Fast Pixel Space Convolution for CMB Surveys with Asymmetric Beams and Complex Scan Strategies: FEBeCoP

TL;DR

The paper introduces FEBeCoP, a fast pixel space convolution method that accurately simulates CMB observations considering asymmetric beams and scan strategies, aiding in precise cosmological analysis.

Contribution

We developed a computationally efficient algorithm to generate effective beams for CMB experiments, accounting for real beam shapes and scan histories, improving systematic error modeling.

Findings

Efficient computation of effective beams for Planck data.

Enhanced accuracy in CMB anisotropy analysis.

Application to point source detection and characterization.

Abstract

Precise measurement of the angular power spectrum of the Cosmic Microwave Background (CMB) temperature and polarization anisotropy can tightly constrain many cosmological models and parameters. However, accurate measurements can only be realized in practice provided all major systematic effects have been taken into account. Beam asymmetry, coupled with the scan strategy, is a major source of systematic error in scanning CMB experiments such as Planck, the focus of our current interest. We envision Monte Carlo methods to rigorously study and account for the systematic effect of beams in CMB analysis. Toward that goal, we have developed a fast pixel space convolution method that can simulate sky maps observed by a scanning instrument, taking into account real beam shapes and scan strategy. The essence is to pre-compute the "effective beams" using a computer code, "Fast Effective Beam Convolution in Pixel space" (FEBeCoP), that we have developed for the Planck mission. The code computes effective beams given the focal plane beam characteristics of the Planck instrument and the full history of actual satellite pointing, and performs very fast convolution of sky signals using the effective beams. In this paper, we describe the algorithm and the computational scheme that has been implemented. We also outline a few applications of the effective beams in the precision analysis of Planck data, for characterizing the CMB anisotropy and for detecting and measuring properties of point sources.