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Optical Shop Testing

Chapter 2.4.2 - Testing of Lenses

2.4.2.   Testing of Lenses

One of the early applications of the Twyman–Green interferometer was the testing of
lenses and camera objectives (Twyman, 1920), including the measurement of the
chromatic aberration (Martin and Kingslake, 1923–1924). Any of the arrangements
in Figure 2.25 can be used to test a convergent lens. A convex spherical mirror with

 FIGURE 2.24. Testing a dispersive prism and a diffraction grating.

FIGURE 2.25. Three possible arrangements to test a lens.

its center of curvature at the focus of the lens is used for lenses with long focal lengths
and a concave spherical mirror for lenses with short focal lengths. A small flat mirror
at the focus of the lens can also be employed to great advantage, since the portion of
the flat mirror being used is so small that its surface does not need to be very accurate.
However, because of the spatial coherence requirements described in Section 2.3.1,
the same arrangement or a cube corner prism must be employed on the other
interferometer arm, if a laser is not used. Another characteristic of this method is
that asymmetric aberrations like coma ere canceled out, leaving only symmetric
aberrations like spherical aberration and astigmatism.

When a lens is to be tested off axis, it is convenient to mount it in a nodal lens
bench as shown schematically in Figure 2.26. The lens L under test is mounted in a
rotating mount so that the lens can be rotated about the nodal point N. Since the focal
surface is usually designed to be a plane and not a sphere, mirror M is moved

 FIGURE 2.26. Testing a lens with a nodal bench.

backward a small distanceby pushing the mirror support against a metallic bar
,fixed with respect to lens L. Interferograms obtained with lenses having third
order aberrations will be shown in Section 2.7.

Testing a large lens on the Twyman–Green interferometer requires the use of a
beam-splitter plate even larger than the lens. To avoid this difficulty, according to
Burch (1940),Williams suggested later by Hopkins (1962) for use with a gas laser in
an unequal path configuration.

Complete small telescopes can also be tested with good results as shown by
Ostrovskaya and Filimonova (1969).



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