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A simple Foucault and Ronchi tester is not difficult to construct and if you are thinking of making a mirror, such a tool is a must for regular checks when figuring. There are plenty of links on the web on how to construct a tester. You do not need the micrometer on the Foucault tester if you do not want to quantify the results as a visual inspection of the shadowgram will soon reveal the surface you have and any defects present. All you need is a light source (25W small globe covered by a small box with a 10mm opening to let the light through), a knife edge (Stanley knife razor edge) and a slit (two razor edges or a some aluminium foil with a 5mm slit cut into it sandwiched between a slide mount (no glass). You will also need a mirror holder to hold the mirror vertical when testing. For the Rhonchi test you replace the knife edge with a grating or very very fine wire mesh.
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