Objective lens beam tilt

Coma is due to the electron beam being tilted away from the optical axis of the objective lens, the coma-free axis. The coma results in lattice fringes related to +g and -g being shifted by the objective lens. Lattice fringes belonging to different beam pairs, +g and -g, are shifted differently resulting in an asymmetry of the HRTEM image. 

Tilt the white light swing arm back and focus the laser beam in the back focal plane of the objective by moving the fiber optic mount holder in the TIR-FM illumination port. When focused, the excitation light exits the lens as a column of light and will project as a tight spot on the ceiling Notes 9-11). 

Fig. 6. Laser diffraction microscope (LDM) and images of dislocations in an fee colloidal crystaL (a) Schematic diagram of the microscope. A laser beam with wave vector ko passes through the crystaL A diffracted beam, with wave vector k is imaged on the screen with an objective and projector lens, (b and c) LDM images of dislocations. The upper left insets show the diffraction pattern (° transmitted beam arrow the (220) diffraction spot used for imaging). These insets show the relation between the scattering vector q = k—ko and the reciprocal lattice vector g. In (c), the sample is tilted to produce an excitation error s = q—g that produces an inversion of the diffraction contrast. From Schall et aL [27].

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