Making maps from Planck LFI 30 GHz data with asymmetric beams and cooler noise

Abstract

The Planck satellite will observe the full sky at nine frequencies from 30 to 857 GHz. Temperature and polarization frequency maps made from these observations are prime deliverables of the Planck mission. The goal of this paper is to examine the effects of four realistic instrument systematics in the 30 GHz frequency maps: non-axially-symmetric beams, sample integration, sorption cooler noise, and pointing errors. We simulated one year long observations of four 30 GHz detectors. The simulated timestreams contained cosmic microwave background (CMB) signal, foreground components (both galactic and extra-galactic), instrument noise (correlated and white), and the four instrument systematic effects. We made maps from the timelines and examined the magnitudes of the systematics effects in the maps and their angular power spectra. We also compared the maps of different mapmaking codes to see how they performed. We used five mapmaking codes (two destripers and three optimal codes). None of our mapmaking codes makes an attempt to deconvolve the beam from its output map. Therefore all our maps had similar smoothing due to beams and sample integration. This is a complicated smoothing, because every map pixel has its own effective beam. Temperature to polarization cross-coupling due to beam mismatch causes a detectable bias in the TE spectrum of the CMB map. The effects of cooler noise and pointing errors did not appear to be major concerns for the 30 GHz channel. The only essential difference found so far between mapmaking codes that affects accuracy (in terms of residual root-mean-square) is baseline length. All optimal codes give essentially indistinguishable results. A destriper gives the same result as the optimal codes when the baseline is set short enough (Madam). For longer baselines destripers (Springtide and Madam) require less computing resources but deliver a noisier map.

Planck pre-launch status: Expected LFI polarisation capability

“…The B-mode polarisation spectrum is similar to E-mode, as might be expected since both are dominated by polconversion from I. The flattening at > ∼ 1000 is due to pixel aliasing, see Ashdown et al (2009). (Note: WMAP data show that the foreground polarisation in these simulations is too bright by nearly an order of magnitude, or nearly 10 2 in C ).…”

“…The LFI design dramatically reduces this raw 1/ f noise, driven by fractional gain errors of order 10 −4 ; and residual 1/ f noise is effectively dealt with by our mapping algorithms (e.g. Ashdown et al 2009). But extrapolating to long timescales, the gain drifts eventually become significant for their effect on the differential signal (as opposed to the T sys offset), and need to be corrected by calibration.…”

“…To assess the impact of random errors in the gain calibration on the polarisation maps, we re-analysed the "Trieste" simulations made by Ashdown et al (2009). These were simulated observations by the Planck 30-GHz system, with a fairly realistic scan strategy in a 1-year survey.…”

“…The data for each polarisation of each detector were multiplied by the appropriate gain factor to simulate random calibration errors, and theĨ andQ H signal streams were used to create maps of (I, Q, U), using the madam destriping mapmaker (Keihänen et al 2005;Keihänen et al 2010), as in the original analysis by Ashdown et al (2009). We also produced and mapped a second set of timelines where the calibration factors had been averaged for 6 days by simple binning.…”

Residual noise covariance forPlancklow-resolution data analysis

“…In addition to these modes, we include a nutation of the spin axis and slight variations to the 1 rpm spin rate. Details of the scanning simulation can be found from Ashdown et al (2009) where it was used in a mapmaking study.…”