Chapter 9: Interferometry and Aperture Synthesis

How can we get higher angular resolution than the simple full width at half maximum of the Airy function,/D, where D is the largest that we can afford? One answer is indicated in the bottom panel of Figure 8.18. If we mask off all of our telescope mirror except two small apertures of diameter d at the edge, the “central peak” of our image becomes twice as sharp as the Airy function but with huge sidelobes, that is fringes. The figure oversimplifies the situation, since the telescope will still make superimposed images of diameter /d through each of the apertures, called the primary beam of the interferometer. If the apertures are separated by a distance D’, the fringe spacing projected up onto the sky will be /D’, or for convenience if we assume D’ ~ D, /D. Since this angular distance corresponds to adjacent maxima, the fringe half width at full maximum (or “beam” width) is ~ /2D; that is, we have doubled the resolution (at the expense of losing a lot of light). In principle, we could try to determine if a source was resolved by measuring the width of the fringe “peaks”, but it is easier and more quantitative to base the measurement on the filling-in of the dark fringes as the source becomes larger, the fringe contrast. We describe this in terms of the visibility: