8.3.
OBJECTIVE APERTURE ALIGNMENT
275
Depending upon the mechanical system of the particular microscope,
an experienced worker and the beginner may alike be frustrated at times
in trying to get any illumination after installing a new aperture. It may
be so far out of alignment that nothing appears on the fluorescent screen
even when the beam is maximally spread out. The operator may attempt
moving the aperture about with its controls with the hope of picking up
illumination. He may be successful but is apt to become completely lost,
and there will be no alternative but to start again with the aperture
centered as best one can by eye. Under these circumstances, there is a
temptation to make other adjustments in the microscope, to move the
stage or to move the gun translation. In general, these temptations should
be resisted, for one is apt to become hopelessly lost. Aperture alignment
should begin with a system known to be illuminable from top to bottom
excepting only that the aperture disk may be interposed. It is only when
one has the assurance that nothing but the aperture disk intervenes in
the system that one can proceed with confidence even when the world
literally looks dark.
The whole procedure depends upon the operator knowing what images
are required and how to get them. The novice should, by all means, work
backwards and practice with an old aperture to explore the variables.
Figures 31(d)-(f) indicate some of the image patterns he may see. To
obtain a satisfactory image of the cross-over beam within the outline of
the aperture, the following conditions must be fulfilled. One must begin
with a satisfactorily diffracting object. The beam current should be
substantially reduced (otherwise the central illumination is so brilliant
that little else can be seen). The cross-over beam must be focused to a
minimal size by the selected area diffraction control knob [otherwise
distorted out-of-focus images of the whole field will be visible as in
Fig. 31(f)]. To avoid images such as those of Figs. 31(d) and (e) it is
sometimes necessary to hide part of the cross-over beam behind a specimen
wire, for otherwise it may not prove possible to obtain a small enough
spot to avoid the interfering "tails."
When microscopes are used as service instruments for a variety of tasks,
it may be necessary to take the aperture out quite frequently. All who use
the microscope must be made aware of the necessity of protecting the
aperture from intense beam bombardment. Also, one should not attempt
voltage alignment of the objective lens when a good aperture is in place,
for when the accelerating voltage is reduced, the intense beam spirals
away from the original optical axis in a manner which can be expected
OBJECTIVE APERTURE ALIGNMENT
275
Depending upon the mechanical system of the particular microscope,
an experienced worker and the beginner may alike be frustrated at times
in trying to get any illumination after installing a new aperture. It may
be so far out of alignment that nothing appears on the fluorescent screen
even when the beam is maximally spread out. The operator may attempt
moving the aperture about with its controls with the hope of picking up
illumination. He may be successful but is apt to become completely lost,
and there will be no alternative but to start again with the aperture
centered as best one can by eye. Under these circumstances, there is a
temptation to make other adjustments in the microscope, to move the
stage or to move the gun translation. In general, these temptations should
be resisted, for one is apt to become hopelessly lost. Aperture alignment
should begin with a system known to be illuminable from top to bottom
excepting only that the aperture disk may be interposed. It is only when
one has the assurance that nothing but the aperture disk intervenes in
the system that one can proceed with confidence even when the world
literally looks dark.
The whole procedure depends upon the operator knowing what images
are required and how to get them. The novice should, by all means, work
backwards and practice with an old aperture to explore the variables.
Figures 31(d)-(f) indicate some of the image patterns he may see. To
obtain a satisfactory image of the cross-over beam within the outline of
the aperture, the following conditions must be fulfilled. One must begin
with a satisfactorily diffracting object. The beam current should be
substantially reduced (otherwise the central illumination is so brilliant
that little else can be seen). The cross-over beam must be focused to a
minimal size by the selected area diffraction control knob [otherwise
distorted out-of-focus images of the whole field will be visible as in
Fig. 31(f)]. To avoid images such as those of Figs. 31(d) and (e) it is
sometimes necessary to hide part of the cross-over beam behind a specimen
wire, for otherwise it may not prove possible to obtain a small enough
spot to avoid the interfering "tails."
When microscopes are used as service instruments for a variety of tasks,
it may be necessary to take the aperture out quite frequently. All who use
the microscope must be made aware of the necessity of protecting the
aperture from intense beam bombardment. Also, one should not attempt
voltage alignment of the objective lens when a good aperture is in place,
for when the accelerating voltage is reduced, the intense beam spirals
away from the original optical axis in a manner which can be expected