8.3.
OBJECTIVE APERTURE
ALIGNMENT
273
be made to coincide with the center of rotation of the objective lens
which then represents alignment.
Naturally when a between-the-lens aperture is first installed it may be
so far out of alignment that no illumination can reach the stage. It is
wise to have anticipated this, and thus be able to set up the operation of
the microscope in a manner that avoids damage to the misaligned
aperture. For this, one also needs assurance that the screen could be
weakly illuminated if the aperture were only centered. Therefore, before
starting to replace an aperture, the beam should be spread widely, and
perhaps the beam current reduced, and these settings recorded so they
can be duplicated. If a specimen is in the microscope, be sure it is not
moved subsequently, or a grid wire may block illumination. Return to
these original conditions before starting to search for the aperture. Have
confidence in them, and do not start making other adjustments. It is
only after a reasonable degree of alignment is achieved that the beam
may safely be brought close to cross-over.
A rear focal plane aperture can be aligned more readily and exactly
than a between-the-lens aperture since it is possible to get a quite satisfactory image of its rim, and the position of the beam within it. To do this,
one resorts to an optical trick. The microscope is set up as for selected
FIG. 31. When the lens system of an electron microscope is used as for selected area
diffraction, and a very small beam is allowed to pass through a diffraction material,
one can see the position of the cross-over beam in relation to the silhouetted rim of
an objective aperture. The image one actually sees is about the size of (a). The other
figures
are greatly enlarged. The small beam is aligned with the center of the aperture
in (a) and (b). In (c) it is decidedly off-center, bombarding the aperture rim, and
requires adjustment.
In order to set up the optimal conditions which make possible alignment of the
aperture, the beam must be as small as possible. A cross-over beam is used, but with
very low beam currents. The beam must be focused using the diffraction focus control
so that it becomes as small as possible. When the beam is slightly out of focus, or is
not at the cross-over position, one may see images such as those of (f) in which a distorted image of a hole in the specimen screen is apparent with a torn portion of a
section within it. Obviously beam conditions in the upper right hand corner were
approaching those necessary for alignment. All that was required was further focusing.
In (d) and (e) the cross-over spot was too large and could not be focused to yield the
necessary thin pencil of electrons, [(d) is focused, (e) is off-focus]. Under these circumstances success may be achieved by hiding part of the cross-over beam behind a specimen wire, and thus reducing its width.
OBJECTIVE APERTURE
ALIGNMENT
273
be made to coincide with the center of rotation of the objective lens
which then represents alignment.
Naturally when a between-the-lens aperture is first installed it may be
so far out of alignment that no illumination can reach the stage. It is
wise to have anticipated this, and thus be able to set up the operation of
the microscope in a manner that avoids damage to the misaligned
aperture. For this, one also needs assurance that the screen could be
weakly illuminated if the aperture were only centered. Therefore, before
starting to replace an aperture, the beam should be spread widely, and
perhaps the beam current reduced, and these settings recorded so they
can be duplicated. If a specimen is in the microscope, be sure it is not
moved subsequently, or a grid wire may block illumination. Return to
these original conditions before starting to search for the aperture. Have
confidence in them, and do not start making other adjustments. It is
only after a reasonable degree of alignment is achieved that the beam
may safely be brought close to cross-over.
A rear focal plane aperture can be aligned more readily and exactly
than a between-the-lens aperture since it is possible to get a quite satisfactory image of its rim, and the position of the beam within it. To do this,
one resorts to an optical trick. The microscope is set up as for selected
FIG. 31. When the lens system of an electron microscope is used as for selected area
diffraction, and a very small beam is allowed to pass through a diffraction material,
one can see the position of the cross-over beam in relation to the silhouetted rim of
an objective aperture. The image one actually sees is about the size of (a). The other
figures
are greatly enlarged. The small beam is aligned with the center of the aperture
in (a) and (b). In (c) it is decidedly off-center, bombarding the aperture rim, and
requires adjustment.
In order to set up the optimal conditions which make possible alignment of the
aperture, the beam must be as small as possible. A cross-over beam is used, but with
very low beam currents. The beam must be focused using the diffraction focus control
so that it becomes as small as possible. When the beam is slightly out of focus, or is
not at the cross-over position, one may see images such as those of (f) in which a distorted image of a hole in the specimen screen is apparent with a torn portion of a
section within it. Obviously beam conditions in the upper right hand corner were
approaching those necessary for alignment. All that was required was further focusing.
In (d) and (e) the cross-over spot was too large and could not be focused to yield the
necessary thin pencil of electrons, [(d) is focused, (e) is off-focus]. Under these circumstances success may be achieved by hiding part of the cross-over beam behind a specimen wire, and thus reducing its width.