Planck's large telescope collects the light from the Cosmic Microwave Background and focuses it onto the focal plane of the scientific instruments on board.
The primary mirror, 1.9 x 1.5 m in diameter, weighs only about 28 kg; the effective telescope aperture is 1.5 m. It has been designed to be robust enough to withstand the 'shake-and-bake' stresses of launch, and the temperature difference between launch, when it is at an ambient temperature of about 300 K, and operation, at about 40 K.
It is made of carbon-fiber-reinforced plastic and is coated with a thin reflective layer (reflectivity >99.5%) of aluminium - so smooth that any bumps in the coating are less than 5 microns in size. The telescope is surrounded by a large baffle that minimises stray light interference from the Earth, Sun and Moon, and cools it by radiating heat into space.
Seen in the microwave range, the CMB is only 1% as luminous as Earth, so straylight is a particular concern for any space-based telescopes observing the CMB in the microwave. This is why Planck will be strategically positioned at L2. From this vantage point in space, Earth will appear to be the same size as the full Moon seen from Earth.
Planck will also be better sheltered from the heat emitted by Earth, the Moon and the Sun. In addition, the telescope is surrounded by a large baffle to minimise straylight interference from the Earth, Sun and Moon.