8.9.
SOURCE ALIGNMENT
285
electrostatic deflection. Thus the axis of inclination can be placed on
any meridian.
The second step in the alignment of the illuminating system is termed
"condenser alignment." In essence the optical axis of the source,
already parallel with that of the objective lens, is made to coincide with
the latter. In the unaligned instrument the illumination sweeps, perhaps
greatly, as the intensity (condenser) control is varied. In the aligned
instrument the illumination expands symmetrically on either side of the
cross-over image of the source. This is a condition of much practical
convenience even though not of great importance in relation to resolution.
8.9. Source Alignment
Source alignment (at least in RCA microscopes) is best accomplished
by taking advantage of an optical trick. One begins with a microscope
lacking a specimen or objective aperture, but with a large, mechanically
centered, condenser aperture in place. The objective lens current is
greatly reduced (coarsest focus control), and the condenser set so that the
source is viewed in focus (cross-over position). The beam current is then
reduced substantially below the bias position, and "clouds" or "halos"
are observed breaking away from the main beam as in Fig. 33. The
position and character of these clouds provide the necessary information
about the source alignment.
If two clouds, nearly identical in size and luminosity, break away
symmetrically from the ends of the cross-over ellipse as in Figs. 33(a) and
(b), the source is properly aligned. However, if the clouds are quite unequal, or if there is but a single cloud at one end of the ellipse as in
Fig. 33(c), there is misalignment in the direction of the long axis of the
ellipse. If one or two clouds are seen at one side of the ellipse, as in
Figs. 33(d) and (e), the misalignment coincides with the short axis of
the elipse. Of course intermediate relationships can exist.
In the unaligned position, the effective tilt of the system is in the wrong
direction, and one must experiment with other tilts until alignment is
achieved by successive approximation. In the older RCA EMU-1 and -2
microscopes, the gun tilt can be changed mechanically. In some foreign
microscopes the filament is moved about within the anode cap which
accomplishes the same thing. In RCA EMU-3 and EML microscopes,
SOURCE ALIGNMENT
285
electrostatic deflection. Thus the axis of inclination can be placed on
any meridian.
The second step in the alignment of the illuminating system is termed
"condenser alignment." In essence the optical axis of the source,
already parallel with that of the objective lens, is made to coincide with
the latter. In the unaligned instrument the illumination sweeps, perhaps
greatly, as the intensity (condenser) control is varied. In the aligned
instrument the illumination expands symmetrically on either side of the
cross-over image of the source. This is a condition of much practical
convenience even though not of great importance in relation to resolution.
8.9. Source Alignment
Source alignment (at least in RCA microscopes) is best accomplished
by taking advantage of an optical trick. One begins with a microscope
lacking a specimen or objective aperture, but with a large, mechanically
centered, condenser aperture in place. The objective lens current is
greatly reduced (coarsest focus control), and the condenser set so that the
source is viewed in focus (cross-over position). The beam current is then
reduced substantially below the bias position, and "clouds" or "halos"
are observed breaking away from the main beam as in Fig. 33. The
position and character of these clouds provide the necessary information
about the source alignment.
If two clouds, nearly identical in size and luminosity, break away
symmetrically from the ends of the cross-over ellipse as in Figs. 33(a) and
(b), the source is properly aligned. However, if the clouds are quite unequal, or if there is but a single cloud at one end of the ellipse as in
Fig. 33(c), there is misalignment in the direction of the long axis of the
ellipse. If one or two clouds are seen at one side of the ellipse, as in
Figs. 33(d) and (e), the misalignment coincides with the short axis of
the elipse. Of course intermediate relationships can exist.
In the unaligned position, the effective tilt of the system is in the wrong
direction, and one must experiment with other tilts until alignment is
achieved by successive approximation. In the older RCA EMU-1 and -2
microscopes, the gun tilt can be changed mechanically. In some foreign
microscopes the filament is moved about within the anode cap which
accomplishes the same thing. In RCA EMU-3 and EML microscopes,