2010a), giving offsets between lead and trail scans of
c ESO 2010
Astronomy & Astrophysics
Pre-launch status of the Planck mission
Special feature
A&A 520, A8 (2010) DOI: 10.1051/0004-6361/200912855
Planck pre-launch status: Expected LFI polarisation capability J. P. Leahy1,2 , M. Bersanelli3,4 , O. D’Arcangelo5 , K. Ganga6 , S. M. Leach7,8 , A. Moss9 , E. Keihänen10 , R. Keskitalo10,11 , H. Kurki-Suonio10,11 , T. Poutanen10,11,12 , M. Sandri13 , D. Scott9 , J. Tauber14 , L. Valenziano13 , F. Villa13 , A. Wilkinson1 , A. Zonca3,4 , C. Baccigalupi7,8,15 , J. Borrill16,17 , R. C. Butler13 , F. Cuttaia13 , R. J. Davis1 , M. Frailis2 , E. Francheschi13 , S. Galeotta2 , A. Gregorio18 , R. Leonardi19 , N. Mandolesi13 , M. Maris2 , P. Meinhold19 , L. Mendes20 , A. Mennella3,4 , G. Morgante13 , G. Prezeau21 , G. Rocha21,22 , L. Stringhetti13 , L. Terenzi13 , and M. Tomasi3 (Affiliations can be found after the references) 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 0.◦ 5, 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
