The low frequency instrument on-board the Planck satellite: Characteristics and performance

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Abstract

Planck is the third generation space mission, after COBE/DMR and WMAP, devoted to image the CMBR anisotropies. The low frequency instrument (LFI) will simultaneously observe the sky in three frequency bands centered at 30, 44 and 70 GHz. It is composed by 11 pseudo-correlation receivers, actively cooled to 20 K, able to detect both orthogonal polarisation of the incoming signal. The LFI will be located, along with the high frequency instrument (HFI), in the focal region of a 1.5 m aperture telescope. The LFI will produce full-sky maps of the anisotropies of the CMBR with a FWHM angular resolution of 33′, 27′ and 14′ for the 30, 44 and 70 GHz LFI bands, respectively.

Introduction

The Planck1 mission, whose launch is foreseen in 2008, will extend our knowledge of the Cosmic Microwave Background beyond the actual limits set by past and present experiments (see for instance WMAP,2 B2K,3 CBI,4 ACBAR.5)

From the second Lagrangian point of the Sun–Earth system, Planck will produce a survey that will cover the whole sky with unprecedented sensitivity, angular resolution and frequency coverage, and it will likely lead us to the final comprehension of the CMB temperature anisotropies. A critical issue for any CMB experiment is the accurate removal of the foregrounds. While WMAP is not sensitive at frequency higher than ∼100 GHz, the Planck instruments will produce cross calibrated full sky maps spanning a very large frequency range. The HFI, operating between 100 and 857 GHz, is able to monitor, for instance, the dust contamination, the LFI, covering from ∼27 to ∼77 GHz, is sensitive to the synchrotron and free–free emission. The combination of the two instrument will therefore produce the cleanest image of the CMB ever obtained. Moreover, the wide frequency range covered, delivering all-sky maps for each channel, will provide at the same time a gold-mine of astrophysical information (see Table 1).

Comparing the three generation of space mission devoted to the CMB anisotropies, COBE/DMR6 (Smoot et al., 1992) first mapped the temperature anisotropies; WMAP, after successful ballon-borne and ground-based experiments (see among others de Bernardis et al., 2000, Dickinson et al., 2004, Kuo et al., 2004) determined with high accuracy the temperature power spectrum (Bennett et al., 2003, Hinshaw et al., 2003) up to the third peak (Hinshaw et al., 2006) and improved (Page et al., 2006) the first determination of the TE and EE power spectrum (along with other experiments Readhead et al., 2004a, Readhead et al., 2004b, Leitch et al., 2005, Piacentini et al., 2005, Piacentini et al., 2006). Planck will not only extend the high precision determination of the TT-spectrum up to ℓ  2000, but it will determine the EE-spectrum with high sensitivity up to ℓ  1000 (The Scientific Programme of Planck, 2005). Planck will represent for the E-mode CMB polarisation what WMAP is for the temperature spectrum. Moreover, it will also reach the sensitivity to detect the B-modes under certain theoretical assumptions (relatively large tensor to scalar perturbation ratio and Thomson scattering optical depth).

The LFI is described from the general point of view in many papers (see for instance The Scientific Programme of Planck, 2005, Bersanelli and Mandolesi, 2000, Mandolesi et al., 2002, Villa et al., 2002c, Lawrence, 2003, Mennella et al., 2004, Sandri et al., 2004a, Bersanelli et al., 2005, Terenzi et al., 2006) and in some specific design issues (Seiffert et al., 2002, Mennella et al., 2002, Maris et al., 2006). We limit ourselves here to review to some specific topics, mainly related to the 4K reference load unit, LFI optics, the sorption cooler and the LFI contribution to Planck science.

Section snippets

LFI characteristics

The design philosophy for the LFI is to minimise any systematic effect, from instrument intrinsic effects and from astrophysical origin, in the design of the instrument instead of removing it in data reduction. It results in a very complex instrument, especially for the radiometric (Seiffert et al., 2002), optical (Burigana et al., 2001) and thermal aspects (Mennella et al., 2002). Moreover an enormous effort is made to model the impact of any known effect on the final maps. The goal of the LFI

Sorption cooler

The vibration-less sorption cooler (Wade et al., 2000, Bhandari et al., 2000, Bhandari et al., 2001, Morgante et al., 2002) for the Planck Satellite is developed by the Jet Propulsion Laboratory (JPL). The Planck sorption cooler is a closed-cycle, continuous cryocooler designed to provide more than 1 W of heat lift at a temperature of less than 20 K using isoenthalpic expansion of hydrogen through a Joule–Thompson valve (J–T). Some of this cooling power will be provided to cool the low-frequency

The LFI contribution to the Planck science

Since the performance of the LFI radiometers, sensitive also to polarization, is the best ever obtained in a CMB space mission at ν  70 GHz, LFI data will play a crucial role in the context of the Planck scientific aims and, possibly, in the context of future CMB space missions dedicated to a more precise measure of CMB polarization anisotropies.

Recent WMAP results show that the minimum foreground contamination to CMB observations, both in temperature and polarisation, is in the 60–70 GHz range (

Conclusions

Planck is the most ambitious space mission devoted to map the CMBR anisotropies for the next years. The two instrument exploit their technology at its best. We reviewed in this paper some topics related to the Low Frequency Instrument. In particular, the 4K reference load unit, the study on the optical properties of the Planck telescope and of the LFI feed horns, the sorption cooler. The last section illustrates how the LFI will contribute to Planck science. All this work reflects the main

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