Mike Dworetsky wrote:

It's thermal radiation from the Earth, not from the Sun,
that causes problems for infrared telescopes in low orbits, where the 300K
Earth fills nearly half the sky. The problem is general heating of the
spacecraft, which makes it use up helium coolant much faster. It is very
hard to reflect away this infrared radiation, which is mostly at wavelengths
of about 0.01mm. Most of the Sun's radiation is visible and near-IR light,
which is easily reflected by a sunshade.

Clean, polished gold and silver have *infrared*
emissivity in the range of 0.01 to 0.03. They
are good enough to reach the microkelvin
temperature range in cryogenic equipment.
A passive tube-shaped insulation surrounding
the telescope in low Earth orbit could reduce
its temperature to 70 K or so. It seems that
NASA is going to send JWST to L2 because they
cannot design decent thermal insulation.