This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
 

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Roughing pumps are the vacuum pumps that are used to initially evacuate a vacuum system. This is the first stage to obtain high vacuum or ultra high vacuum. Actually, it is named by ‘roughing’ because this is the very first, initial step of pumping to low vacuum. Roughing pumps generally are mechanical (rotary) pumps. However, there are some 'dry' roughing pumps which do not use oil. They are more expensive and less reliable, and they do not pump to the low pressures that rotary pumps can provide.

Roughing pump(s) should remove 99.999% of the gas molecules from the vacuum chamber. A roughing pump gives a vacuum pressure between 1 and 10⁵ Pa. With roughing the chamber, the flow of the molecules is changed from a viscous flow to a molecular flow.

In general, the vacuum systems of standard EMs are composed of mechanical pump(s) (rotary pumps, rough pumps, or forepumps), diffusion pump(s), vacuum gauges, switching valves, and a network of connecting pipes.