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Optical Shop Testing

Chapter 11 - Star Tests

11.1.   INTRODUCTION

This chapter has been rewritten and updated by the first author, largely based on the
previous version by late Prof. W. T. Welford. The star test is conceptually, perhaps,
the most basic and simplest of all methods of testing the image-forming optical
systems. We examine the image of a point source formed by the system and judge the
image quality according to the departure from the ideal image form. In principle, the
test can be made quantitative by, for example, photoelectric measurement techniques,
but in practice the star test in the workshop is almost always carried out visually
and semiquantitatively.We shall mainly discuss visual techniques.We can divide star
testing methods into two groups: (a) those in which very small aberrations, near or
below the Strehl tolerance limit, are examined and (b) those in which relatively large
aberrations are studied. Group (a) is typified by tests on microscope and telescope
objectives and group (b) by tests on camera lenses in which, for example, the star test
is used to plot the astigmatic field surfaces or to estimate transverse chromatic
aberration.

The monochromatic image of a point source, or point spread function as it is often
called, has a very complex structure, particularly in the presence of aberrations. The
structure depends in a complicated way on the geometrical aberrations, but it would
be out of place in a book on practical methods to go deeply into this aspect. Moreover,
it is always possible in principle to calculate the point spread function from the
aberrations, although the calculation may in practice be costly in computer time; but
for the present purposes we should like to be able to estimate the aberrations from the
form of the point spread function. This, however, is generally impossible in principle.
If the aberration is axially symmetrical, this calculation could be done from very
careful measurements of the light intensity in the star image. We have to make
estimates based on experience and on the many examples of point spread functions
that have been computed and photographed from known aberrations. Thus, the star
test is semiquantitative, and considerable experience is needed to get the best results
from it.

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