Planck payload module design and performance

Abstract

Planck associated to Herschel is one of the next ESA scientific missions. Both satellites will be launched in 2007 on a single ARIANE V launcher to the 2nd Lagrange libration point L2. Planck is a Principal Investigator Survey mission and the Planck spacecraft will provide the environment for two full sky surveys in the frequency range from 30 to 857 GHz. Planck aims to image the temperature anisotropies of the Cosmic Microwave Background (CMB) over the whole sky with a sensitivity of ΔT/T = 2 .10-6 and an angular resolution of 10 arc-minutes. This will be obtained thanks to a wide wavelength range telescope associated to a cryogenic Payload Module. The Planck mission leads to very stringent requirements (straylight, thermal stability) that can only be achieved by designing the spacecraft at system level, combining optical, radio frequency and thermal engineering. The PLANCK Payload Module (PPLM) is composed of a cryo-structure supporting and a 1.5 m aperture off-axis telescope equipped of two scientific instruments HFI (High Frequency Instrument) and LFI (Low Frequency Instrument). The LFI detectors are based on HETM amplifier technology and need to be cooled down to 20 K. The detectors for the HFI are bolometers operating at 0.1 K. These temperature levels are obtained using 3 different active coolers, a 20K sorption cooler stage, which need pre-cooling stages for normal operation (the coldest one is around 60 K). Finally, the telescope temperature must be lower than 60 K. To meet those requirements, a specific cryo-structure accommodating a multi-stages cryogenic passive radiator has been developed. The design of this high efficiency radiator is basically a black painted open honeycomb surface radiatively insulated from the warm spacecraft by a set of angled shields opened towards cold space, also called V-grooves. The coldest stage offers a ~1.5 W net cooling capacity around 55 K. Specific design are implemented to guarantee the straylight performance. The impacts of these elements on the Planck straylight performance have been assessed. The Payload Module design, the thermal performances (temperature level and stability) and RF performances as well as the integration logic are presented in this paper.