Editing Amateur telescope making (section)

====Aluminizing or "silvering" the mirror====
Once the mirror surface has the correct shape a very thin coating of a highly reflective material is added to the front surface.

Historically this coating was silver. [[Silvering]] was put on the mirror chemically, typically by the mirror maker or user. Silver coatings have higher reflectivity than aluminum but corrode quickly and need replacing after a few months.

Since the 1950s most mirror makers have an [[aluminum]] coating applied by a [[thin-film deposition]] process (work is done by a firm specializing in the process). Modern coatings usually consist of an aluminum layer overcoated with protective transparent compounds.

The mirror is aluminized by placing it in a vacuum chamber with electrically heated tungsten or nichrome coils that can [[evaporation (deposition)|evaporate]] aluminum.<ref>{{Citation 
|last=Strong 
| first=John 
|editor-first=Albert G.
| editor-last=Ingalls
|title=[[Amateur Telescope Making|Amateur Telescope Making Advanced]] 
|publisher=Scientific American 
|date=1959
|pages=467–482 
|chapter=Aluminizing Mirrors
|editor-link=Albert Graham Ingalls}}</ref> In a vacuum, the hot aluminum atoms travel in straight lines. When they hit the surface of the mirror, they cool and stick. Some mirror coating shops then evaporate a layer of quartz onto the mirror, whereas others expose it to pure oxygen or air in an oven so that the mirror will form a tough, clear layer of aluminum oxide.