What interferes in an astronomical interferometer? As I understand it, in an astronomical interferometer, both telescopes need to be aimed at the same light point in space to get an interference pattern. But I also understand that a single photon cannot be detected at two points simultaneously; it registers in one receptor or the other, not both. So what is interfering in the interferometer? Two photons in sequence from the same source?
 A: If the angular extent of the source is sufficiently small the light from it can be considered coherent.  This means that the wavefront will have the same phase as seen from both of the receptors independent of when the signals arrive.
Often times it is easier to think of these situations from a classical perspective and consider just the wave nature of light and self-interference.  Of course you would have to arrive at the right answer when considering quantum effects and single photons.  That would make this example here similar to the double-slit experiment where you can observe interference fringes with only s single photon at a time.
Typically in optical systems there is only one sensor and the light beams are combined optically before detection.  In radio interferometers the detection is typically done per antenna where the phase of the received signal to a reference oscillator is measured and then the interference computed on digitized signals.
More information can be found in this Wikipedia article: Astronomical interferometer.
