Foucault knife-edge test
The Foucault knife edge method is a test method for imaging optical surfaces. It is also suitable for analyzing the quality of a complete optical systems and is used in astrophotography for focusing the observation instrument. It was developed in 1856 by Jean Bernard Léon Foucault.
The test procedure is also known as knife cutting method, cutting method, Foucault test and other related names.
The Foucault knife edge method is extremely sensitive. It requires very little technical effort and immediately makes a qualitative statement about the investigated optical system, with the smallest deviation from the ideal shape can be seen directly with the naked eye. The quantitative values of the deviations can be determined to within a few nanometer precision over various computational methods.
Application of cutting method using the example of a spherical mirror
The test is best done in a darkened room. As an aid, a very sharp edge ( knife blade, razor blade ) and an artificial star are required. The cutting should be able to be positioned as sensitive (already somewhat putty is suitable for some caution ). The artificial star consists of a tiny hole ( pinhole ) in an opaque foil ( aluminum or brass), which is illuminated by a bright lamp. It is favorable to set for reasons of space in front of the artificial star a small deflection.
The artificial star is positioned approximately ( double focal length of the mirror ) in the center of curvature of the mirror. The mirror forms the artificial star again in the vicinity of this point. The edge should now be set precisely to that figure. A visual inspection of this position is easily possible. One eye has a layer behind the cutting edge have that all the light from the mirror can fall into the pupil.
The mirror will appear as a bright area, which is covered in part by the cutting edge. The cutting edge is too far in the direction of the mirror, the side of the mirror is obscured, on which the cutting is actually located. But the cutting edge is too close to the eye, the other side of the mirror is obscured. If the cutting so far that it covers just the image of the artificial star, the edge of the blade is no longer visible. Instead, the mirror appears slightly darker on the whole surface. Only the parts of the mirror, which deviate from the ideal shape to appear as a side- lit relief either lighter or darker. The spherical shape of the mirror is not visible because the image only shows the deviations from the sphere and not the sphere itself
Such as the example of the spherical mirror is apparent based Foucault knife edge method that all of the light coming from a point light source, is once again collected in one point. Only the light rays coming from a faulty part of the specimen surface, not passing through this point and thus can arrive at the blade passing into the eye.
Testing of other types of optical surfaces
For the testing of lenses, individual lenses or parabolic mirrors as aids another optics necessary. This ensures that the light is converted into parallel light of the artificial star and then combined by the device under test in the focus. Such auxiliary optics must have a perfect shape, so that their errors do not distort the test results.
For lens optics is also the method of auto-collimation offers. Here, a plane mirror is provided behind the test piece, so that the light for a second time the DUT passes through before it is again concentrated in a point.
Examination of astronomical instruments at a bright star
A bright star in the sky is of course also suitable as test equipment. The cutting edge can be secured to a Okularsteckhülse. Also, a special audit approach, which is often called Foucault tester is useful. If such a push-on attachment for an observation instrument designed so that its length is tuned to a specific camera and have the camera and testing approach, the same connections, the testing approach can also serve as a focusing for astrophotography.