Single slit diffraction

This is a ripple tank picture of plane waves (ie long straight wavefronts) coming down to hit a barrier with a gap in it. The gap is about one wavelength wide. The waves fan out below in semi-circles. I have never managed to do it well enough to be able to see the dead spots in the semi-circles in the ripple tank. Try https://johnvagabondscience.wordpress.com/2008/10/25/huygens-and-single-slit-diffraction/ for a better picture. But it did work quite well with a laser.

The single narrow beam is spread out into a blurred line - that's the semi-circle Then we could see two dimmer patches off either end of the main blob. The gap to the dimmer patch is the dead spot that I'm struggling to see in my ripple tank.