I'm no expert here.
You mention "either side" as if the light came from both the left and the right. But I think the light would be coming along a circle all around the sun depending on exact position.
So the trick here is that if you are at the focus point, you get all that light in a small area "for free". But if you try to catch the light on the way, you now need to catch eveywhere in a whole massive circle, which is basically impossible, so you only catch a minuscule amount of the light. And then have to deal with interferometry.
So the trick here is that if you are at the focus point, you get all that light in a small area "for free". But if you try to catch the light on the way, you now need to catch eveywhere in a whole massive circle, which is basically impossible, so you only catch a minuscule amount of the light. And then have to deal with interferometry.