There is an incredible amount of interest in testing mirrors using Interferometers and we are always being asked if testing with Interferometers is better than a conventional Null test? W2}%zux  
The answer is a resounding "No" {D;Xa`:O  
Interferometers cannot compete with the "Time Honoured" method of a knife edge and the human eye in a Double Pass Null test for accuracy. If they could, we would be using an Interferometer instead of conventional methods. aeG#: Ln+{  
But it is amazing the faith placed in an Interferometer result! We are occasionally getting challenged about the specification of our mirrors on the basis of a poor Interferometer test. There seems to be a semi religious belief that the Interferometer result is correct and our method is wrong, - when in reality it is the opposite way round! 2>!_B\%)