8.14.
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297
may so overload the other circuits that electronic instability is present as
well. A contaminated condenser aperture may be responsible, although
these are so heated by electron bombardment that they tend to be selfcleaning. An unstable condenser lens circuit may produce flickering.
Some flickering can be tolerated for a while if it is ascertained that the
image is stable in spite of this. Flickering associated with an unstable
image is apt to mean contamination both at the specimen level and above
it, most likely a dirty objective aperture as well as a dirty anode.
(3) Fixed sharp silhouette of contaminant! Objects protruding into
the optical path below the level of the objective lens will appear in
silhouette. They are most apt to be lodged in the projector pole-piece.
They may rhythmically charge and discharge so that the image oscillates.
(4) Extremely out of focus images of contaminants! Contaminants
which protrude into the electron beam above or within the objective
lens may produce vague images that are far out of focus. The objective
and condenser apertures should be particularly suspect.
(5) Fixed image distortion! A contaminant below the level of image
formation which produces an electrostatic field close to the optical pathway will produce a distortion of the image. The distortion will remain
fixed as the specimen is moved about. The electrostatic field may be so
localized, yet so intense, that images looking like silhouettes of globules
may protrude into the field of view. These probably will seem to grow
or diminish as the beam is altered. These are not true images of the offending particulate matter. Things introduced into the column such as
a mechanical shutter or a photocell may give this sort of trouble.
(6) Particular parts of a grid image showing distortion! The distortion
moves as the stage is moved. The contamination is on the specimen screen
itself, the grid cap, or the specimen holder, and may be associated with
image instability. The contamination may not be in the specimen plane,
and so be completely out-of-focus. A piece of lint sticking through a hole
in a supporting screen is a common cause. It may move about as the beam
is varied and its electrostatic charge changes, and so the distortion also
will move as the condenser control is varied.
(7) An unstable focus! When a high magnification image will not hold
focus for a reasonable length of time, it is apt to mean the electronic
instability of the high-voltage circuit. Reference batteries should be
suspect, and also corona discharge or other high tension leakage. High
voltage is capable of jumping considerable gaps. So a circuit may actually
be mechanically open and yet behave more or less erratically as if closed.
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297
may so overload the other circuits that electronic instability is present as
well. A contaminated condenser aperture may be responsible, although
these are so heated by electron bombardment that they tend to be selfcleaning. An unstable condenser lens circuit may produce flickering.
Some flickering can be tolerated for a while if it is ascertained that the
image is stable in spite of this. Flickering associated with an unstable
image is apt to mean contamination both at the specimen level and above
it, most likely a dirty objective aperture as well as a dirty anode.
(3) Fixed sharp silhouette of contaminant! Objects protruding into
the optical path below the level of the objective lens will appear in
silhouette. They are most apt to be lodged in the projector pole-piece.
They may rhythmically charge and discharge so that the image oscillates.
(4) Extremely out of focus images of contaminants! Contaminants
which protrude into the electron beam above or within the objective
lens may produce vague images that are far out of focus. The objective
and condenser apertures should be particularly suspect.
(5) Fixed image distortion! A contaminant below the level of image
formation which produces an electrostatic field close to the optical pathway will produce a distortion of the image. The distortion will remain
fixed as the specimen is moved about. The electrostatic field may be so
localized, yet so intense, that images looking like silhouettes of globules
may protrude into the field of view. These probably will seem to grow
or diminish as the beam is altered. These are not true images of the offending particulate matter. Things introduced into the column such as
a mechanical shutter or a photocell may give this sort of trouble.
(6) Particular parts of a grid image showing distortion! The distortion
moves as the stage is moved. The contamination is on the specimen screen
itself, the grid cap, or the specimen holder, and may be associated with
image instability. The contamination may not be in the specimen plane,
and so be completely out-of-focus. A piece of lint sticking through a hole
in a supporting screen is a common cause. It may move about as the beam
is varied and its electrostatic charge changes, and so the distortion also
will move as the condenser control is varied.
(7) An unstable focus! When a high magnification image will not hold
focus for a reasonable length of time, it is apt to mean the electronic
instability of the high-voltage circuit. Reference batteries should be
suspect, and also corona discharge or other high tension leakage. High
voltage is capable of jumping considerable gaps. So a circuit may actually
be mechanically open and yet behave more or less erratically as if closed.