Foucault Simulation
The Foucault test is an optical bench test for evaluating astronomical mirrors. It is named for its inventor, french
optician Leon Foucault in 1859. It is a null test for a sphere. In other words, it is most accurate and simple in
measuring spherical mirrors, but is also commonly used to test parabolic mirrors. The Foucault test is cheap and easy
to do, which accounts for its almost universal use among amateur telescope makers. It basically consists of a light
source, usually a small light bulb, LED or laser pointer, stopped down to a diameter near the mirror’s diffraction
limit by a pinhole, and a knife edge, which may be a razor blade.
The light from the pinhole is directed onto the mirror, and the reflected image of the pinhole is actively "cut off" by the knife edge. The idea is that a perfectly spherical mirror will form an image of the pinhole at a single point. When the knife edge is positioned with its edge at this point, a little motion of the knife edge at right angles to the beam will either let all of the light pass the knife edge and proceed into the eye, or will block all of the light. The mirror will appear to go from a bright disk to a dark disk. However, if the mirror’s surface is not entirely spherical, but rather has some "zones" whose slopes differ from those of the surrounding mirror, then light from these zones will not be returned precisely to that point. It may pass beyond the knife’s edge when light from the rest of the mirror has been cut off, and its source zone will appear bright when the rest of the mirror is dark.
Couder Mask
Foucault testing of telescope mirrors using a Couder mask is well described in Jean Texereau’s book "How to Make a Telescope". A Couder mask has several openings in pairs, each displaying a part of an annular zone of the mirror under test. The Foucault tester has a narrow light source and an occluding "knife edge". This straight edge is movable along the optical axis as well as across it. You hold your eye a little behind the edge, looking past it at the mask, to compare the brightness of the holes. For each hole pair in turn, we determine the position along the optical axis where the holes are equally dimmed by the knife edge as it is moved sideways to occlude part of the returned light. Finally for each zone, the knife edge position determines the average slope, and we can use the technique described by Texereau.
The light from the pinhole is directed onto the mirror, and the reflected image of the pinhole is actively "cut off" by the knife edge. The idea is that a perfectly spherical mirror will form an image of the pinhole at a single point. When the knife edge is positioned with its edge at this point, a little motion of the knife edge at right angles to the beam will either let all of the light pass the knife edge and proceed into the eye, or will block all of the light. The mirror will appear to go from a bright disk to a dark disk. However, if the mirror’s surface is not entirely spherical, but rather has some "zones" whose slopes differ from those of the surrounding mirror, then light from these zones will not be returned precisely to that point. It may pass beyond the knife’s edge when light from the rest of the mirror has been cut off, and its source zone will appear bright when the rest of the mirror is dark.
Couder Mask
Foucault testing of telescope mirrors using a Couder mask is well described in Jean Texereau’s book "How to Make a Telescope". A Couder mask has several openings in pairs, each displaying a part of an annular zone of the mirror under test. The Foucault tester has a narrow light source and an occluding "knife edge". This straight edge is movable along the optical axis as well as across it. You hold your eye a little behind the edge, looking past it at the mask, to compare the brightness of the holes. For each hole pair in turn, we determine the position along the optical axis where the holes are equally dimmed by the knife edge as it is moved sideways to occlude part of the returned light. Finally for each zone, the knife edge position determines the average slope, and we can use the technique described by Texereau.