Planck   pre-launch status: Expected LFI polarisation capability

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 <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.

Planckintermediate results

“…Planck observes the sky in nine frequency bands covering 30-857 GHz with high sensitivity and angular resolution from 31 to 5 . The Low Frequency Instrument (LFI; Mandolesi et al 2010;Bersanelli et al 2010;Mennella et al 2011) covers the 30, 44, and 70 GHz bands with amplifiers cooled to 20 K. The High Frequency Instrument (HFI; Lamarre et al 2010; Planck HFI Core Team 2011a) covers the 100, 143, 217, 353, 545, and 857 GHz bands with bolometers cooled to 0.1 K. Polarisation is measured in all but the two highest frequency bands (Leahy et al 2010;Rosset et al 2010). A combination of radiative cooling and three mechanical coolers produces the temperatures needed for the detectors and optics (Planck Collaboration 2011b).…”

Planckpre-launch status: ThePlanckmission

“…the sensitivity to radiation polarised orthogonally to the principal plane). Both these parameters have been measured on the ground, with accuracies described in Leahy et al (2010) and Rosset et al (2010); a summary for both instruments is provided in Tauber et al 2010. These measurements will be complemented in flight with observations of a bright and strongly polarised source, the Crab Nebula (Tau A).…”

“…This compact source has well-known polarisation characteristics whose knowledge is now being improved specifically for Planck (Aumont et al 2010). The details of the polarisation measurement and calibration scheme are developed further in Leahy et al (2010) and Rosset et al (2010).…”

“…The relative angle can be determined by observation of regions of brightly polarised foreground emission at high ecliptic latitudes (which are observed many times with a wide range of scan angles). Some further details of the calibration scheme and its accuracy are described in Tauber et al (2010) and Leahy et al (2010).…”

“…telescope plus instruments; - Mandolesi et al (2010), describing programmatic aspects of the LFI and its development; - Bersanelli et al (2010), describing in detail the design of the LFI; - Mennella et al (2010), describing the test and calibration programme of the LFI at instrument and system levels prior to launch; - , describing the test and calibration of the LFI radiometer chains; - Sandri et al (2010), describing the design and test of the LFI optics; - Leahy et al (2010), describing the polarisation aspects of the LFI, and its expected performance in orbit; - Lamarre et al (2010), describing in detail the on-ground design, manufacture, test and performance of the HFI; - Pajot et al (2010), describing the test and calibration programme of the HFI prior to launch; - Ade et al (2010), describing the design, test and performance of the cryogenic elements of the HFI focal plane; - Holmes et al (2008), describing the design, manufacture and test of the HFI bolometers; - Maffei et al (2010), describing the design and test of the HFI optics; - Rosset et al (2010), describing the polarisation aspects of the HFI.…”