HFI design, qualification, and performance

This section provides an overview of the High-Frequency Instrument and of its different sub-systems. Two papers that include and detail this information are available: Planck-PreLaunch-III and Planck-Early-IV. Additional detailed information potentially useful for users of HFI data is included in this section or annexed to it.

HFI electronics in the satellite

The HFI instrument is designed around 52 bolometers. Twenty of the bolometers (spider-web bolometers or SWBs) are sensitive to total power, and the remaining 32 units are arranged in pairs of orthogonally-oriented polarisation-sensitive bolometers (PSBs). All bolometers are operated at a temperature of ~0.1 K by means of a space qualified dilution cooler coupled to a high precision temperature control system. A 4He-JT system provides active cooling for 4 K stages using vibration controlled mechanical compressors to prevent excessive warming of the 100 mK stage and minimize microphonic effects in the bolometers. Bolometers and sensitive thermometers are read using an AC-bias scheme through JFET amplifiers operated at ~130 K that provide high sensitivity and low frequency stability. The HFI covers six bands centred at 100, 143, 217, 353, 545, and 857 GHz, thanks to a thermo-optical design consisting of three corrugated horns and a set of compact reflective filters and lenses at cryogenic temperatures.

HFI focal plane optics and 4K thermo-mechanical stage.

The whole satellite is organized to provide thermal transitions between its warm part exposed to radiation from the Sun and Earth, and the focal plane instruments that include the cold receivers (Sections HFI_cold_optics and HFI_detection_chain). The various parts of the HFI are distributed among three different stages of the satellite in order to provide each sub-system with an optimal operating temperature. The "warm" parts, including nearly all the electronics and the sources of fluids of the 4K and 0.1K coolers, are attached and thermally linked to the service module of the satellite. The first stage of the preamplifiers is attached to the back of the passively cooled telescope structure. The focal plane unit is attached to the 20K stage cooled by the sorption cooler. This is detailed in Section HFI_detection_chain.

The telescope and horns select the geometrical origin of photons. They provide a high transmission efficiency to photons inside the main beam, while photons coming from the intermediate and far-side lobes have very low probability of being detected. The essential characteristics are determined by a complex process mixing ground measurements of components (horns, reflectors), modelling the shape of the far sidelobes, and measuring bright sources in-flight, especially planets.

The filters and bolometers define the spectral response and absolute optical efficiency, which are known mostly from ground-based measurements performed at component, sub-system, and system levels, reported in this document. The relationships between spectral response and geometrical response are also addressed.

Photons absorbed by a bolometer include the thermal radiation emitted by the various optical devices: telescope, horns, and filters. They are transformed into heat that propagates to the bolometer thermometer to influence its temperature, which is itself measured by the readout electronics. Temperatures of all these items must be stable enough not to contaminate the scientific signal delivered by the bolometers. How this stability is reached is described in Section HFI_cryogenics.

The bolometer temperature depends also on the temperature of the bolometer plate, on the intensity of the biasing current, and on any spurious inputs, such as cosmic rays and mechanical vibrations. Such systematics are included in a list discussed in Section HFI-Validation.

Since the bolometer thermometer is part of an active circuit that also heats it, the response of this system is complex and has to be considered as a whole. In addition, due to the modulation of the bias current and to the sampling of the data, the response signal of the instrument when scanning a point source is more complex still. Item in Sub-Section Time_response and Annex HFI_time_response_model are dedicated to the description of this time response.

HFI signal formation.

Logic of the formation of the signal in HFI. This is an idealized description of the physics that takes place in the instrument. The optical power that is absorbed by the bolometers comes from the observed sky and from the instrument itself. The bolometers and readout electronics, acting as a single and complex chain, transform this optical power into data that are compressed and transmitted for science data reduction.