Planck 2015 results. VI. LFI mapmaking

TL;DR

This paper details the mapmaking process for Planck LFI data, producing sky maps including polarization, with validation and systematic control for accurate cosmological analysis.

Contribution

It introduces an enhanced destriping mapmaking algorithm with noise priors and polarization map production at LFI frequencies, validated through simulations.

Findings

First polarization maps at LFI frequencies.

Validated noise covariance matrices for low-resolution maps.

Controlled systematics for accurate polarization analysis.

Abstract

This paper describes the mapmaking procedure applied to Planck LFI (Low Frequency Instrument) data. The mapmaking step takes as input the calibrated timelines and pointing information. The main products are sky maps of $I,Q$, and $U$ Stokes components. For the first time, we present polarization maps at LFI frequencies. The mapmaking algorithm is based on a destriping technique, enhanced with a noise prior. The Galactic region is masked to reduce errors arising from bandpass mismatch and high signal gradients. We apply horn-uniform radiometer weights to reduce effects of beam shape mismatch. The algorithm is the same as used for the 2013 release, apart from small changes in parameter settings. We validate the procedure through simulations. Special emphasis is put on the control of systematics, which is particularly important for accurate polarization analysis. We also produce low-resolution versions of the maps, and corresponding noise covariance matrices. These serve as input in later analysis steps and parameter estimation. The noise covariance matrices are validated through noise Monte Carlo simulations. The residual noise in the map products is characterized through analysis of half-ring maps, noise covariance matrices, and simulations.