Mosby Null Test

The Ronchi test has been popular for many years for testing spherical surfaces as the test produces nice straight lines which are easy to understand. This method has also been used for testing aspherical surfaces but, as the fringes thus produced at the detector are not straight, thier shapes must be calculated to get the correct surface shape. However, this procedure does not give accurate results mainly because of the following reasons:

The technician figuring the surfaces has no easy way to match a curved fringe to a theoretical curve line. I would be a lot simpler to match a straight fringe to a straight line.
Since the fringes are curved, the diffraction effects tend to diffuse the fringes, making the measuring procedure very uncertain and inaccurate.

The above disavantages can be overcomed by means of a special kind of Ronchi ruling with curved lines, the curvature of which compensates for the asphericity of the surface to produce straight fringes. This method is called Mosby Null Test. The only restriction for the test is that it is specific for a particular focal length and size of mirror. If you are interested in doing several mirrors of the same focal length, this is a nice test as it is an easy to run on the mirror and is accurate in determining if the mirror is good or not. It is possible to make the grating large enough and to apply circles of different radius, so we can test different diameters, one for each size we have put on the grating. Similar to the Mozby is the Terebizh Null Test, that use gratting with concentric spaced rings.

Testing

Null Test

Terebizh

Mosby

Simulation

Data Reduction

Telescopes

Mosby Null Test