F30 gun tilt alignment

Bill Tivol tivol at caltech.edu
Fri Jul 29 11:21:14 PDT 2005


Dear Lists,
	We have had a peculiar situation for a while with the gun tilt 
alignment on the F30.  When using the Alignments panel, which sets the 
mag and spot size to default values, and measuring the intensity with 
the Measured Exposure value, we have seen that the maximum intensity 
can vary by a factor of about 2.  Of course we have saved an alignment 
file on the days that the maximum intensity is good--a measured 
exposure of <~1 sec--and when the alignment maxes out at > 1 sec, we 
try loading the gun alignments from the file.  Sometimes that works, 
but sometimes it doesn't.  I'm sure that the poor intensity max comes 
from finding a local max rather than the global max, and I can usually 
keep working until I find the global max, but I don't know why the good 
alignment file stops working, and that is bothersome.  A FEG should be 
more stable than a thermionic filament with respect to alignments since 
it doesn't evaporate, and the beam is remarkably stable during use, so 
the alignments shouldn't change much.  Have any of you had the same 
experience with your FEG instruments?
Yours,
Bill Tivol, PhD
EM Scientist and Manager
Cryo-Electron Microscopy Facility
Broad Center, Mail Code 114-96
California Institute of Technology
Pasadena CA 91125
(626) 395-8833
tivol at caltech.edu





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