This is a follow up question to an earlier one asked. I understand now that natural sunlight cannot be focused to heat a target up to the temperatures needed to ignite the target to fuse. But, that leads to the next part. Incoherent radiation has a focal limit. From my understanding of it, because the surface of the Sun has a temperature of 5800 K, the light cannot be focused to an intensity more than that temperature unless the light is coherent.
Am I understanding this correctly? That the only way to concentrate the sunlight to a intensity where the temperature is higher than 5800 K is to somehow make the radiation coherent? (I.E. a laser or electron current of some type)? Also, how does one calculate the maximum intensity that incoherent radiation can be concentrated to?