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Glossary terms about Aperture

In electron microscopy: a minute hole in a strip of metal that is placed in the path of the electron beam in order to restrict or limit electron progress down the machine column. An aperture stops off-axis or off-energy electrons.

17 pages mention Aperture

Acceleration voltage vs. specimen type
Beam Adjustment
This might include changing an aperture to a larger or smaller size, adjusting the gun controls to provide the brightest image, and adjusting the “wobbler” to align the beam axis. 
See Principles of operation – Electromagnetic lenses, apertures and beam size for more on SEM theory. 
Electromagnetic lenses, apertures and beam size
A series of electromagnetic lenses and apertures are used to reduce the diameter of the source of electrons and to place a small, focused beam of electrons (or spot) onto the specimen. 
Electron-matter interactions
Environmental scanning electron microscope (ESEM)
Generating an image
Magnetic lens system
This aperture is used to reduce or exclude extraneous (scattered) electrons. 
Principles of SEM operation
Electromagnetic lenses, apertures and beam size  
Role of sample height
This can be offset by using a smaller objective aperture and putting up with the reduction in electrons that comes with this choice (grainier image). 
Sample preparation
Saturating the filament
Spot size
A smaller objective lens aperture produces a smaller spot size. 
Troubleshooting: edge effect, charging, sample damage
The number of electrons relates to a number of parameters including, beam current, the emission level of the gun, the spot size, and the apertures between the gun and the specimen. 
What is an SEM (Parts of the machine)?
What is resolution?
NA is the numerical aperture of the objective lens.