By Professor Jean Kovalevsky (auth.)
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Left: pincushion distortion. 7 Chromatic Aberration Until now, in the presentation of instrumental defects, we have assumed monochromatic light. The actual generalised image is the superposition of monochromatic images which are not necessarily identical and situated in the same place. This is generally the case when the optical system includes lenses whose refractive index is wavelength dependent so that its dispersion is also colour dependent. This effect can be reduced by coupling a convergent lens made of a glass with high refractive index and small chromatic dispersion with a divergent lens made of a glass with low refractive index and large chromatic dispersion.
In this formula, ro is the radius of the Earth and h the mean altitude of the ionosphere. However, this assumption is only approximate and, in general, all astrometric observations are performed in dual frequency mode. Plasma refraction may be very large. For instance, delays in pulsar radio pulses observed at 1400 MHz due to interstellar and interplanetary plasmas are of the order of a millisecond when the frequency is shifted by only ten megahertz. 4 Heterogeneity of the Atmosphere In the preceding sections, we have assumed that the atmosphere has smooth optical properties so that it could be treated as an additional refracting optical medium.
This time interval is the coherence time. The interpretation of this result is the following. In a wave train having an energy spectrum represented in Fig. t. Now, since f = cl).. D is the coherence length. f. f. To take an example consider a bandwidth of 1 nm (10 angstrom filter) at a wavelength of ).. 5 Mm (green light). 3 1O- 13 s. The very small coherence length is one of the major difficulties in optical interferometers (see Chaps. 9 and 10). 24 2 Image Formation Finally, let us note that the notion of coherence time and length was first introduced in radio techniques.