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Barry,
I think it is incorrect to assume you can "focus" the temperature of a remote object through an optical system. The temperature of an object is a measure of the energy of the atoms, molecules and sub-atomic particles that make up that object; what we perceive as radiant energy is carried only by photons emitted by the object, not by the excited atoms and molecules themselves. The frequency / wavelength / energy of the emitted photons is a function of the temperature of the object (among other things), but the photons do not have "temperature" in the same way that the atmosphere of a star does.
What we can do is measure the Sun's radiant power (W/m2) and focus this into a smaller area to increase the energy flux. We can then try to work out what that focussed energy does - by way of heating the target and surrounding air, and being re-emitted by reflection, radiation, and convection. That is what merlin66's cited paper and spreadsheet attempt, and my thermal analysis tries to better allow for the thermal properties of paper vs wood (on which the cited paper is based).
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