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Very high magnification |
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Best LM, perhaps 1500x |
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First-generation TEM, 50,000x |
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Current instruments > 300,000x |
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More limited on biological specimens |
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Very high resolution |
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Much more important than magnification! |
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Best LM, 0.1 mm |
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First-generation TEMs, < 20 Å (200 nm) |
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Current instruments < 3.0 Å |
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TRANSMISSON electron microscope |
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SCANNING electron microscope |
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Similar in principles, to some extent |
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Different in operation, capabilities &
applications |
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RELATIVE SIZE SCALE: |
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Mark I Eyeball, 0.5 mm |
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LM, »0.1 mm |
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TEM, |
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<1.0 nm |
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Most often used in life sciences |
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Similar to LM in layout |
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Optical principles and formulae same as LM |
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COLUMN |
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Illuminating system |
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Imaging system |
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Camera system |
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VACUUM SYSTEM |
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Pumps & cooling apparatus |
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CONTROL SYSTEM |
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Interface between operator & electronics |
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TEM has same basic layout |
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Usually “inverted” compared to LM |
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Differ in many details! |
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Electron Gun |
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Filament |
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Cathode cap |
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Anode |
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Gun aperture |
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Condenser lenses |
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Stigmator |
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Condenser aperture |
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Specimen stage |
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Side entry |
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Top entry |
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Objective lens |
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Objective aperture |
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Objective stigmator |
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Intermediate lens |
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Projector Lens |
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Viewing chamber |
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CATHODE |
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Filament is the cathode |
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CATHODE CAP |
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Filament cap, “Wehnelt Cylinder” |
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More negative than filament |
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ANODE PLATE |
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At ground potential (0 volts) |
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Source of electrons |
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Usually tungsten |
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Sometimes not |
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LaB6 most common alternative |
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Filament life important indicator |
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Voltage across gun is 50kV to 200 kV |
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Resistance is very high |
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Current is in microamps |
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Filament heated white-hot with DC |
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Electrons excited out of outermost orbital shell |
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Leave filament at tip |
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Most guns are SELF-BIASED |
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“Bias” is D between filament & cap |
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Cap is connected directly to – term of
high-voltage line |
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Filament is connected through a resistor |
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Because cap is more negative than filament,
electrons are repulsed from it |
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Centers the first beam & regulates escape
from cap |
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As accelerating voltage the bias also due to wiring |
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At saturation, no more electrons escape even
with
filament current |
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As filament temp emission of electrons as well |
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Too high a temp will shorten life |
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SATURATION is optimum balance between electron
yield and filament life |
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Rays passing through are brought to coincidence
at the focal point |
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Rays from objects at back focal point are
projected into a plane |
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A measure of resolving power |
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Formula: |
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n (sin a) |
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Where: |
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n is refractive index of the medium |
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In a vacuum = 1.0 |
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a is
the aperture angle of the lens |
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The half-angle of illumination a lens can accept |
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As aperture size decreases, so does a |
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As aperture size decreases,
resolution increases |
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Obey the same laws of optics as glass lenses |
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Are subject to the same defects |
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Carry out the same functions in a compound TEM
as in a compound LM |
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Differ in structure and operation |
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Function to strengthen magnetic field |
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Soft iron core |
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North and South poles |
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Gap of non-magnetic material |
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Source of astigmatism |
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Machining is never perfect |
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Electrons are charged |
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Pass through a magnetic field in a spiral about
the field axis |
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Strength of field determines focal length and
plane |
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Beam displays wave behavior |
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Can calculate l for the beam |
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Adjusting FL adjusts focal plane |
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Wavelength (l) is the key to high resolution! |
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l is
the distance between two “peaks” of a wave |
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Electron beams are particulate but have l! |
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r = 0.612 x l |
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NA |
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Where: |
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is the illuminating wavelength |
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NA is Numerical Aperture of the lens |
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Formula to calculate the wavelength of an
electron…and an electron beam: |
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l = h/mv |
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Where: |
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h = Planck’s constant (6.626 x 10-23
ergs/sec) |
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m = mass of the electron |
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v = electron velocity |
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Simplified version: |
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l = 1.23 |
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Ö V |
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Where V = accelerating voltage |
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Hence as voltage increases, l decreases and
resolution improves |
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r = 0.612 x l |
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NA |
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Where: |
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is the illuminating wavelength |
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NA is Numerical Aperture of the lens |
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TOTALLY INDEPENDENT OF MAGNIFICATION! |
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Distance between closest and farthest point of
acceptable focus |
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Applies to specimen (Depth of field) and to
image (Depth of focus) |
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Controlled with apertures |
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Smaller apertures have greater depth |
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Df = l |
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___________ |
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Sin a |
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Where l = Wavelength of illumination |
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a =
Aperture angle |
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Hence smaller apertures enhance DOF |
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Focuses e-beam on specimen |
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Typical 2-lens system: |
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Lens C1 demagnifies “spot” from gun |
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Lens C2 enlarges “spot” |
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Controls aberrations and illumination level |
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Illuminates only are being examined |
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Minimizes damage and contamination |
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Spot size is adjusted with CL current control |
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High mag requires small spot size: but… |
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Small spots are dim! |
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Aperture angle due to ¯ focal length |
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Loss of electrons |
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OBJECTIVE LENS |
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Makes primary image |
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Most important lens |
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1-2 mm focal length |
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Least variable in strength |
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Image is improved with objective aperture |
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INTERMEDIATE LENS |
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Part of two-stage magnification system |
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Receives image from Objective |
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Increases magnification |
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PROJECTOR LENS(ES) |
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Creates final level of magnification |
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P1 and P2 used in various combinations for final
magnification |
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Designed for great depth of focus |
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CHROMATIC ABERRATION |
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SPHERICAL ABERRATION |
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ASTIGMATISM |
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Result in defects of image and loss of
resolution in the image |
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Waves of different l are brought to a focus at different
points; resolution degraded |
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Peripheral rays refracted more than central ones |
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Degraded focus as a “spot” not a “point” |
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Controlled with apertures |
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Smaller apertures have less SA |
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Pincushion distortion |
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Edges more magnified than center |
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Barrel distortion |
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Center more magnified than edges |
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Can offset each other |
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Most projector lens systems do this |
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Most serious lens defect |
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Results from asymmetric magnetic field in the
lens(es) |
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Different focal lengths in different planes |
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Inherent in all lens assemblies |
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Correctable with STIGMATORS |
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Astigmatism due to distorted field(s) |
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Usually due to manufacturing tolerances |
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Correction is by induction of counterdistortions |
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Equal in strength |
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Opposite in direction |
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Done with STIGMATOR devices |
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May be mechanical, or electromagnetic |
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Used on all lenses in the system |
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Objective stigmator most often used |
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Thin collodion film |
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Holes of various sizes |
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Used to check astigmatism on daily basis |
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Can be made or purchased |
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A: No astigmatism |
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B: Astigmatic condition |
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C: Focus changed to direct astigmatism 90° |
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Equal strength, opposite direction |
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D: Corrected condition |
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TEM uses “subtractive” method of contrast |
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Dense materials scatter electrons |
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Contrast depends on imparting differences in
density in the specimen |
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Apertures increase contrast |
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Top or side entry |
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Side entry usually carries 2-6 specimens |
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Top entry less versatile |
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Why a vacuum? |
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Filament |
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Low electron mass |
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Contamination |
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ATMOSPHERE |
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760 mm Hg |
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TORR |
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1 Torr = 1.32x10-3 ATM |
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1 Torr = 1mm Hg |
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PASCAL |
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International Std |
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1 P = 9.92x10-6 ATM |
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1 P = 7.42x10-3 Torr |
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133 Pascals per Torr |
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Rough pump (forepump, mechanical pump) |
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Removes bulk of air from the column |
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Rotary oil-bath most common type |
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Rotating vanes create vacuum on one side |
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Exhausted air is driven out the other side |
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Oil bath for cooling and lubrication |
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Diffusion Pump |
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Finishes job of removing air |
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Exhausts to rotary pump |
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Will pull final vaccum of 10-5 to 10-7m
Torr in modern scopes |
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Water cooled |
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Oil boiled and directed up & out by vanes |
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Condensation on sides |
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Air molecules drift down and are removed |
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Work like a jet engine in reverse |
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Moving rotors alternate with stators |
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“Sweep” air molecules out |
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Advantages over conventional pumps |
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Oil-less |
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No cooling lines & minimal venting |
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THERMOCOUPLE GAUGE |
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Measures temperature of a wire |
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As air removed, insulation of vacuum wire temperature |
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Gauge correlates temperature and vaccum |
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COLD CATHODE GAUGE |
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High voltage ionizes gas molecules |
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Drawn to cathode |
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Ion current is read & translated to vacuum
units |
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IONIZATION GAUGE |
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Heated wire emits electrons |
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Electrons drawn to grid (+ charged) |
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Gas molecules ionized by “beam” |
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Gas ions drawn to central wire, current
proportional to number of gas molecules |
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Useful in high-vacuum situations |
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Column, camera, specimen port isolated by
airlocks |
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Undesirable to vent entire system |
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Column normally under vacuum at all times |
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Air inlet closed |
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Main valve open |
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Column completely |
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evacuated |
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Rotary pump backs oil pump |
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Camera & specimen airlocks open |
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Air inlet open |
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Camera lock closed |
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Camera backing valve closed |
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Diffusion pump on column |
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Camera door closed |
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Air inlet closed |
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Camera valve closed |
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Backing valve closed |
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Air inlet closed |
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Column evacuated |
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Rotary pump backs oil pump |
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Camera & specimen airlocks open |
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No lens or polepiece is perfect: optical &
physical axes usually not in the same place |
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Alignment is necessary for optimal quality of
images & convenient operation |
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Definition: |
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Condition in which the optical axes of lenses
are coaxial; all apertures are physically centered WRT the optical axis;
and all lens astigmatism is corrected |
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On installation |
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After major cleaning or repairs |
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After filament replacement |
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VOLTAGE ALIGNMENT |
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Filament voltage is varied |
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Image rotates around a central point |
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Lens is shifted to bring center of rotation to
center of screen |
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CURRENT ALIGNMENT |
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Lens current is varied |
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Image rotates |
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Lens is shifted to bring center of rotation to
center of screen |
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Repeated for each lens |
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The two rarely coincide but can be close! |
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Final portion of the column |
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Necessary to visualize image |
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Control of photographic recording |
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Polished bronze plate |
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Phosphorescent coating |
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Glows when impinged on by electrons |
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Energy absorbed, lost and emitted at longer
wavelengths |
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May be in vacuum |
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Some advantages, some drawbacks |
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May be external to vacuum |
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Not most common arrangement |
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Film formats standardized |
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Electrons initiate development centers in silver
halide crystals |
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Crystals catalyze precipitation to metallic
silver |
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Unexposed grains do not precipitate |
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Development is proportional to exposure |
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Stop bath ends further development |
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Unexposed grains removed by fixer |
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Negative image formed by precipitated metallic
silver |
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Jones & Bartlett Publishers |
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Chapman-Hall Inc. |
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Cambridge University Press |
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Eastman Kodak |
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VMRCVM Morphology Lab |
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University of Toronto |
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Dr. Ihab El-Zoghby |
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Dr. Amal A.M. Ahmed |
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JEOL Inc. |
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Notes