Foucault+Tester

The old Foucault Test has served us well since the making of the TROK. This was a fixed source light source with knife edge and Ronchi screen for testing.

In January 2013, it has been rebuilt with a fixed source adjacent the knife edge. This allows a Digital Laser distance finder (Bosch DLE 40) to measure the radius of curvature of the mirror under test to +/- 1.5mm.

This should be sufficient accuracy of ROC for the Bath Interferometer using OpenFringe.

The Knife or Ronchi can still be used. The Ronchi can be mounted in front of the Knife so that the stage only needs minor tilt, to bring the returning beam onto the Ronchi, and display 6 or more lines without axial movement.

The light source is an LED (Maplin N28FN, 3.6V forward, 5V reverse, 100mA forward) in line with a 120 Ohm 1W resistor and a 10K linear pot with built in switch. Power is a 9V battery.

A digital micrometer is fitted to the optical axis (10 micro scale). The tester is fitted to a tilt table for axial alignment of the laser to the centre of the mirror before testing to ensure proper alignment.