[3dem] Magnification anisotropy at low mag settings on Titan Krios
Marin van Heel
marin.vanheel at googlemail.com
Thu Apr 3 23:10:49 PDT 2014
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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--
================================================================
Marin van Heel
Professor of Cryo-EM Data Processing
Leiden University
Cell Observatory
NeCEN Building Room 05.27
Einsteinweg 55
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
Biochemistry Building (Room 512)
South Kensington Campus
London SW7 2AZ, UK
Tel. UK: + 44 (0) 20 75945316
Mobile: + 44 (0) 7941540625
email: marin.vanheel(A_T)gmail.com
email: m.vanheel(A_T)ic.ac.uk
and: mvh.office(A_T)gmail.com
I receive many emails per day and, although I try, there is no guarantee that I will actually read each incoming email. Moreover, our Spam filters can be strikt and sometimes make legitimate emails disappear (try the gmail accounts, alternatively)
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