Astronomy Principles and Practice Fourth Edition.pdf

Telescope resolving power 247
Figure 16.7. Intensity scan through a diffraction pattern.
Figure 16.8. The diffraction pattern in the focal plane of the objective.
For a collector in the form of an objective lens, the resultant diffraction pattern produced from
a point object appears as a spot at the centre of a system of concentric rings (see figure 16.6). The
strength, or energy contained in each ring, decreases according to the number of the ring. However,
after the third ring it is difficult to detect the presence of the outer rings. According to theory, 84% of the
energy is concentrated into the central spot. As this type of diffraction pattern was first investigated by
Airy, the central spot is sometimes referred to as the Airy disc. By making a scan along a line through
the centre of the pattern, an intensity profile such as that illustrated in figure 16.7 can be obtained.
The sizes of the diffraction rings are also predicted by theory. By defining α to be the angle
subtended at the collector by the centre of the Airy disc and a point in the diffraction pattern (see
figure 16.8), the positions for minima of intensity are given by
sin α n = m nλ
D
where n is the number or order of the minimum, m is a numerical factor which is obtained by
performing the integration of the interference from points over the telescope aperture, λ is the