An apparatus for determining the orientation and/or position of a structure comprises a light source (28) for generating a light beam. A structure (31) is mounted in the optical path of the light beam such that the position and/or orientation of the structure, relative to the light beam, may be altered. The apparatus further comprises first capture means (32) for capturing a diffraction pattern produced by the first structure. Information about the alignment of the structure may be obtained from the diffraction pattern, and the position and/or orientation of the structure (31) relative to the light beam may be adjusted if necessary.