Quantum mechanical description of a photon arriving at a telescope from extremely far away Typically, telescopes are explained in terms of bouncing light paths around. For example, this image from wikipedia shows "photon tracks" being redirected:

I realize this is a very effective way to model telescopes, but I was wondering if this is actually reflective of what happens or if it's just a really great approximation.
Wouldn't the wave function of a photon arriving from far away be spread out over quite a huge area? So the photon's wavefunction would be arriving as a sort of spread-out wall? But in that case the mirror isn't working by redirecting the path, it's working by creating interference effects depending on how the wall arrives. Or maybe that's totally wrong! I dunno! That's the question!
What is the quantum mechanical description of a photon arriving at a telescope, from extremely far away?
 A: A telescope receiving enough photons to take a picture of a distant galaxy is also receiving enough photons (in terms of sheer numbers!) that the classical description of optics (regarding lenses, mirrors, prisms, etc.) furnishes a perfectly adequate overall picture of the process- no quantum mechanics is needed.
If we look at a single photon departing that galaxy, it is still a single photon by the time it reaches our telescope and its wave function is not "spread out" as such- it's just that there are fewer photons in the beam by the time it enters the telescope.
A: From what I've read scientists have not used QM to describe the transmission of a photon in free space.  Maxwell derived an equation based on electro and magnetic fields that say a simple sine wave is possible.
Feynman used a path integral theory which is related to QM to describe a probability function .... where a photon is likely to go in an experimental setup.
For a single photon the site has many questions about "how big is a photon" but there is no specific answer.
