Optical interferometry – A practice primer

The existing and planned interferometers differ largely in the size of telescope area, baseline length and wavelength at which they operate. In this article we focus on the common principle between all of them and discuss the three main ingredients of an interferometer: the light collectors, the delay lines and the beam combiner. We take a look into the different approaches taken, e.g. beam travel in vacuum or air, the use of spatial filters, interferometer layouts etc. Furthermore, we describe the subsystems which are helping in the formation of fringes, namely adaptive optics, fringe trackers and phase reference systems.