Optical Shop Testing

Chapter 2.4.3 - Testing of Microscope Objectives

2.4.3.   Testing of Microscope Objectives

Twyman (1920, 1923) also used his instrument for the testing of microscope
objectives with good success. The arrangement is essentially the one used for a lens,
but a convex mirror, in general, cannot be employed because of the short focus of the
objective. Since the microscope sometimes works at a finite tube length, a negative lens
is added to change the collimated light and simulate a light source 16 cm away. This
lens must be corrected for spherical aberration, but it is not necessary to correct it
quasi-monochromatic. As shown in Figure 2.27, several arrangements can be used to
test an objective, the most common being a spherical concave mirror with its center of
curvature at the focus of the objective. A solid spherical reflector slightly thicker than a
hemisphere can serve to simulate the presence of a cover glass.

A plane mirror at the focus of the objective can also be used, but in this case the
wavefront is rotated 180o. Therefore, we should either use a laser light source or
rotate the wavefront on the other arm. This can be done by means of a cube corner
prism or with another microscope objective with the same flat mirror arrangement. It
should be pointed out that the interferogram in this case represents the difference
between the aberrations of the two objectives.

A fourth arrangement is formed by two oppositely placed microscope objectives.
In this case, the interferogram represents the sum of the aberrations of the two
objectives. However, when the aberrations to be measured are not small or the
pinhole is not small, the best arrangement is the one with the Dyson’s system
described in Chapter 12.

FIGURE 2.27. Testing microscope objectives.

 

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