[3dem] Magnification anisotropy at low mag settings on Titan Krios

Nikolaus Grigorieff niko at grigorieff.org
Thu Apr 3 17:17:48 PDT 2014


Dear Colleagues,

We have recently noticed a problem with anisotropic magnification on one
of our Titan Krios microscopes. When recording data at a nominal
magnification of 29,000x, there seems to be an image distortion that
produces variable magnification in different directions of the image.
These variations were estimated using diffraction from gold particles to
be about 2%, a significant amount especially when working on large
assemblies such as viruses. The distortions can be approximately
corrected using image interpolation but this is not desirable, of
course. In one case, the resolution of a 700 Angstrom virus
reconstruction with data collected on a Gatan K2 direct electron
detector improved from 7 to 4 Angstrom after correcting for the distortions.

The severity of the distortion depends on the magnification setting. At
37000x magnification the magnification anisotropy is about 1% and
59,000x it appears to be undetectable. Since most Krios microscopes are
only calibrated for magnifications of 59,000x and higher, it is possible
that the problem we have observed also occurs on other instruments. This
will be particularly relevant for instruments that operate with the K2
detector mentioned above. The pixel size of this detector (5 microns)
usually demands magnifications settings of 29,000x and lower where the
distortions are significant. Users of detectors with a larger pixel size
(e.g. the Falcon direct electron detector) are less likely to experience
the distortions since they will typically use magnifications of 59,000x
and higher.

FEI have acknowledged the problem but at this point the cause is not
clear. We hope that the distortions can be corrected with a simple
recalibration of the projector lenses. We would be grateful if other
Titan Krios users could share their experience and possibly check if
distortions are detectable at lower magnification settings.

     Thanks,

          Niko.


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