Astronomy Principles and Practice Fourth Edition.pdf

Measurements at high angular resolution 337
Figure 20.4. The basic geometry of the Fresnel pattern associated with the lunar occulting edge.
For any telescope/photometer system recording the disappearance of the star, the diffraction
pattern is made to sweep across the aperture by the Earth’s rotation and also by the diffracting edge
moving according to the lunar orbital motion, the former being the dominant factor in determination
of the timescales. A simple calculation of the timescale for the passage of a fringe is given by
t ≈
xP ⊕
2π R ⊕
where R ⊕ and P ⊕ are the radius and period of rotation of the Earth respectively. By inserting the
necessary values with their appropriate units
(14·5) × 24 × 3600
t =
2π(6370 × 10 3 ≈ 0·030 s.
)
Thus, the diffraction pattern may be recorded simply by letting it sweep across the telescope
aperture and undertaking the brightness sampling with sufficient time resolution. Figure 20.5 displays
the pattern according to the time of its progression across the photometer.
Because the stellar source has a finite angular size, the recorded pattern does not correspond to
that expected of a point source, the oscillations being not as well defined (see figure 20.5). Differences
between the record and that of a theoretical point source allows determination of the diameter of the
stellar disc. The number of stars that offer diameter determinations in this way is of course limited to
the very bright ones which happen to lie in the zone of sky through which the Moon travels.
20.3.5 Speckle interferometry
When a very large telescope is used with high magnification to observe a star, the image may be seen
to have a rapidly changing grainy structure or exhibit a speckle pattern. Such effects are produced by
the turbulence in the atmosphere above the telescope. Any bright portion in the pattern results from