The Foucault method is the primary method for determining the shape of
a mirror it works by emitting a slit of light at the mirror and coming
back to a straight edge which cuts through the light displaying shadows
indicating defects from a sphere. Ronchi gratings provide a somewhat
easier to read overview of a mirror and it is easier to get by with
less sophisticated equipment it uses a slide of parallel lines with a
light source behind it and then the reflected light has lines running
through it which are bent indicating deviations from a sphere. At
stellafane they use a flashlight with prism duct taped and a Ronchi
screen mounted to that for basic testing. I never had the opportunity
to use a Mobsby test but I have heard good things about them it is
similar to a Ronchi but the lines show deviations from a parabola. a
interferometer is the complex tester with a video display they had at
last year's Stellafane (that's what the picture is of) are to costly
for most amateur mirrors.
