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

[Xing Zhang]

Hi, Niko,

The titan krios in UCLA works fine. We have been using 29,000x and 14,000x
on a  K2 camera, and didn't notice such problem.

Xing

-----Original Message-----
From: 3dem-bounces at ncmir.ucsd.edu [mailto:3dem-bounces at ncmir.ucsd.edu] On
Behalf Of Nikolaus Grigorieff
Sent: Thursday, April 3, 2014 5:18 PM
To: 3dem
Subject: [3dem] Magnification anisotropy at low mag settings on Titan Krios


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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