Interferometric Observation

Abstract

The image of a light source as produced by two narrow apertures in the beam becomes a ‘fringe’ pattern of alternating high and low intensity. Compared with the ‘maximum’ visibility of the fringes as given by a monochromatic point source, a double star does not differ if the line joining the stars is perpendicular to that joining the interferometer slits. Minimum visibility occurs when the separation ρ of the pair (in radians) and the component D of the slit separation in the direction of the position angle satisfy the equation 2 _ρ D = λ. If the wavelength λ is always larger than 2 _ρ D then one pair of minima, aligned with θ(±180°), will result. Otherwise, two pairs of minima are found symmetrically to the true position angle. The wavelength, the distance between the slits, and a correction allowing for their shape enter the calculation of the separation ρ. Limitation of both bandwidth and aperture width restrict interferometry more than other techniques to bright objects.