[3dem] Bright/Dark Spots in a Flat-Field Image

Wim Hagen wim.hagen at embl.de
Tue Mar 26 12:34:21 PDT 2013


I have never seen spots like this but I am used to magnification and illumination conditions influencing the gain reference/flat field validness. For this reason I always take gain reference/flat field images with the exact same conditions used for actual imaging. Once gain reference is acquired I double check by acquiring an image and do an autocorrelation: this should show a small peak in the middle of a noise image. When this looks good, I do not worry about anything until optical conditions are changed for whatever reason.

You might trace the exact origin by checking for a change of the pattern when changing lens values: if it changes in size or rotates, it originates above the lens changed. You indicate that beam brightness changes the pattern so it must start somewhere above your intensity controlling lens. Play with either spot size or intensity separately, while keeping the other value the same and see where things starts in the column. Be aware that a spot size change also changes intensity automatically so compensate for that: keep intensity lens value constant for different spot sizes.
If the pattern changes with spot size, change gun settings: gun lens acts similar to spot size and only changes beam current, extraction voltage will change both beam current and gun brightness.
On any FEI FEG scope I would use at least 4000V extraction voltage.

Best,

Wim Hagen
EMBL Heidelberg


On Mar 26, 2013, at 6:56 PM, Benjamin Bammes wrote:

> I recently used a 200 kV FEG microscope, where all of the bright-field images showed a bright spot and a dark spot (with some separation between them) near the middle of the illumination area (see the attached image). The size of the spots, their rotation, and the distance between them changed with beam brightness and magnification. The spots were present regardless of the condensor aperture, and I also observed the spots on several different cameras. I have not observed this phenomenon on any other similar microscope.
> 
> So, I am curious as to what could be causing this and how important it is (in terms of being able to do single-particle imaging or tomography). Any ideas?
> 
> Thanks!
> 
> --
> Benjamin Bammes, Ph.D.
> Director of Applications & Marketing
> DIRECT ELECTRON, LP
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