Foucault Test

Foucault test is very popular for amateur astronomers to inspect the surface accuracy of mirrors. Some people say that Foucault test can't check enough accuracy because of its geometrical way. Simulaion showed Foucault test's charactaristic.

At first, simulation showed that the shade patern depended on the pin hole size. See the 0.2mm diameter pin hole and the 0.3mm one.
Left : 0.2mm hole, Right : 0.3mm hole

They show that smaller pin hole make a higher contrast image.
Then, let's see a parabola mirror with wave error in one-direction. By rotating of the mirror, it is easy to understanda that Foucalut test can detect only one-direction because the knife cuts only one way.

0,30,45,60,and 90 degree rotated Foucault test images

1, Foucalut test can detect only one dimension error.
2, Smaller pin hole shows higher contrast image.

September 4th, 1999 originally ( Japanese )
June 5th, 2002 ( English )