8.12.
OBJECTIVE LENS COMPENSATION
293
focus image looks rather different [Fig. 34(h)]. The diffraction band is
bright, and is not particularly visible since it does not contrast greatly
with the illuminated background. However, it makes the apparent edge
of the particle or the rim of the hole appear very sharply defined with
great contrast. The in-focus edge [Figs. 34(e) and (f)], in which the true
edge of the particle or hole is seen, will not appear with as much contrast
as the latter. The edge will be sharp, it is true, but relatively indistinct.
Similar Fresnel fringes also are visible in the through-focus series of
micrographs of Fig. 40 which includes a small hole in the section.
When astigmatism exists the focal plane is different for two axes. Thus,
if the poles of one axis are in focus, the poles of the other will show
diffraction images corresponding to either the above or below-focus
patterns [Figs. 34(i)-(k)].
One begins compensation by finding the axis of asymmetry using the
over-focused image [Fig. 34(j)]. One then imposes a rather larger and
easily visible asymmetry on this field using the compensator controls
[Fig. 34(i)]. This field is rotated with the azimuth control so that this
axis of asymmetry is at right angles to the intrinsic asymmetry [Fig. 34(i)].
The imposed voltage is then reduced. The diffraction rings will be seen
to approach the radial symmetry which is desired [Fig. 34(d)]. If the axes
of asymmetry have been established exactly at right angles to one another, no further compensation will be needed. More likely, however,
some fine adjustments of azimuth and magnitude will be needed to obtain the truly symmetrical pattern of good compensation.
The difficulties that the beginner may experience in this procedure
are mainly those of adequate visualization of the diffraction images.
Illumination is apt to be faint at the high magnification where the compensation is done. Microscopes which have a double condenser system
have an advantage in providing brighter illumination. Instruments which
are equipped with a movable condenser aperture may be used with a
large bore aperture in position to increase illumination while compensation is performed. In general, with experience and the knowledge of
what is a suitable specimen, one can compensate with considerable accuracy simply by viewing the specimen screen with the low power microscope used in normal focusing. Certainly this is adequate for routine work
with sections, although for very high resolution work, compensation may
have to be checked photographically by taking a through-focus series of
pictures.
OBJECTIVE LENS COMPENSATION
293
focus image looks rather different [Fig. 34(h)]. The diffraction band is
bright, and is not particularly visible since it does not contrast greatly
with the illuminated background. However, it makes the apparent edge
of the particle or the rim of the hole appear very sharply defined with
great contrast. The in-focus edge [Figs. 34(e) and (f)], in which the true
edge of the particle or hole is seen, will not appear with as much contrast
as the latter. The edge will be sharp, it is true, but relatively indistinct.
Similar Fresnel fringes also are visible in the through-focus series of
micrographs of Fig. 40 which includes a small hole in the section.
When astigmatism exists the focal plane is different for two axes. Thus,
if the poles of one axis are in focus, the poles of the other will show
diffraction images corresponding to either the above or below-focus
patterns [Figs. 34(i)-(k)].
One begins compensation by finding the axis of asymmetry using the
over-focused image [Fig. 34(j)]. One then imposes a rather larger and
easily visible asymmetry on this field using the compensator controls
[Fig. 34(i)]. This field is rotated with the azimuth control so that this
axis of asymmetry is at right angles to the intrinsic asymmetry [Fig. 34(i)].
The imposed voltage is then reduced. The diffraction rings will be seen
to approach the radial symmetry which is desired [Fig. 34(d)]. If the axes
of asymmetry have been established exactly at right angles to one another, no further compensation will be needed. More likely, however,
some fine adjustments of azimuth and magnitude will be needed to obtain the truly symmetrical pattern of good compensation.
The difficulties that the beginner may experience in this procedure
are mainly those of adequate visualization of the diffraction images.
Illumination is apt to be faint at the high magnification where the compensation is done. Microscopes which have a double condenser system
have an advantage in providing brighter illumination. Instruments which
are equipped with a movable condenser aperture may be used with a
large bore aperture in position to increase illumination while compensation is performed. In general, with experience and the knowledge of
what is a suitable specimen, one can compensate with considerable accuracy simply by viewing the specimen screen with the low power microscope used in normal focusing. Certainly this is adequate for routine work
with sections, although for very high resolution work, compensation may
have to be checked photographically by taking a through-focus series of
pictures.