A range of possible null tests.
GOTO RONCHI INDEX
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.
GOTO RONCHI INDEX