[OzEclipse (Joe Cali)]

Hi Neil,

Foucault testers are generally illuminated to be bright enough for diffraction patterns to be visible on an un-silvered polished mirror - reflectivity 1-4%. A 96% mirror will therefore be very bright.

What you are seeing could be correct. I just can't tell from that photo. You could be seeing some minor un-eveness in the figure and a Foucault shadow. It's hard to tell from that photo.

My tester is a dual ronchi / Foucault tester based on Jean Texereau's original model. It uses a razor and a very fine brass mesh at each end of a rotating arm so you can position the ronchi grating or Foucault blade at the optical axis. Keep the light source and viewing point as close together and on the optical axis as possible. The horizontal strands have no optical effect because of the vertical slit over the light source only diffracts with the vertical strands of the mesh.

I'll post some pics of my tester when I get a chance.

There are many ways to determine the exact ROC. You can use the sun to determine the focal length with a piece of card attached to the end of a long stick. Point the stick toward the sun, slide the mirror along and mark the position on the stick where the suns image comes to a focus on the card. With your tester and a a silvered mirror, place a piece of tracing paper at the viewing point. Adjust the position of the tester until the slit comes into sharp focus.

Joe