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Thermo Fisher Scientific

Astigmatism and coma increases while changing the defocus


Astigmatism and coma change while changing the defocus.




These are possible resolutions for the issue:

  • Replaced Extension tube demagnetized in the factory


These are possible causes for the issue:

  • Column contamination

Specimen holder and/or cryo-braids

  • IDU or extension tube magnetized


In first place the system is set to a defocused value, for instance, -2µm. Then, using Sherpa, the astigmatism and coma are corrected. Then at different defocus values the astimatism and coma are mesured. It is seen that astigmatism and coma grows as the defocused value is increased relative to the initial value (-2µm). The astigmatism grows in a clean linear way. Also, the coma increases very rapidly.

  1. WI 1075464. Debris was found at the differential aperture, but this did not solve the problem.
  2. While tilting the sample the beam moved. Specimen holder and braids were changed. Problem still present. 
  3. The extension tube was removed. Problem was not present anymore. Replaced the extension tube, demagnetized in the factory, the problem was solved.


  1. Demagnetizing, in the field, with the welding equipment, results improved, but the astigmatism and coma still increasing. Did not solved the problem. 
  2. With the new extension tube, demagnetized in the factory with the professional equipment, astigmatism and coma remained within specifications:
    • Astigmatism, Sherpa setting: ≤ 6nm
    • Coma, Sherpa setting: ≤ 160 nm

Test performed at 120kX showing the astigmatism and coma increasing while changing the defocus. Experiments were performed in µP/nm at 80 and 200kV. Below it is shown an example. The astigmatism rotation was aprox -90 degrees.


Experiments performed in the factory regarding measuring distortion with and without an extension tube. Also, the demagnetizing experiments with a welding system and with a professional demagnetizing equipment.




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