Editing Amateur telescope making (section)

===Mirror making===
[[File:Grinding a mirror - tool over 300 mm glass (Korenica, Croatia).jpg|thumb|left|200px|Grinding a mirror using an abrasive and a smaller tool over {{val|300|u=mm}} mirror ("ATM Korenica 2006", in [[Korenica]], [[Croatia]])]]
Since the [[Newtonian Telescope|Newtonian reflector]] is the most common telescope built by amateur telescope makers, large sections of the literature on the subject are devoted to fabrication of the [[primary mirror]]. The mirrors start as a flat disk of glass, typically [[Flat glass|plate glass]] or [[borosilicate glass]] (Pyrex).<ref>Gary Seronik, Four Infamous Telescope Myths, skyandtelescope.com, July 28, 2006</ref> The disk is carefully [[Abrasive machining|ground]], [[Polishing|polished]] and [[figuring|figured]] to an extremely accurate shape, usually a [[parabola|paraboloid]]. Telescopes with high [[focal ratio]]s may use spherical mirrors since the difference in the two shapes is insignificant at those ratios. The tools used to achieve this shape can be simple, consisting of a similarly sized glass tool, a series of finer abrasives, and a polishing pitch lap made from a type of tree sap. Through a whole series of random strokes the mirror naturally tends to become spherical in shape. At that point, a variation in polishing strokes is typically used to create and perfect the desired paraboloidal shape.

====Foucault test====
{{main|Foucault knife-edge test}}
The equipment most amateurs use to test the shape of the mirrors, a [[Foucault knife-edge test]], is, like the tools used to create the surface, simple to fabricate. At its most basic it consists of a [[light bulb]], a piece of tinfoil with a pinhole in it, and a razorblade.
[[File:Foucault-Test 1.png|thumb|left|350px|Foucault test setup to measure a mirror]]
[[File:Foucault-Test 2 rotated.jpg|thumb|right|120px|Parabolic mirror showing Foucault shadow patterns made by knife edge inside radius of curvature R (red X), at R and outside R.]]
After the mirror is polished out it is placed vertically in a stand. The Foucault tester is set up at a distance close to the mirror's radius of curvature. The tester is adjusted so that the returning beam from the pinhole light source is interrupted by the knife edge. Viewing the mirror from behind the knife edge shows a pattern on the mirror surface. If the mirror surface is part of a perfect sphere, the mirror appears evenly lighted across the entire surface. If the mirror is spherical but with defects such as bumps or depressions, the defects appear greatly magnified in height.  If the surface is paraboloidal, the mirror looks like a doughnut or lozenge. It is possible to calculate how closely the mirror surface resembles a perfect paraboloid by placing a special mask over the mirror and taking a series of measurements with the tester. This data is then reduced and graphed against an ideal parabolic curve.

Some amateur telescope makers use a similar test called a [[Ronchi test]] that replaces the knife edge with a ''grating'' comprising several fine parallel wires or an etching on a glass plate. Other tests used include the Gaviola or Caustic test which can measure mirrors of fast f/ratio more accurately, and home-brew Interferometric testing made possible in recent years by affordable lasers, digital cameras (such as webcams), and computers.

====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.