Ronchi Simulation
The Ronchi test was first described by Vasco Ronchi (1897-1988) in 1923. It is much like the more commonly used Foucault
test except that the knife edge is replaced by a grating consisting of fine, opaque, equally spaced lines ruled onto a
transparent substrate. The shadows (actually silhouettes) of these fine lines of the grating appear projected onto the
face of the mirror under test. The shape and position of these bands is examined and interpreted to give information about
the shape of the mirror's surface.
The Ronchi grating is plastic or glass plate with parallel lines etched or engraved on it. These lines are very small and perfectly spaced and straight. Commonly used are 5 lines per milimeter (lpm) gratings, but also used are 10 lpm or more for special problems.
When the Ronchi grating is placed near the focus of a converging beam of light, any imperfections in the optical system will show as deviations from the perfectly straight lines on the Ronchi grating. You can tell from a glance exactly what problems exist on the surface of the mirror under test. Notice that the farther you get from the center of curvature the more lines appear. When you are at the center of curvature you see no lines, but rather a messy, confused circle. Also notice that you can move so far away from the center of curvature that the mirror is filled with zillions of lines. This is important: the farther the grating is from the center of curvature, the more lines that are seen. If the mirror is spherical, then these lines will be straight and parallel, no exceptions. Anything else implies a non-spherical mirror.
As light source can be used light emitting diode (LED) or laser pointer. The usages of such light sources is recommend, because they are bright, monochromatic and cool. For Ronchi testing, the stand doesn't need to be complicated, just able to be moved and adjusted. The mirror is set on a stand so that it is facing horizontally.
The applet bellow can help the amateurs by mirror testing. Such programs are used to give you a very good idea of the degree of correction of your mirror. You can use the test to assign an upper bound to the accuracy of your mirror's figure. All that is required is to compare your mirror to not only a perfect virtual mirror but also to a mirror with a known amount of wavefront error generated by the program. To do this, you will need to find the conic constant for a mirror similar to yours that deviates from perfect by the amount of wavefront error you specify.
The Ronchi grating is plastic or glass plate with parallel lines etched or engraved on it. These lines are very small and perfectly spaced and straight. Commonly used are 5 lines per milimeter (lpm) gratings, but also used are 10 lpm or more for special problems.
When the Ronchi grating is placed near the focus of a converging beam of light, any imperfections in the optical system will show as deviations from the perfectly straight lines on the Ronchi grating. You can tell from a glance exactly what problems exist on the surface of the mirror under test. Notice that the farther you get from the center of curvature the more lines appear. When you are at the center of curvature you see no lines, but rather a messy, confused circle. Also notice that you can move so far away from the center of curvature that the mirror is filled with zillions of lines. This is important: the farther the grating is from the center of curvature, the more lines that are seen. If the mirror is spherical, then these lines will be straight and parallel, no exceptions. Anything else implies a non-spherical mirror.
As light source can be used light emitting diode (LED) or laser pointer. The usages of such light sources is recommend, because they are bright, monochromatic and cool. For Ronchi testing, the stand doesn't need to be complicated, just able to be moved and adjusted. The mirror is set on a stand so that it is facing horizontally.
The applet bellow can help the amateurs by mirror testing. Such programs are used to give you a very good idea of the degree of correction of your mirror. You can use the test to assign an upper bound to the accuracy of your mirror's figure. All that is required is to compare your mirror to not only a perfect virtual mirror but also to a mirror with a known amount of wavefront error generated by the program. To do this, you will need to find the conic constant for a mirror similar to yours that deviates from perfect by the amount of wavefront error you specify.