The ‘Junkyard’ Foucault Tester

After making the pitch lap I excitingly did a couple hours of polishing on each mirror. At this time the mirrors had the beginnings of a polish, the once frosty mirrors were now kind of shiny J

Now it was time to build a Foucault tester. A Foucault tester isn’t really all that complicated. Basically you have a pinpoint, or slit, of light shining towards the mirror, The light bounces of the mirror and returns to the observer, where at the Center of Curvature (COC) the light is interrupted by a razor blade.

The razor blade moves left and right. If the razor blade starts at a position where all the returning light is blocked by the razor blade, the mirror appears dark to the observer. As you move the razor blade aside and start letting some light slip by, the mirror starts to light up. If the mirror is a perfect sphere and the razor blade is exactly at the COC of the mirror, the entire surface of the mirror should start to light up evenly. However if the razor blade is a slight distance away from the COC (too close or too far away from the mirror) One half of the mirror will light up before the other half, depending on the side of the mirror that lights up, you can determine if your too close or too far away from the COC.

Now that we know if were too close or too far away from the mirror, we need a way to precisely move the razor blade towards or away from the mirror. Generally this is done with a screw type micrometer.

So, we need to build a platform that moves Towards and away from the mirror, and the distance moved is measurable down to one thousandths of an inch. The platform also need to be able to move a razor blade left and right with respect to the mirror surface. To move the razor blade we have 2 choices. We can move the entire platform left/right, or we can rotate the platform on an axis parallel with the mirrors optical axis. Most homebuilt Foucault testers rotate the platform on metal rod, which is the same rod that the platform slides on to adjust the distance between the razor blade an the mirror.

Here is what I hacked together. In this picture the mirror is behind us and to the left.

(Red Lines are direction of travel, Item A is the knife edge adjustment bolt)

(round thing is the home made micrometer)

Let me try to explain this thing… The tester has two major parts.

1. The base which holds a metal rod and the micrometer.
2. The platform which slides and rotates on the metal rod, and is pulled towards the micrometer with a spring. The platform also holds the light source, razor blade and video camera (which can be removed and you can stick your eyeball down there instead)

The micrometer is just a ¼-20 threaded rod with a plastic disk divided into 50 increments. One revolution of the rod equals 50 thousandths of an inch. As you turn the rod it moves the platform

The light source is a series of different colored LEDs stacked on top of each other. Half of the LED is covered by the knife-edge and only one LED is illuminated at a time.

The whole thing is mounted on a tripod for easy setup.

This will probably be the first of many Foucault testers I build as this one has some slop in it.