All astrometric techniques use the properties of incoming electromagnetic radiation emitted by celestial bodies in order to determine the direction from which they where emitted or to describe the emission structure in some small portion of the sky. The problem is twofold: to collect light and measure its properties at the receiver end of an astronomical instrument and to interpret the measurements in terms of the properties of the actual emitting bodies in the sky. These two aspects of astrometry imply that one is able to describe exactly the transformation undergone by the light (or radio waves) between the celestial body down to the sensitive part of the instrument. Actually, except in one particular case of pulsar timings, one may assume that no significant effect on light is produced when crossing the interstellar and interplanetary media. So one has to consider the instrument on one side, the atmospheric effects on the other. The latter will be examined in Chap. 3. In the present chapter, we introduce the study of instrumental effects by presenting some basic results in optics that will be used throughout the book.
KeywordsCoherence Eter Refraction Fermat sinO
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