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? <.6$zcW  
The answer is a resounding "No" d4-cZw}+  
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. ;,LlOR  
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! DfP vi1  
Even firms in the optics industry who should know better are being taken in by Interferometer results. We are making some information available on the level of accuracy that can be expected from our tests compared with Interferometer tests. !O8vr4=  
"Conventional Methods" - The Double Pass Null Test }Rq-IRa'  
What follows is a description of the Double Pass Null Test as carried out at Oldham Optical. This is the basic test we use on all large parabolic mirrors. The description is very simplified and is aimed at peak to valley (PV), measurement, but all the Double Pass strengths are brought out to illustrate why it is such a good test of a mirror. This test is also known as "Auto-collimation" and most professional mirror makers agree with us that it is the definitive test of a parabolic mirror. JSiLG0  
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The diagram adjacent shows the basic arrangement of a parabolic mirror set up under test facing an optical flat that has a central hole. A point light source is set up near the focal point of the mirror and shines through the central hole onto the surface of the parabolic mirror. K<tkNWasQ  
The light reflects back parallel to the axis of the system to the optical flat which reflects it back along the same path to the parabolic mirror again. $yg}HS7HC  
It reflects off the parabolic mirror a second time and returns to a focus near the original light source. In practice the light source has to be set up just slightly off axis so the focus of the reflected light can be accessed. eTa[~esu.  
A knife edge is set up at the exact point of focus. The knife has micrometer adjustments to allow it to be adjusted slowly and accurately into the returning light cone.