Untitled. The ATM's Workshop Project Index. Foucault test. TelescopeѲptics.net ▪ ▪ ▪ ▪ ▪▪▪▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ CONTENTS ◄ 4.5.1.
Testing optical quality ▐ 4.5.3. Ronchi test ► Invented by the French scientist Leon Foucault in 1858, this ingenious test uses point source of light placed at the center of curvature of a concave mirror (in practice, slightly to the side, so that the mirror focus is separated from the source, and focusing light can be intercepted without cutting off source of illumination), as illustrated on FIG. 52. The combination of simplicity and accuracy has made it the single most used test in the amateur telescope makers' circles.
FIGURE 52: The principle of the Foucault test. If it is a sphere, a straight-edge shadow moves over the surface as the KE cuts through the light converging to or diverging from the focus (A'); or the uniform, light-grayish shadow spreads over the entire surface when the KE intercepts converging cone at the focus, producing so called null (A). LA = KDρ2/8F = -K(ρd)2/R, LA = KDρ2/16F = -K(ρd)2/2R. Automated foucault testing. A semi-automated Foucault test 1.
Introduction After encountering the usual frustrations trying to figure a relatively fast mirror with a crude home made foucault test apparatus and couder mask I decided that on my next ATM project I would use a real quantitative mirror test. This is especially necessary since my next two planned projects are going to have f/3 primaries, which are generally considered to be untestable in a conventional foucault setup. Occasional discussions on the ATM mailing list and in the literature suggested that it might be possible to turn the foucault test or one of its variants into a useful quantitative test. The method described here was developed independently from a similar approach described by Dick Suiter in an article in Amateur Telescope Making Journal #13, and at the time I posted the original version of this note I had not read Suiter's article. 2.
Figure 1 Figure 2. Automated foucault testing. Homemade Foucault Tester. Foucault Tester To test the optics that a mirror maker is creating, the best tool is a Foucault tester.
This can easily be made by anyone with a minimum of tools: a saw, a drill, hammer and screwdriver and a soldering iron. First of all, I should mention this design should not be copied slavishly. It is a workable tester and is quite easy to build. But there is quite a bit of latitude in design. This page is under construction, as is the tester. Parts: Maglight (tm) light bulb from their small penlights. Basics of the Tester Before I go into the Construction and photos, here is the basics on how the test is used.
The mirror is on the left. Here is a schematic view of the tester as seen from the eye position, looking towards the mirror, explaining the parts of the tester. To perform the Foucault test or to adjust the Ronchi bands to appear where desired, the tilt screw is used to tip the entire top portion of the platform, as seen below: This is an exaggerated view. Construction Photos: Various AtM related projects.