F30H objective aperture & low dose--solution

[Shi, Dan (NIH/NCI)]

I did a little more investigation about the phenomenon by comparing Polara
with T12 at diffraction mode. I tried to adjust C1 and C2 (mainly C2) to get
obj. aperture and DP focused at same plane for both Polara and T12, the
difference was when both the aperture and DP were in focus, the C2 was
under-focused (convergent beam on specimen)for Polara, and C2 was
over-focused (divergent or parallel beam) for T12. It indicates the obj.
aperture of Polara may be located between the back focal plane and the lens,
and the T12's obj. aperture may be very close to the back focal plane. When
I set the beam near parallel for Polara, and separately focused the aperture
and the DP, it was found the estimated distance between the aperture and the
back focal plane might be in mm range (over 250 um over-focus of the Obj.
lens could not compromise the difference). The aperture shifted against the
DP when the beam was shifted. The distance may vary among the microscopes.
On the other hand, when I centered the obj. aperture of T12 at DP mode with
parallel beam, the aperture did not move with few um of beam shift in SA
mode.  The attachment is a simple drawing to explain why one might observe
the obj. aperture move during beam shift for Polara. 
Because the focal length would mainly related the obj. lens current and the
wave length of electron beam, Polara with 300 kv HT may have much longer
focal length comparing to T12, if the obj. lens currents are similar for
both T12 and Polara. FEI could eliminate the problem by lowering the obj.
aperture or increasing the maximum obj. lens current, the second one may
need to change lens program to match all the image planes of diff, inter
medium and projection lenses.
The following paragraph is an explanation for 'back focal plane' and
'parallel beam', you can omit it.

'Because the back focal plane would be changed by adjusting any lens'
current of the whole lens set including gun lenses, C1, C2 and mini condense
lens and the current of objective lens itself, there would be no really
fixed back focal plane for objective lens. If one would define the back
focal plane of objective lens as parallel beam illumination onto a sample
located at Eucenttric height, which would be unique position. As we know
theoretically, the parallel beam on specimen means the beam should be
focused at the front focal plane of the last condense lens (mini condense
lens?), which FEI has distributed the settings to some customers for
obtaining parallel beam.'

Sincerely

Dan Shi, PhD
Kelly Service Inc 

-----Original Message-----
From: Bill Tivol [mailto:tivol at caltech.edu] 
Sent: Monday, June 27, 2005 5:43 PM
To: microscopy at msa.microscopy.com; tecnai at wadsworth.org; 3dem at ucsd.edu
Subject: F30H objective aperture & low dose--solution

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

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