Trying to bake a 'matte' screen for my horizon landscape, help!

SOLVED: I was trying to bake all my far horizon scenery into a cylindrical object…found a simpler (or probably more conventional ) solution. I moved all the horizon stuff including the main sun lamp into a ‘library’ blend. There I rendered all as an EXR equirectangular panorama image. In the main blend I linked that as a World (Environment) image. I also linked (not Import) the sun lamp. This way my main lighting will always match the objects rendered into my sky image.
I lose control of editing my sky but if I did it probably wouldn’t have matched my imagined cylindrical horizon object anyway.

Original post:
2.74, cycles, lots of memory/gpu/cpu resources on Win 7.

hi. I’ve setup a three-ring far-horizon landscape with lots of verts. All coloring is procedural. I want to bake the whole thing from view of the camera in the middle and put it on a simple matte-screen mesh; the green mesh is an example.

The main landscape (not shown) is about 1.5 miles square in the middle, and the horizon rings are about 20 mile diameter, so I shouldn’t have any perspective problems if the camera wanders around the central landscape.

The first image shows what the camera sees at the horizon.
The second image shows my three-layer setup to make the horizon. The outer rings have a Transparent shader mixed in to create the haze effect, which is SO wrong but easy to do and I made it work. Having the sky visible through the horizon meshes might be a baking issue though.

I unwrapped the green ring (green is for this post only) to a 4096^2 image.

I set the baking to Composite, but I’m not getting much, hard to tell what is going on. I’m not sure I even understand how the baking knows what is getting baked and where it is going. Is it always from the view of the active camera? Does it matter whether the green ring is inside or outside the landscape meshes? BTW, for baking purposes I use a transparent shader on the green ring so it wouldn’t get in the way.
Do I need to highlight the mesh being baked to; does it need to be in Object mode? I feel like I don’t understand some fundamentals.