Editing Amateur telescope making (section)

====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.