8.6.
ALIGNMENT, GENERAL REMARKS
281
to master these all at once. He can rest assured that in practice alignment
is not a serious problem. One has to realign the whole column only when
it has been torn down. As a general rule, only one or two alignment
procedures are necessary to put the microscope back in operation after a
limited maintenance operation on one or another part of the column.
Thus, when a filament is replaced, or the objective pole-piece removed
to clean the aperture, the alignment of most parts of the column is not
affected at all, and realignment operations are limited to the gun in the
first case and the aperture holder in the second. It may be many months
before the whole column need be disassembled and require complete
realignment. From a practical point of view alignment consists of many
little problems rather than one big one.
The newcomer to electron microscopy should ask two questions. First,
why is alignment necessary, and second how exact must it be in practice
for reasonable performance?
There are several reasons for column alignment, some practical, some
theoretical. An important theoretical reason is that both magnetic and
electrostatic lenses are fundamentally spherical lenses. Inevitably such
lenses distort. The distortion becomes increasingly severe as one moves
away from the central axis. There is no practical probability that this
spherical aberration can be corrected in electron microscopes as can be
done when complex glass lenses are used in light microscopes. In practice, however, one need not be very concerned about spherical aberration
unless one is trying to make accurate measurements, for it is not particularly noticable except at very low magnifications.
A very important theoretical and practical reason for (voltage) alignment is that it minimizes the effects of circuit instability. Electrical
stability is a relative thing. A slight ripple of the accelerating voltage
produces electrons moving at different speeds. This corresponds to using
light of different wavelengths. It leads to the electron equivalent of
chromatic aberration. The aberrant wavelengths focus at different levels
and tend to blur the image, thus destroying resolution. Furthermore,
electrons going through a magnetic or electrostatic lens move in spiral
paths, and not as light waves do through glass lenses. Thus, if there is
any change in the accelerating voltage, or if lens currents change, the
image turns about the optical axis. Naturally such a movement, even
though slight, would tend to destroy resolution, and the effect increases
rapidly as one moves away from the optical axis. If there is a reasonable
degree of electrical stability in the microscope circuits, this effect is not
ALIGNMENT, GENERAL REMARKS
281
to master these all at once. He can rest assured that in practice alignment
is not a serious problem. One has to realign the whole column only when
it has been torn down. As a general rule, only one or two alignment
procedures are necessary to put the microscope back in operation after a
limited maintenance operation on one or another part of the column.
Thus, when a filament is replaced, or the objective pole-piece removed
to clean the aperture, the alignment of most parts of the column is not
affected at all, and realignment operations are limited to the gun in the
first case and the aperture holder in the second. It may be many months
before the whole column need be disassembled and require complete
realignment. From a practical point of view alignment consists of many
little problems rather than one big one.
The newcomer to electron microscopy should ask two questions. First,
why is alignment necessary, and second how exact must it be in practice
for reasonable performance?
There are several reasons for column alignment, some practical, some
theoretical. An important theoretical reason is that both magnetic and
electrostatic lenses are fundamentally spherical lenses. Inevitably such
lenses distort. The distortion becomes increasingly severe as one moves
away from the central axis. There is no practical probability that this
spherical aberration can be corrected in electron microscopes as can be
done when complex glass lenses are used in light microscopes. In practice, however, one need not be very concerned about spherical aberration
unless one is trying to make accurate measurements, for it is not particularly noticable except at very low magnifications.
A very important theoretical and practical reason for (voltage) alignment is that it minimizes the effects of circuit instability. Electrical
stability is a relative thing. A slight ripple of the accelerating voltage
produces electrons moving at different speeds. This corresponds to using
light of different wavelengths. It leads to the electron equivalent of
chromatic aberration. The aberrant wavelengths focus at different levels
and tend to blur the image, thus destroying resolution. Furthermore,
electrons going through a magnetic or electrostatic lens move in spiral
paths, and not as light waves do through glass lenses. Thus, if there is
any change in the accelerating voltage, or if lens currents change, the
image turns about the optical axis. Naturally such a movement, even
though slight, would tend to destroy resolution, and the effect increases
rapidly as one moves away from the optical axis. If there is a reasonable
degree of electrical stability in the microscope circuits, this effect is not