A method for aligning an image generated by a head or helmet-mounted image display system, including an image intensifier disposed within a line of sight of a user of the image display system, including the steps of: a) displaying to the user a first symbol on an image display; b) aligning, by head movement, the first symbol with a viewable external reference symbol defining a known line of sight of the user; c) recording movement of the image display to align the first symbol with the reference symbol; d) generating a first stabilised symbol and displaying it to the user on the image display at an initial position substantially coincident with an aligned position of the first symbol and the reference symbol from step b), the position of the first stabilised symbol appearing on the image display to remain substantially unchanged relative to the user's view of the reference symbol during movement of the user's head; e) aligning the first stabilised symbol with the reference symbol while keeping the first displayed symbol substantially aligned with the reference symbol and determining the orientation of the image display when the first stabilised symbol is aligned with the reference symbol; f) displaying to the user at least one alternate symbol on the image display; g) aligning, by head movement, the alternate symbol with the reference symbol; h) recording movement of the image display to align the alternate symbol with the reference symbol; i) generating at least one alternate stabilised symbol and displaying it to the user on the image display at an initial position substantially coincident with an aligned position of the alternate symbol and the reference symbol from step g), the position of the alternate stabilised symbol appearing on the image display to remain substantially unchanged relative to the user's view of the reference symbol during movement of the user's head; j) aligning the alternate stabilised symbol with the reference symbol while keeping the alternate symbol substantially aligned with the reference symbol and determining the orientation of the image display when the alternate stabilised symbol is aligned with the reference symbol; k) determining magnification of a forward scene induced by the image intensifier from the recorded movement and orientation of the image display from steps c), e), h) and j). a) displaying to the user a first symbol on an image display; b) aligning, by head movement, the first symbol with a viewable external reference symbol defining a known line of sight of the user; c) recording movement of the image display to align the first symbol with the reference symbol; d) generating a first stabilised symbol and displaying it to the user on the image display at an initial position substantially coincident with an aligned position of the first symbol and the reference symbol from step b), the position of the first stabilised symbol appearing on the image display to remain substantially unchanged relative to the user's view of the reference symbol during movement of the user's head; e) aligning the first stabilised symbol with the reference symbol while keeping the first displayed symbol substantially aligned with the reference symbol and determining the orientation of the image display when the first stabilised symbol is aligned with the reference symbol; f) displaying to the user at least one alternate symbol on the image display; g) aligning, by head movement, the alternate symbol with the reference symbol; h) recording movement of the image display to align the alternate symbol with the reference symbol; i) generating at least one alternate stabilised symbol and displaying it to the user on the image display at an initial position substantially coincident with an aligned position of the alternate symbol and the reference symbol from step g), the position of the alternate stabilised symbol appearing on the image display to remain substantially unchanged relative to the user's view of the reference symbol during movement of the user's head; j) aligning the alternate stabilised symbol with the reference symbol while keeping the alternate symbol substantially aligned with the reference symbol and determining the orientation of the image display when the alternate stabilised symbol is aligned with the reference symbol; k) determining magnification of a forward scene induced by the image intensifier from the recorded movement and orientation of the image display from steps c), e), h) and j).