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A matter of some importance in EM’s mainly for depth of focus.

Need considerable DOF to make image fall on photo plate and be in focus still.

CHROMATIC ABERRATION in the TEM:

In LM results in “color fringe” in cheap lenses.

In TEM, short-wavelength, high energy electrons have a longer focal point than long-WL low-energy ones.

Result in degraded focus.

Corrected by using only a single WL of electrons, with a stable acceleratiing V and good vacuum.

THICK SPECIMENS can cause CA by absorbing electron energies unequally.

PERIPHERAL ELECTRONS are more refracted than central ones.

Peripheral image is at a greater mag than central image.

Peripheral image is at a different focal plane as well

Result is PINCUSHION DISTORTION

Exaggerated by very low magnification

In DEMAGNIFYING MODE center is more magnified than edges.

Result is BARREL DISTORTION.

MOST MODERN SCOPES PLAY THESE OFF TO NEUTRALIZE THEM.

APERTURES improve contrast and depth of field/focus.

Aperures may be a source of astigmatism!

Need vaccum for three reasons:

1.Filament life…will blow without vaccum

2.Reduce discharge from cathode to earth

3.Increase “mean free path” of electrons, distance w/o encountering gas molecules

4.Decrease contamination of specimen by water vapor and organics in air