QUESTION:
Why are there mirrors on the telescope if it's an IR telescope and
measuring heat?

ANSWER from Thomas Roellig on November 8, 1995:
Although we cannot see infrared light with our eyes, it still reflects from
metals just like visible light does. As a result, we can still use mirrors to
collect the infrared light and focus it just like we would do with visible
light. If a telescope is designed to work mostly at infrared wavelengths,
sometimes its mirrors will be made of a different material that does a
better job of reflecting infrared light. For example, most telescopes that
are designed to primarily do visible wavelength observations will coat
their glass mirrors with shiny aluminum. This is the same way the mirrors
that you would find in your house are made. Now aluminum will reflect
infrared light as well as visible light, but not as well as gold does for
example. Therefore, some telescopes that are designed to be used primarily
in the infrared have gold-coated mirrors. One additional way to get a big
advantage from a dedicated infrared telescope is to make the mirrors
colder. Since infrared light is heat radiation, it is hard to see a dim
infrared object in the sky if its signal is being drowned out by infrared
radiation coming from the telescope parts. The best telescopes optimized
for the infrared fly in space, where the vacuum of space allows them to be
cooled to -450 F degrees using very cold materials like liquid helium.

Yours truly,

Dr. Thomas L. Roellig
NASA Ames Research Center