8.12.
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289
should consider whether or not the anode cap should be cleaned. In the
long run, it is apt to save time and effort to do this routinely with every
filament replacement. As insulating contamination films build up around
the aperture of this cap, bias relations are altered and beam current soon
increases to dangerous levels. Furthermore, the insulating properties of
these films establish aberrant, asymmetrical, electrostatic fields which
distort the beam. The insulation is apt to break down from time to time
allowing discharge. This causes flicker, which is often rhythmical, and
can become severe. This leads to yet another consequence. If the objective aperture is at all asymmetrically contaminated, its electrostatic
charge is sensitive to the amount of electron bombardment, and alterations due to beam flicker will cause image movements.
The anode cap can be cleaned with levigated alumina powder. The
critical region is around the aperture, and particularly the side walls of
the aperture. A finger dampened and coated with alumina powder is
enough to clean the flat surfaces, but the walls of the aperture must be
cleaned with a dampened cotton swab with a little abrasive. The alumina then must be removed by vigorous sudsing, followed by adequate
rinses. The cap may be blotted with towels, but it should be wiped
finally with a chamois skin or lens paper which is not apt to leave lint.
For this and other similar purposes we recently have been using "Kimwipes" with good success. These are disposable wipers manufactured by
the Kimberly-Clark Corp., Neenah, Wisconsin, for industrial purposes.
They are practically lint-free, as well as being absorbent and strong.
They are commonly sold through chemical supply houses. However,
before final assembly of the gun, the aperture area of the anode cap
should be observed with a binocular dissecting microscope to be sure
that no lint remains near the aperture.
An unduly short filament life, probably accompanied by an obviously
rapid contamination of the anode cap, suggests a small vacuum leak in
the gun area. This may not be apparent on the gauges for they usually
record from the lower portion of the column.
8.12. Objective Lens Compensation
Every time a new objective lens aperture is installed the lens must be
compensated. It must be emphasized that resolution cannot be better
than the properties of the objective lens. Astigmatism resulting from
aperture contamination is the big "bugaboo" which can appear rapidly
OBJECTIVE LENS COMPENSATION
289
should consider whether or not the anode cap should be cleaned. In the
long run, it is apt to save time and effort to do this routinely with every
filament replacement. As insulating contamination films build up around
the aperture of this cap, bias relations are altered and beam current soon
increases to dangerous levels. Furthermore, the insulating properties of
these films establish aberrant, asymmetrical, electrostatic fields which
distort the beam. The insulation is apt to break down from time to time
allowing discharge. This causes flicker, which is often rhythmical, and
can become severe. This leads to yet another consequence. If the objective aperture is at all asymmetrically contaminated, its electrostatic
charge is sensitive to the amount of electron bombardment, and alterations due to beam flicker will cause image movements.
The anode cap can be cleaned with levigated alumina powder. The
critical region is around the aperture, and particularly the side walls of
the aperture. A finger dampened and coated with alumina powder is
enough to clean the flat surfaces, but the walls of the aperture must be
cleaned with a dampened cotton swab with a little abrasive. The alumina then must be removed by vigorous sudsing, followed by adequate
rinses. The cap may be blotted with towels, but it should be wiped
finally with a chamois skin or lens paper which is not apt to leave lint.
For this and other similar purposes we recently have been using "Kimwipes" with good success. These are disposable wipers manufactured by
the Kimberly-Clark Corp., Neenah, Wisconsin, for industrial purposes.
They are practically lint-free, as well as being absorbent and strong.
They are commonly sold through chemical supply houses. However,
before final assembly of the gun, the aperture area of the anode cap
should be observed with a binocular dissecting microscope to be sure
that no lint remains near the aperture.
An unduly short filament life, probably accompanied by an obviously
rapid contamination of the anode cap, suggests a small vacuum leak in
the gun area. This may not be apparent on the gauges for they usually
record from the lower portion of the column.
8.12. Objective Lens Compensation
Every time a new objective lens aperture is installed the lens must be
compensated. It must be emphasized that resolution cannot be better
than the properties of the objective lens. Astigmatism resulting from
aperture contamination is the big "bugaboo" which can appear rapidly