Since light is quantized into photons, how can a single rare photon entering one side of a large, [say, 100 meter wide], interferometer from a very dim star, a phton which is only in the interfereometer for a microsecond or so, possibly "interfere" with anything which has entered the other side of the interferometer at exactly the same instant?? Does the photon, arriving still in its wave-packet form, simply interfere with itself? That would imply that the quantized wave-packet, unlike the sub-microscopic photon particle which finally "collapses" and is recorded on the CCD camera, must be at least as wide as the interferometer's separated telescopic inputs, if not, indeed, as wide as the boundary of the entire sphere of the universe surrounding the wave's source at the same distance as the interferometer; something which certainly strikes me as preposterous!! So, how then IS such interference, and therewith, the vastly improved angular resolution of even very dim sources, whose photons may arrive only occasionally over time, actually possible??
Yes, as you write, a single photon can interfere with itself.
The wavefunction discribes the probability distribution of the photon's spatial position (and other characteristics) for all of space.
Now in your case the photon is with a high probability to be found at a certain side of the interferometer, let's say that that would be the point where the photon as particle would interact with the metal lattice of the antenna, and be absorbed by an atom inside the lattice.
Now it is a misconception to think that the photon was traveling in a straight (spatial 3D) path to that point of interaction from the source. The photon in reality is traveling as a wave, and its spatial position's distribution is described by the wavefunction for all of space.
Now in your case the wavefunction would give you a high probability for the photon to be found close to that point on one side of the interferometer. But the probability to find the photon on the other side of the interferometer is not 0. Actually it is not 0 anywhere in space. The photon is spread out in space (as the other answer suggests) everywhere, with different probabilities.
As the photon travels as a wave and reaches the interferometer, the partial waves interact with themselves and create a single photon interference pattern. If you change the interferometer with a screen, you would get bright areas where the interference was constructive and dark areas where the interference was destructive.
You are saying that the photon is only in the interferometer for a microsecond, and how can something at the same instant interfere on the other side of the interferometer. Well this is in part explained by the photon traveling as a wave and partial waves interfere with the interferometer everywhere at the same instant.
It is a misconception to think of the photon as another particle that would have rest mass and would experience time as we do. The photon does not have a rest mass, does not have a rest frame, and you cannot talk about what happens when you are in the photon's frame and move along with it. There is no such frame. The photon moves in the spatial dimensions with speed c (in vacuum, when measured locally) and moves in the time dimension with speed 0. It does not experience time as we do. We do have a rest mass, and we (or another particle with rest mass) move in the time dimension with speed > 0. Thus, we move in the spatial dimensions with speed less then c. Now the photon does not experience time as we do, you could say that it sees the entire time dimension in one, but in reality nobody knows.
What you can say is that the photon travels (if there would be a frame where you would go with the photon) from the source to the point of interaction (absorption) in 0 spacetime distance. You could say that the path the photon travels is lightlike.