Technology of tomorrow

Suomeksi

Carefully designed instruments are required for the accurate measurement of the CMB. Planck satellite and its instruments have strict specifications. For example, its receivers are of latest and finest technology yet. First of all, the satellite must be able to scan the whole sky -the goal is to achieve 90 percent coverage. Second, the pointing accuracy of the satellite must be excellent so that the tiny CMB anisotropies can be measured. Third, all instruments must produce negligible interference with the operation of the satellite.

Planck satellite operates between frequencies 30 and 857 GHz. The frequency range should be as wide as possible so that the foreground noise can be efficiently eliminated. This means that the receivers must be based on two different techniques. At lower frequencies they are "normal" radio receivers and at higher frequencies, bolometers. Bearing in mind the minute nature of the CMB temperature anisotropies, the instruments should be as sensitive as possible. Planck will be enormously more sensitive than COBE, and at least ten times more sensitive than WMAP.

Telescope

The main reflector mirror of the Planck satellite is 1.5 metres in diameter and the sub-reflector is about 0.8 metres in diameter. The design is a so called off-axis tilted Gregorian system. A shield surrounds the telescope and focal plane instruments. Its purpose is to reduce the contamination level of straylight and help keep the instruments cool. Straylight is caused by the Sun and the planets, and also by the satellite itself.

Receivers LFI and HFI

The Low Frequency Instrument (LFI) operates on frequencies 30, 44 and 70 GHz. There are several individual receivers at each frequency (4, 6, and 12, respectively). The High Frequency Instrument (HFI) operates at frequencies 100, 143, 217, 353, 545, and 857 GHz. The number of receivers at each frequency is 4, 12, 12, 6, 6 and 6, respectively. The HFI instrument will be installed inside the LFI instrument.

The twelve 70 GHz receivers are being developed and built in Finland (in cooperation between Millilab, Ylinen Electronics Co., and Metähovi). They use Monolithic Microwave Integrated Circuits (MMIC) and are about a size of a matchbox! The CMB radiation must be amplified by a factor of one million as the differences in the temperature structure are so very small. Because the CMB is very cold (3 Kelvins or -270 degrees Celsius), the receivers must be also cooled, otherwise the required sensitivity will not be reached.

Orbit

The Planck satellite was launched on Ariane 5 launch vehicle together with the Herschel satellite in 14.5.2009. The baseline orbit chosen for Planck is an elliptical orbit around the L2 Lagrangian point of the Earth-Sun system. This point is at a distance of about 1.5 million kilometres from Earth, away from the direction of the Sun. At this location it is possible to point the satellite towards the empty space and away from most sources of interference. Thus the amount of straylight will be minimal, and the thermal environment stable.


The telescope is at an angle of 70 degrees to the body of the satellite. As the satellite spins around, the telescope scans a narrow slice of the sky. Every day the attitude of the satellite changes a little, and in about six months the sky has been covered once. This full-sky measurement is then repeated to improve sensitivity. The whole mission is predicted to last, from the launch to the repeated measurement of the last slice of the sky, about 15 months.