project the images onto the optics display; moving the display module, while the display module is joined to the optics housing and at least a portion of the display module is positioned within the second open end of the optics housing, relative to the optics display so that a first central axis (A2) extending through a mechanical center of the display module is aligned with a second central axis (A3) extending through the mechanical center of the optics display, wherein the display module is free to move in six degrees of freedom within the second open end of the optics housing for aligning the first central axis with the second central axis; and securing the display module to the optics housing at an interior edge provided at the second open end of the optics housing, when a desired alignment is determined so that the display module and optics display are optically aligned and fixed relative to one another. joining an optics display to a first open end of an optics housing , wherein the optics display is configured to be positioned in front of an eye of a user and to display images to the user that are projected onto the optics display through the optics housing; joining a display module to the optics housing so that the optics housing connects the display module and the optics display together and positioning at least a portion of the display module within a second open end of the optics housing, the display module being configured to project the images onto the optics display; moving the display module, while the display module is joined to the optics housing and at least a portion of the display module is positioned within the second open end of the optics housing, relative to the optics display so that a first central axis (A2) extending through a mechanical center of the display module is aligned with a second central axis (A3) extending through the mechanical center of the optics display, wherein the display module is free to move in six degrees of freedom within the second open end of the optics housing for aligning the first central axis with the second central axis; and securing the display module to the optics housing at an interior edge provided at the second open end of the optics housing, when a desired alignment is determined so that the display module and optics display are optically aligned and fixed relative to one another. <EOS>