[3DEM] Why FEG?

[Henning Stahlberg]

Hello,

The CTFEXPLORER defines the effective convergence angle from the 
magnification. The lower the magnification the lower becomes the 
effective convergence angle. At a magnification of 12k the effective 
convergence angle almost vanishes in the CTFEXPLORER, so that virtually 
any resolution becomes possible at high defocus and with any microscope 
settings.

As Dieter Typke once mentioned, the effective convergence angle on a 
TEM also depends indirectly upon the exposure time: If the illumination 
system is set so that only low light arrives onto the specimen (long 
exposure time), then the "magnification" between electron source and 
sample is high, which reduces the effective convergence angle.  Using 
this, with a non-FEG instrument you can indeed record high-resolution 
data at high defocus and normal magnification (50kx), if you set the 
illumination so that your exposure time becomes 1 minute or so. This, 
however, then becomes a challenge for cryo sample holders.

In my EXCEL sheet, the user defines the effective convergence angle 
manually. I used 0.7 mrad for a thermal source, and 0.1 mrad for a FEG, 
independently of the magnification. I estimated these values for 
illumination settings for an exposure time of 0.5 seconds.
Another difference between my EXCEL version and the CTFEXPLORER might 
be that I include an adjustable amplitude contrast portion (sample 
dependent, e.g. 7%), and the scattering profile for carbon, which 
contributes an additional envelope function.

Henning.

Henning Stahlberg, Ph.D.,
Molecular & Cellular Biology, Briggs Hall 115B,
UC-Davis, 1 Shields Ave., Davis, CA 95616, USA
Tel: +1 (530) 752 82 82      Fax: +1 (530) 752 30 85
mailto:HStahlberg at ucdavis.edu	
http://www.amp.ucdavis.edu	
--------------------------------------------------------------------

On Apr 26, 2004, at 5:53 AM, Philip Koeck wrote:

> To get similar curves with CTFexplorer I have to select a magnification
> Between 300K and 400K. At magnifications between 30K and 60K the
> difference
> between a FEG and a LaB6 is much smaller according to CTFexplorer.
> (Of course I don't know whether the curves given by CTFexplorer are
> realistic.)
>
> yours,
>
> Philip
>
> -----Original Message-----
> From: owner-3dem at ncmir.ucsd.edu [mailto:owner-3dem at ncmir.ucsd.edu] On
> Behalf Of Henning Stahlberg
> Sent: 23 April 2004 19:00
> To: Philip Koeck
> Cc: 3dem at ucsd.edu
> Subject: Re: [3DEM] Why FEG?
>
> **** Messages to this list are automatically archived ***
> **** Please limit quoting of previous postings to the bare minimum ****
>
>
> Hi,
>
> The biggest difference between a FEG and an LaB6 performance seems to
> me to arise from the effective electron source opening angle, when
> short cryoEM exposure times are considered. I used 0.1 mrad for a FEG
> and 0.7 mrad for a thermal electron source, see also Jong & van Dyck,
> Ultramicroscopy 49, 66-80 (1993).
> http://dx.doi.org/10.1016/0304-3991(93)90213-H
>
> Even when only interested in 0.5 nm resolution, the difference is
> significant, see
> http://www.amp.ucdavis.edu/index.php?p=download
>
> Those are CTF simulations made with a simple MS-Excel sheet, which is
> also on that server.
>
> Henning.
>
>
> Henning Stahlberg, Ph.D.,
> Molecular & Cellular Biology, Briggs Hall 115B,
> UC-Davis, 1 Shields Ave., Davis, CA 95616, USA
> Tel: +1 (530) 752 82 82      Fax: +1 (530) 752 30 85
> mailto:HStahlberg at ucdavis.edu	
> http://www.amp.ucdavis.edu	
>
>