Tecnai on-line help manual -- Alignments - UT Southwestern

<strong>Tecnai</strong> on-line help Alignments 20<strong>Tecnai</strong> 12 Software version 2ProcedureThe gun tilt pivot point alignment consists of four steps:• A step for preparation.• Two steps to align the pivot points for the x and y directions.And a final step to redo the gun tilt itself (because the pivot points affect the gun tilt, the latter step canbe necessary).4.3 Gun shiftPurpose: Shift the electron beam sideways so that it comes down along the optical axis.Importance: ESSENTIAL for minimizing movements between different spot sizes and for having thebeam correctly along the optical axis for all spot sizes.Method: Minimize spot displacement when spot size (that is focal length of C1 lens) is changed.ProcedureThe gun shift alignment consists of two steps:• A first step in which spot 9 is centered with the beam deflection coils.• A second step in which spot 3 is centered with the gun deflection coils.The two steps are repeated until the change in gun shift is very small. In the gun shift procedure thespot-size dependent gun shift values for spots 3 and 9 are reset to zero (for proper alignment the spotsizedependent gun shift should therefore be done after the gun shift procedure).DescriptionGun alignment consists of two parts: gun tilt and gun shift. The gun tilt alignment makes sure that theelectron beam enters the microscope (that is, the C1 - spot size - lens) parallel to the optical axis, whilegun shift ensures that the beam goes through the center of the C1 lens.When spot size 9 is used (or any spot size above that), the C1 lens is strong. Under these conditions allaberrations of the microscope system above it are demagnified by the lens (C1 works in the oppositeway of the magnification system; instead of magnifying the image, it makes the image of the source - theelectron gun - smaller). Thus the demagnification of the misalignment of the gun shift is much smaller forspot 9 than for spot 3, so spot 9 gives the reference ('no' gun shift) and spot 3 defines the gun shift itself.