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

[Henning Stahlberg]

Hi Niko,
Is this a Cs corrected instrument?

Henning Stahlberg
http://c-cina.org
(sent from iPhone)

> On Apr 4, 2014, at 3:30, "Nikolaus Grigorieff" <niko at grigorieff.org> wrote:
> 
> 
> 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.
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