Foucault- tester

The french physicist Leon Foucault we owe the brilliant test for checking the surface- shape of a telescope mirror. The basic principals and how the test is done is described in an excellent manner by David Anthony Harbour ( english ) and by Martin Trittelvitz ( german )

My Foucault- testing device is not a beauty, but due to constant made improvements it works very well.
In combination with webcam and computer I can measure differences of the longitudinal aberration within a tolerance of 5/100 mm reproduceable.

For the precission of measuring it is very important to have a steady picture for judging.
I fastened the webcam in a manner, that it can be shifted in two directions and swivelt around the vertical axis. This allows to find the best position for the camera before measuring begins.

The knife egdge can be easily interchanged with a ronchi- grating.
I replaced the origninal lens of the webcam by a 24mm/f2,8 wide angle lens from my Nikon SRC. This enables the webcam to take full size pictures of a f/5 mirror. As a benefit the resolution and sharpness are much better than before. The wide angle lens is connected to a spacer and an adapter with the thread of the webcams´ lens.

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