[3DEM] Diffraction of underfocused image for search on Tecnai

[Hong-Wei Wang]

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Dear colleagues,

I'm bringing up a phenomenon on our Tecnai microscopes which you may have 
encountered and may give me some suggestion to solve the problem.

I was trying to use underfocused diffraction pattern for search at low dose 
mode on our Tecnai-12 and Tecnai F20. Ideally, I would like to use the 
diffraction pattern of a highly defocused image of low magnification (say 
4,000 ~ 6,000) in order to get a good contrast for search, while the focus 
and exposure at high magnification (30,000 ~ 100,000) are much closer to 
focus. For those who use the similar setting on JEOL or CM series, this is 
practical. However, I noticed the following phenomenon which prevented this 
function on our Tecnai microscopes:
With the Low Dose on, at Search mode, I adjusted the defocus to highly 
underfocus (tens to hundreds of micrometers), pressed the diffraction 
button, adjusted the camera length and focus of the diffraction pattern. It 
gave a very good contrasted diffraction image of the sample at low mag. I 
then went to the Focus mode and Exposure mode, focusing on the image at 
these two modes (1-2 micrometers defocus). When I switched back to Search 
mode, the current of objective lens didn't recover as I set originally but 
stayed as the value under Focus and Exposure, thus I was looking at the 
diffraction pattern of a close-to-focus low mag image rather than the one of 
a highly defocused image. I can turn off and on the diffraction button one 
cycle to recover my original setting. But this, firstly is inconvinient, and 
even worse, simply turn off the objective lens after several such cycles so 
that my alignment of the low dose are totally messed up.

Since this phenomenon was observed on both of our Tecnai microscopes, I am 
wondering if it is a shortcoming of the design or a bug in the software 
interface. If anyone had this experience and found a way to solve this 
problem, could you please let me know?

Thank you in advance!

Hongwei

-----------------------------------
Hongwei Wang, Ph D
MS 20A-355, LSD, LBNL
1 Cyclotron Rd, Berkeley,
CA 94720

510-642-2222 (O)
510-642-8806 (FAX)
hwwang at lbl.gov 

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