The photograph above was taken by Monica Alcazar-Duarte when she visited us in 2012. It shows Terry Pearce using a simple tester to examine the surface of a mirror. The pictures on the right were taken using this tester

This image shows the surface with the tester set for the focus of the centre zone of the mirror
 
  This image shows the surface, with the tester set for focus at 70% out from the centre of the mirror
 
 
This image shows the surface with the tester set for the focus of the edge zone of the mirror