[3dem] Parallel Beam Illumination

[Farzad]

Dear Friends and Colleagues,

We have a JEOL JEM 3200 FSC, a 300keV TEM with cryo-stage and energy
filter. This microscope has just 2 condenser  lenses. I mean there is no
even condenser micro/mini-lens and and no objective micro/mini-lens and
after the condenser system we have just a twin lens type objective lens.

This classical optical system means that your smallest beam diameter in
parallel beam is equal to the smallest C2 aperture and for avoiding
irradiation we have to converge the beam a bit. I have not yet measured the
convergence angle but it should be maximum around 100 n.Rad in worse case.
We use objective lens wobbling for making the axis of the cone of the beam
parallel to the optical axis and not more.

My questions are as follows:

1- what would be the effect of this deviation from the beam parallelism on
the Phase Contrast? I always read it leads to a general phase shift (it
means but not contrast?).

2- What is the effect of this deviation from the bam parallelism on the
resolution? My first guess would be we will have coma on the edges of the
image.

3- What are the other problems with beam convergence? Somebody told me it
may affect pixel size accuracy, but I could not get a valid argument for
that yet.

I would appreciate if you can share your knowledge on the beam parallelism
issue in cryo-microscopy here.

Kind Regards,

Farzad Hamdi
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