Love this.I'll try to cover everything here - The white disc at the top has a hole in it - the sun shines thru to a target at the back to tune it in. To heat up, tune it in dead on - to cool down, go off target. There is a thermometer that goes into the hot water can.
The unit has to be tuned in every hour or so.
My sight is just a tall nail with a big head perpendicular to the base and I minimize the shadow to max it out. Yours appears a bit more refined.