How to produce the mirror
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| How to produce the mirror |
In modern aluminum silvering, a sheet of glass is placed in a vacuum chamber with electrically heated nichrome coils that can evaporate aluminum. 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 makers evaporate a layer of quartz or beryllia on the mirror; others expose it to pure oxygen or air in an oven so that it will form a tough, clear layer of aluminum oxide.
Mirrors made by this method are classified as either back-silvered, with the silvered layer viewed through the glass; or front-silvered, (called a first surface mirror) with the reflective layer on the surface towards the incoming light or image. Most common household mirrors are back-silvered since this protects the fragile reflective layer from corrosion, scratches, and other damage. However, precision optical surfaces normally need the reflective material on the front surface of the glass to avoid introducing optical aberrations. First surface mirrors use the substrate to keep form. There are optical mirrors such as Mangin mirrors that are back-silvered (reflective coating on the rear surface) as part of their optical design.
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| How to produce the mirror |
Although the silvering on a second surface mirror such as a household mirror is often actual silver the "silvering" on precision optical instruments such as telescopes is usually aluminum. Even though silver has the best initial front-surface reflectivity in the visible spectrum it is unsuitable for optical mirrors because it quickly oxidizes and absorbs atmospheric sulfur to create a dark, low-reflectivity tarnish. Although aluminum also oxidizes quickly, the thin aluminum oxide (sapphire) layer is transparent, and so the high-reflectivity underlying aluminum stays visible.
The "silvering" on infrared instruments is usually gold. It has the best reflectivity in the infrared spectrum and has high resistance to oxidation and corrosion.
