[3DEM] Why FEG?

Tue Apr 27 03:51:11 PDT 2004

I think it's time to get this question out of the way once and for all.

The point that Henning makes indicates that CTF explorer is unrealistic
since such low convergence angles require very long exposure times. That
means CTF explorer can be quite misleading if you use it to judge the
suitability of a microscope for structural biology.

On the other hand I don't think Hennings simulation at
http://www.amp.ucdavis.edu/index.php?p=download do a LaB6-instrument
complete justice.
If I type in parameters for a Philips CM120 at 1500 nm defocus, using
0.7 mrad convergence angle the plot shows an information limit of about
16 Angstrom. With our old CM120 I have recorded images of carbon film at
45K magnification, 1500 nm defocus and 1 second exposure time where the
CTF shows oscillations out to beyond 10 Angstrom. That certainly isn't
the limit for that microscope either, since the filament wasn't
especially new at the time and I recorded quite a small area on a 1K CCD
camera.
Maybe the convergence angle for a LaB6 should be a bit smaller than 0.7
mrad.

What would really settle it for me is some experimental data.

Does anybody have a direct comparison between a FEG and a LaB6 that are
otherwise equivalent, for example images of carbon film taken around 50K
magnification and 1500 nm defocus with identical settings for exposure
time, apertures etc? Maybe the same with a well-used LaB6 filament to
see how quickly a LaB6 deteriorates?
The EM manufacturers ought to have data like that!? Or does anybody know
of a publication?

Philip

-----Original Message-----
From: Henning Stahlberg [mailto:HStahlberg at ucdavis.edu] 
Sent: 26 April 2004 19:38
To: Philip Koeck
Cc: 3dem at ucsd.edu
Subject: Re: [3DEM] Why FEG?

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	
>
>