[3dem] FEI LowDose over-exposing film?

[Bob Grassucci]

Gabe,
I am wondering if there is something going wrong with your beam 
blanker.  Perhaps the beam blanker calibration is off or the beam is not 
deflected enough when the problem occurs.  In the low dose calibration 
page check the calibration.  I have seen instances where it does not 
deflect the beam enough even when it says it is.  As I understand it the 
CCD and film use different axis deflectors which would account for it 
never happening with your ccd.  Since the low dose is controlled by a 
GUI and not directly it could flake out from time to time.  On our 
tecnai there is a way to monitor the current going to the correct 
deflectors with FEIspy but I understand things changed with the Titan 
series.  How often is this over exposure happening?  I hope that helps.
Bob
On 5/14/2012 4:55 PM, Gabriel Lander wrote:
> We are encountering a serious issue collecting film using our Titan (not Krios) with the Tecnai low dose kit. Certain images appear to be severely over-exposed, while others appear to be normal, high quality, low-dose exposures.  Here are two examples of images collected during the same session, from the same cartridge of film, one right after another.
> http://www.ocf.berkeley.edu/~glander/pic/img001_FFTs.jpg
>
> The top image is of good quality, exhibits Thon rings to around 5Å, while the one below it has no discernible Thon rings, and is overall much darker in appearance.  We can only assume that this is due to overexposure of the film to the beam, but not to the extent that we see obvious radiation damage to the sample (no bubbling). We've monitored the blanker during the exposures, and according to all the readouts in the Tecnai software it is working properly. The film is inserted into the microscope and waits for 10 seconds before exposing the film. Multiple users are having this issue.
> Also, we never seem to have this problem when collecting CCD images.
> If anyone has ever experienced this issue or has any insights into what is happening, we would be most appreciative.
> - Gabe Lander&  Greg Alushin, Nogales Lab_______________________________________________
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> 3dem at ncmir.ucsd.edu
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>
>


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