RONCHI

Null Testing

A range of possible null tests.

Copyright – P. J. Smith

But permission is given to distribute this material in unaltered form as long as it is not sold for profit.

BRIEF EXPLANATION OF NULL TESTING

A Null test is one done on a spherical wavefront. Since there is no residual spherical aberration in the final image, the tester may concentrate fully on removing errors of smoothness which otherwise could be seriously masked.

Tests are far more sensitive in the absence of any spherical aberration.

Testing a sphere at the Center of Curvature and a paraboloid with parallel light from a star are both Null tests.

It is possible to test surfaces using auxiliary optical elements that deliberately introduce an equal and opposite amount of Spherical Aberration. When tested and figured to the null, the surface under test is then both extremely smooth and has the correct amount of asphericity

The nulling elements must, of course, be of guaranteed quality, but they may be much smaller than the surface under test and use spherical surfaces, so are easier to make to a high precision.

Professionals use this technique a lot. Amateurs tend to struggle through by ingenious but less effective methods and forget that both Foucault and Ronchi tests are really null tests. Most of the typical ATM criticism of Ronchi testing is invalid when applied to Null tests.

In many, but not all null tests, the rays retrace their path so source and image are superimposed - or at least are very close to each other. In this situation, if a quality grating is available, the Grating/Grating variation of Ronchi testing may be convenient. A Slit/Grating works at least as well but is more awkward. Pinhole/gratings are also useful. The Laser Diode source is interesting because of its small footprint – it may be easily placed in inaccessible places. Another useful option is piping light in via optic fibers.

Be aware that, depending on the optical arrangement, Ronchi patterns may be the inverse from expected.

Anyone who is serious about any null testing these days should obtain a Raytracing program. This will alleviate the painful computations, which were once necessary for most of the more complex nulls.

Examples

What follows is a selection of possible useful nulls. They are included to give some idea of what is possible but the selection is by no means complete. This is a topic in itself.

In some cases, the diagrams show a knife edge. This may, of course, be replaced with a Ronchi grating and expert operators will probably use both.

At top is a null test for a paraboloid using a flat. If the flat is perforated, the small diagonal may be dispensed with.

The bottom image is of the Ross null lens used to produce a paraboloid. A Dall null test is similar but the light only passes through the lens only once.

The famous Hindle test (above) uses the geometry of conics to produce a perfect convex Hyperboloid using a nulling sphere.

Unfortunately, the nulling sphere must be large

Paraboloids or a limited range of other conics may be tested using a nulling sphere as above. This is often referred to in the ATMing community as the Waineo test. Although it was around long ago, it may be an appropriate name because Tom Waineo did vigorously push its advantages.

This is often used to test the smoothness of a very weak lens or parallel plate by placing it in front of a concave sphere. Often used as a test for glass homogeneity.

The above may be used to test convex surfaces if the lens is perfect. This can be realized by using a quality microscope objective for the lens. Because there is a limited range of tests available for convex spheres this is very useful.

It is possible to test a convex surface through the back of a lens. This can produce a range of gentle hyperboloids. Of course, the glass and the front surface must be almost perfect.

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