What temperature can you attain with a solar furnace?

A solar furnace is a device that concentrates the sun's light on a small point to heat it up to high temperature. One can imagine that in the limit of being completely surrounded by mirrors, your entire $4\pi$ solid angle will look like the surface of the sun, at about 6000K. The target will then heat up to 6000K and start to radiate as a blackbody, reaching thermal equilibrium with the sun.

The question is: is there any way to surpass this temperature, perhaps by filtering the light to make it look like a BB spectrum at higher temp, then concentrating it back on the target?

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That would violate the second law of thermodynamics, assuming all the optics are passive. – Mark Eichenlaub Jan 10 '11 at 15:34
If you assume equilibrium, then there is no way. Black body must always emit as much radiation as it receives. The upper limit is given by total Sun's output corresponding to 6000K. – Marek Jan 10 '11 at 17:03
By the way, you might want to read up on Dyson sphere. – Marek Jan 10 '11 at 17:04
If you surround the Earth-sun system with ideal mirrors, everything on the inside will eventually heat up well beyond 6000K. Just divide the total mass by the heat derived from fusing everything to iron. – Scott Carnahan Jan 11 '11 at 13:11