I have discussed a method of inserting a mechanical iris in front of the Null lens in order to easily adjust it to its correct position.
A Null lens correction of a wavefront is a measuring technique, where a specifically designed lens is inserted into the light path.
The wave travels through the lens, reflects from test optics, passes through the lens again. The reflected wavefront transforms according to the test optics surface. Assuming the test optics shape is as desired, the null lens transforms it back to spherical (or flat) wavefront, measurable with an interferometer. Errors seen in interferometer are errors on the mirror, even when the mirror is not supposed to be spherical or flat. Null lenses are designed specifically for their test optics.
Problem with the technique is adjusting for the correct distance from the optics under test. Usually the tolerance for error is hundred parts of a millimeter over several meters. The other distances are relatively small and can be adjusted and verified with contact measurement techniques.
Here (pdf) I have discussed a method of inserting a mechanical iris in front of the Null lens. It's purpose is to throttle the numerical aperture of the beam into a tight aberrationless beam that will only illuminate the central parts of the test optics, where the difference between spherical and aspherical wavefronts are not yet detectable.
Using this property, one may have a rough idea of where the correct position might be and adjust distance accordingly, because the tolerance for aberrationless wavefront would be very relaxed. Then, by opening the iris slightly and thus tightening the tolerance, one can adjust the distance with higher accuracy than with previous numerical aperture. This cycle of opening the iris and adjusting to minimum aberration would end up with the original numerical aperture illuminating the whole aperture of the test optics and the definite wavefront aberration.
Null lenses can be used with Väisälä two-slit interferometric test as well as other types of interferometers.