Planck   pre-launch status: Expected LFI polarisation capability

¹ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, M13 9PL, UK e-mail: j.p.leahy@manchester.ac.uk
² Osservatorio Astronomico di Trieste - INAF, via Tiepolo 11, 34143 Trieste, Italy
³ Università degli Studi di Milano, Dipartimento di Fisica, Italy
⁴ IASF - Sezione di Milano, INAF, Milano, Italy
⁵ Istituto di Fisica del Plasma - CNR, via Cozzi 53, 20125 Milano, Italy
⁶ Laboratoire APC/CNRS, Bâtiment Condorcet, 10 rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France
⁷ SISSA/ISAS, Astrophysics Sector, via Beirut 2-4, Sezione di Trieste, 34014 Trieste, Italy
⁸ INFN, Sezione di Trieste, 34014 Trieste, Italy
⁹ Department of Physics & Astronomy, University of British Columbia, Vancouver, BC, V6T 1Z1 Canada
¹⁰ Department of Physics, University of Helsinki, PO Box 64, 00014 Helsinki, Finland
¹¹ Helsinki Institute of Physics, PO Box 64, 00014 Helsinki, Finland
¹² Metsähovi Radio Observatory, TKK, Helsinki University of Technology, Metsähovintie 114, 02540 Kylmälä, Finland
¹³ Istituto di Astrofisica Spaziale e Fisica Cosmica - Sezione di Bologna, INAF, Bologna, Italy
¹⁴ European Space Agency (ESA), Astrophysics Division, Keplerlann 1, 2201 AZ, Noordwijk, The Netherlands
¹⁵ INAF - Trieste, 34131 Trieste, Italy
¹⁶ Computational Cosmology Center, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
¹⁷ Space Sciences Laboratory, University of California Berkeley, Berkeley CA 94720, USA
¹⁸ Dipartimento di Fisica, Università degli Studi di Trieste, Italy
¹⁹ Department of Physics, University of California, Santa Barbara, CA 931106, USA
²⁰ Planck Science Office, European Space Agency, European Space Astronomy Centre, PO Box – Apdo. de correos 78, 28691 Villanueva da la Caada, Madrid, Spain
²¹ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
²² Department of Physics, California Institute of Technology, Pasadena, CA 91125, USA

Received: 8 July 2009
Accepted: 15 May 2010

Abstract

We present a system-level description of the Low Frequency Instrument (LFI) considered as a differencing polarimeter, and evaluate its expected performance. The LFI is one of the two instruments on board the ESA Planck mission to study the cosmic microwave background. It consists of a set of 22 radiometers sensitive to linear polarisation, arranged in orthogonally-oriented pairs connected to 11 feed horns operating at 30, 44 and 70 GHz. In our analysis, the generic Jones and Mueller-matrix formulations for polarimetry are adapted to the special case of the LFI. Laboratory measurements of flight components are combined with optical simulations of the telescope to investigate the values and uncertainties in the system parameters affecting polarisation response. Methods of correcting residual systematic errors are also briefly discussed. The LFI has beam-integrated polarisation efficiency >99% for all detectors, with uncertainties below 0.1%. Indirect assessment of polarisation position angles suggests that uncertainties are generally less than 05, and this will be checked in flight using observations of the Crab nebula. Leakage of total intensity into the polarisation signal is generally well below the thermal noise level except for bright Galactic emission, where the dominant effect is likely to be spectral-dependent terms due to bandpass mismatch between the two detectors behind each feed, contributing typically 1–3% leakage of foreground total intensity. Comparable leakage from compact features occurs due to beam mismatch, but this averages to < 5 × 10^-4 for large-scale emission. An inevitable feature of the LFI design is that the two components of the linear polarisation are recovered from elliptical beams which differ substantially in orientation. This distorts the recovered polarisation and its angular power spectrum, and several methods are being developed to correct the effect, both in the power spectrum and in the sky maps. The LFI will return a high-quality measurement of the CMB polarisation, limited mainly by thermal noise. To meet our aspiration of measuring polarisation at the 1% level, further analysis of flight and ground data is required. We are still researching the most effective techniques for correcting subtle artefacts in polarisation; in particular the correction of bandpass mismatch effects is a formidable challenge, as it requires multi-band analysis to estimate the spectral indices that control the leakage.