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

[Nikolaus Grigorieff]

I just noticed that I use the phrase "diffraction" from gold particles.
What I meant was an FFT of images of gold particles. What I am
describing is a defect in the images, not in the diffraction patterns.

   Sorry for the confusion,

       Niko.


On 4/3/2014 8:17 PM, Nikolaus Grigorieff wrote:
> 
> 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.
>