[Microscopy] RE: RE: Why FEG?

[Philip Koeck]

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Hi,

In biology we use the microscope quite differently than material
scientists generally do. We never work at Scherzer defocus and we try to
get as close as possible to the information limit by averaging and
CTF-correction based on the weak phase / weak amplitude object
approximation. I think that should get rid of delocalization. You can
look up the method for example at
http://ncmi.bcm.tmc.edu/~stevel/EMAN/doc/.

By resolution I don't mean point resolution. The resolution we aim at is
close to the information limit at the highest underfocus we used to take
the images. We can only achieve this resolution in the final average
(actually a 3D model) but not in an individual image.
For that sort of imaging the FEG does have advantages because the
information limit is better (but I'm not sure whether the advantages are
big enough to justify the higher price). Max Sidorow shows that nicely
at http://clik.to/ctfexplorer (click on "science behind it").

To clarify: We have to use high defocus because we need good contrast
transfer at very low spatial frequencies to be able to see the protein
molecules.

We use film and scan it with 7 micron pixels.



-----Original Message-----
>From: A.Chuvilin [mailto:andrey.chuvilin at tu-ilmenau.de] 
Sent: 27 April 2004 03:00
To: Philip Koeck
Subject: [Microscopy] Re:   RE: Why FEG?

Sorry Philip, I'm novice in the field, could you please explain a number
of
points?
As far as I understood so far FEG is not good for point-to-point
resolution
because of its high coherence and related delocalisation problems.
Moreover
2A should not be a problem with any modern TEM even with W cathod. What
advantages do you plan to get from FEG?
How are you getting 2-3-5-10A resolution (point-to-point as I
understood) so
far from focus?
At x60K 2A fringes are about 15 microns, what recording media is used to
record them?

Thank you
Andrey


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>
> Hi again,
>
> I think I have to be a bit more specific. The question is whether a
FEG
> is worth the extra investment for the
> sort of work we are planning.
>
> On the one hand we image individual proteins with a magnification of
> 20K- 50K and defocus between 1000 nm
> and 2000 nm. There we want to get resolutions between 5 and 10
Angstrom.
> Judging from Uwe's plot that
> shouldn't be a problem with a LaB6.
>
> On the other hand we image 2D crystals at about 60K magnification
aiming
> at a resolution of about 2 to 3 Angstrom.
> Here we work much closer to focus (500 nm), which should improve the
> envelope of the LaB6 quite a bit.
> (It would be nice, Uwe, if you could send me a plot for 500 nm defocus
> for comparison. I can't reproduce your plots
> with the parameters you give.)
>
> For these two types of application, is there any point in investing in
a
> FEG?
> >From what I've seen so far there doesn't seem to be.
>
> Yours sincerely,
>
> Philip