Greg Arnold wrote:
...
Hmm, I wonder why astronomers use those huge telescopes. Or why a huge
array of mirrors is used for attempts to harness the sun's energy.
...

Astronomers use huge telescopes because they are primarily concerned
by resolution and resolution is better with big mirrors.

Attempts to harness the sun's energy are primarily concerned by the
amount of energy collected rather than by its concentration. The amount
of energy collected is proportional to the surface of the mirror(s).
The concentration, i.e. ratio of energy per surface unit on the target,
is proportional to the (square of the) focal ratio. This is because the
energy collected is proportional to the collecting surface, i.e. to
the square of the mirror's radius, and the surface on which it is
collected is the image of the sun, this surface is proportional to
the square of its radius, itself proportional to the focal length.