NI ring adjustment 2

After the work done yesterday (NI ring adjustment) that did not seem to be enough, this morning we (Laura and I) tried again to improve the NI ring.
First we tried Eric suggestion: to put the iris where the beam is focused (between L1 and L2). Unfortunately, this did not help, since the beam is so small that the residual hole in the iris is far too large. A real pin-hole would be needed.
So we implemented the back-up solution: we increased the size of the mask (see figure 1) to remove as much junk as possible from the centre of the ring.
Figure 2 shows the ring with the old mask and figure 3 with the new mask. There is clearly less power in the centre, but there is no improvement in the power distribution of the ring.