[Wavytone]

Bojan solar furnaces routinely exceed 2,200 degrees so to be honest I'd be careful putting any thermometer in the image from a sizeable mirror.

Ken - a significant aspect is the total energy focussed in the spot. An all-reflecting system with aluminium first surface mirrors will focus everything from the far IR into the UV. Lenses on the other hand don't transmit so well in UV or IR and the chromatic issues mean the heat isn't all that well focussed as it is in a reflector.

Bojan is right in that f/ratio plays a role, but a simpler view is this:

- calculate the total incoming energy (entering the scope primary aperture) in watts, full spectrum;
- calculate the size of the solar image;
- calculate the energy density at the image (in watts per square cm).
- allow for thermal losses and inefficiencies, probably 50%.