Biggest Space Telescope
With a primary mirror measuring 3.5 meters in diameter, Herschel's Ritchey-Chr�tien telescope will be larger than any previous space-based telescope. Shielded from the sun, it will passively cool to around 80 K (80 degrees Celsius above absoulte zero) by radiating its heat into space.
The mirror will be made of silicon carbide, a lightweight ceramic material resistant to stress, fatigue, and extreme temperatures. It can be polished as if it were glass, deviating from perfect smoothness by no more than one micron - a requirement to avoid distorting the images it receives.