In-flight main beam reconstruction for Planck-LFI

TL;DR

This paper presents a method for in-flight main beam reconstruction of the Planck-LFI instrument using observations of external planets, achieving accurate, robust results that improve beam shape modeling and enable precise planet temperature measurements.

Contribution

It introduces a new approach to reconstruct the main beam using planetary observations, including a bivariate Gaussian approximation, applicable across frequency channels.

Findings

Accurate main beam reconstruction from Jupiter and Saturn observations.

Bivariate Gaussian model significantly improves beam shape representation.

Planet temperature can be measured with percent-level accuracy during in-flight calibration.

Abstract

In-flight measurement of the antenna main beams of the Planck instruments is a crucial input to the data analysis pipeline. We study the main beam reconstruction achievable through external planets using a flight simulator to model their observation. We restrict our analysis to the 30 GHz LFI channel but the method can be easily extended to higher frequency channels. We show that it is possible to fit the antenna response from Jupiter and Saturn to obtain an accurate, robust, simple and fast reconstruction of the main beam properties under very general conditions, independently of the calibration accuracy. In addition, we find that a bivariate Gaussian approximation of the main beam shapes represents a significant improvement with respect to a symmetric representation. We also show that it is possible to combine the detection of the planet's transit and Planck's very accurate in-flight calibration to measure the planet's temperature at millimetric wavelengths with an accuracy at the % level. This work is based on Planck-LFI activities.