F30H objective aperture & low dose--solution

[Bill Tivol]

Dear Lists,
	Thanks everyone for the many helpful comments and suggestions.  After 
a series of investigations, we tried something that, in retrospect, was 
obvious.  When a smaller objective aperture was inserted, the problem 
became less severe, leading to the conclusion that somehow the larger 
objective aperture was out of position.  Some of the possibilities are 
that the aperture was not placed properly in the holder, that the foil 
came loose from the body of the aperture, or that the tip was being 
displaced when moved to the farthest-in position. In our case the we 
still do not know exactly what occurred, but the apertures were 
properly positioned and looked fine.  This can occur in a microscope 
where the aperture is in the back focal plane, as well as one like the 
F30 where the aperture is not, so if you experience a similar problem 
with your aperture not being centered in Focus state when it is 
centered in Exposure state, see if this occurs just with one aperture 
or is present for all the apertures.
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