[3dem] Newbie question - Defocus mistmatch

Eduardo Sanz Garcia esanzgar at gmail.com
Fri Oct 24 17:17:15 PDT 2008


Dear 3DEM users,

Has anyone observe a mismatch between the defocus values reported by the 
electron microscope software and the real defocus values computed from 
the powerspectrum?

Example of a pair of micrographs:

First micrograph (this one is the reference):

    * Reported defocus value by the microscope: -2 microns
    * Defocus value computed from the powerspectrum: -2.2 microns

I use the "series" option to change the defocus and take the other 
micrograph.

Second micrograph:

    * Reported defocus value by the microscope: -4 microns
    * Defocus value computed from the powerspectrum: *-2.6* microns
      (here is the problem, it should be closer to -4 microns).

We use a Tecnai F30.
Is this a problem of  hysteresis?
 
Thank you very much.

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