HFI INSTRUMENT

HFI instrument (High Frequency Instrument), which is the french supply to the scientific payload of the PLANCK mission:

is composed of bolometers 83 GHz - 857 GHz functioning at a temperature of 0.1 K,
its angular resolution is of 5 arcmin,
its temperature sensibility is of 5 µK at 100 GHz.

It was designed to measure the cosmic background in the spectrum part where it is maximum and where the contamination of the measurement by first plane sources is minimal. Its six bands have been optimized to identify as best the high frequency sources to eliminate their residual contribution. The instrument is based on the use of bolometers at very low temperature (0.1 Kelvin), which sensibility will mainly be limited by the photons noise of the diffuse background itself and, at short wavelength, by the noise of the photons of the thermal radiation of the telescope, even if it has a low emissivity and is cooled at 50 K. It thus approachs the theorical sensibility limit for this kind of measurements. Moreover, the reading system of the bolometers, based on a modulated polarization, presents a very low noise level at low frequency. It enables a measurement strategy based on the satellite rotation at 1 turn by minute and on a sweeping of cercles in the sky in an direction approximately perpendicular to the direction of the Sun, which enable to cover two times the sky in (a little more than) a year.

The best detectors available have an absorbent of the spider web type and a thermometer in germanium doped by transmutation. They are developed by Caltech/JPL. the reading electronics are developed in France (CESR) and constitute an essential contribution to the project. the bolometers cooling at 100mK, needed for the required performances, is ensured by an open loop dilution cooler (invention CNRS/CRTBT covered by a CNES patent). Helium 3 and helium 4 reserves compressed in spheres give an autonomy of at least 14 months to the cooler.

From this 0.1K requirement at the focal plane follows all the HFI instrumental architecture. It is obtained by the passive precooling of the payload and the telescope at less than 60K (-213°C). Two active coolers (a 20K sorption compressors + 4K mechanical compressors) in cascade again lower this temperature to 4K, temperature from which begins the dilution cooler to 0.1K.

The telescope, with a 1.5m of useful diameter, is an off-axis gregorian type without obstruction, to minimize the stray light produced by diffraction. The coupling of the detectors with the telescope is ensured by corrugated horns, which define the gaussian beams, and enable the strict control of the stray light. Interferential filters enable to control the spectral domain reaching the bolometers. Their repartition on the cryogenic levels at 0.1K, 1.6K, and 4K enable to limite the thermal charges on the more sensible cryogenic levels.

HFI focal plane - Credits HFI

HFI instrument is built by a consortium directed by Jean-Loup Puget of the Institut d'Astrophysique Spatiale (IAS, Orsay, France).

Credits HFI