Dealing With the Horizon

I'm writing up this issue to discuss how we should deal with the horizon in simulation.

There are a lot of factors to consider here, and I'm just going to mention a few of them, and hopefully we generate some discussion that leads to implementation. Factors include:

1. How the beam behaves at the horizon. Is it correct to say that the beam has a hard cut at the "horizon". A compact beam in real space is non-compact in UV space. In which space is it more physical to be compact?
2. What is the horizon? Should we be more careful about defining the local horizon given terrain etc?
3. How do point sources behave at the horizon? Is there diffraction? Is there some (small) smoothing scale over which the point source disappears? Do we need to account for the extended nature of point sources when we're this close to the horizon?
4. How do we represent diffuse structure in terms of the horizon? Having "pixels" that gradually move over the horizon means we should really only account for a fraction of their flux when they straddle the horizon.
5. Are there "effective" ways of dealing with some of these issues that could be applied as something like modifications to the beam and could be simulator-agnostic?

In terms of 4 and 5, I wrote up some basic ideas in the attached document.
vissim-horizon.pdf

Start by reading the attached vissim-horizon.pdf and the five horizon questions in the issue. Clarify which horizon behavior and simulation scope should be addressed before implementation. Done means the project has an agreed approach that resolves the selected horizon cases and leads to an implementation plan.