Saint Petersburg Astronomy Club - Mirror Lab

Robo Foucault

We have constructed a robotic foucault tester which is superior to the subjective manual foucault test. The robotic foucault test is faster, non-subjective, and more accurate than traditional testing by hand.

The Foucault Test

"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." - From lab member James Lerch's website

Leon Foucault
Jean Bernard Leon Foucault ("Foo-KOH") was born on September 18th 1819 and died February 11th 1868. Besided the Foucault test he is known for inventing the Foucault pendulum, the gyroscope, the Foucault polarizer and supposedly discovered eddy currents.

Leon Foucault invented this mirror testing technique in 1858. It tests how much a mirror deviates from spherical. In 1862 Foucault used a variation of his test to determine the speed of light to within 0.6% of todays accepted value.

The Software
Real Time Automated Foucault Tester or RTAFT is the software program that runs everything. (written by James).

In addition James made another program called figureXP as a replacement for and old dos program figure45.

Mirror Lab downloads
figureXP (4.2MB)

Dave Bevels RoboFoucault page