[3DEM] RE: Why FEG?

[Bill Tivol]

On Apr 23, 2004, at 7:08 AM, Philip Koeck wrote:

> From your plots it seems that you need a FEG to reach resolutions 
> beyond about 3 A,
>
> But I can’t see any great advantage up to about 5 A resolution.
>
> Is that correct? (and what is all the excitement about then?)

Dear Philip,
	Since we look at biological cryo-specimens at large values of defocus 
in order to get contrast, the first zero of the contrast transfer 
function (CTF) may not be at high enough spatial frequency to give the 
resolution we want.  If the beam is coherent--a FEG is the source that 
provides the best coherence--we can compensate for the CTF and get 
information at higher resolution than that first zero.  Uwe Lukens' 
plot was for a 2um underfocus, and we often take data at more than 10 
um underfocus, where the difference between the coherent and 
non-coherent plots is significant much sooner than 0.5 nm.
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
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: text/enriched
Size: 1371 bytes
Desc: not available
Url : http://mail.ncmir.ucsd.edu/pipermail/3dem/attachments/20040423/d1f2f309/attachment.bin