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? mE>{K  
The answer is a resounding "No" Y|J=72!]  
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.