Principle of interferometry

Radio interferometry is based on the interference of electromagnetic waves. Consider the reception of a plane radio wave from a distant point source that is directly above two radio telescopes, separated by a short distance B (baseline; Fig. 1a). The wavefronts in Fig. 1a arrive at the two telescopes in phase because the source is directly overhead. If the coaxial cables carrying the signals (electromagnetic waves) from the telescopes bring the signals together, the electric and magnetic fields are added to yield radio-frequency oscillations of large amplitude. This is constructive interference.

If earth rotation causes the source direction to be displaced from the zenith by the angle 9 = X/2d (Fig. 1b), the wavefronts arrive at the two telescopes exactly out of phase. The addition of the signals ideally yields a zero output (destructive interference). At increasing angular displacement A9, this pattern repeats with constructive interference at A9 ~ X/B,2X/B,3X/B, etc., for approximately normal incidence (9 ~ 90°). The angular separation between adjacent constructive directions is, for

If X = 1 m and B = 100 m, then A9 = 1/100 « 0.5°; the earth rotates 0.5° in 2 minutes.

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