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

[Marin van Heel]

Dear All,

This could also be due to astigmatism in the illumination system. Such 
astigmatism would mean that the illumination is not parallel to the 
optical axis in (at least) one direction leading to anisotropic 
magnification effects as per our paper:

G. van Duinen,  M. van Heel, and A. Patwardhan,* Magnification 
variations due to illumination curvature and object defocus in 
transmission electron microscopy, */Opt. Express /*13 *(2005) 9085-9093*. *

Hope this helps,

Marin

=======================================================

On 04/04/2014 02:17, 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.
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> 3dem at ncmir.ucsd.edu
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-- 
================================================================

     Marin van Heel


     Professor of Cryo-EM Data Processing

     Leiden University
     Cell Observatory
     NeCEN Building Room 05.27
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     2333 CC Leiden
     The Netherlands
      
     Tel. NL:     + 31 (0) 715271424
     Mobile NL:   + 31 (0) 652736618
     Skype:       Marin.van.Heel

     and:

     Professor of Structural Biology

     Imperial College London
     Faculty of Natural Sciences
     Division of Molecular Biosciences
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     email:  marin.vanheel(A_T)gmail.com
     email:  m.vanheel(A_T)ic.ac.uk
     and:    mvh.office(A_T)gmail.com


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