In-flight measurement of Planck telescope emissivity

TL;DR

This paper reports the first in-flight measurement of the Planck telescope's emissivity, comparing it with ground tests to assess potential degradation over the mission lifespan.

Contribution

It introduces a novel in-flight measurement method for telescope emissivity and evaluates its stability throughout the mission.

Findings

End of Life emissivity matches ground test results

No significant degradation of emissivity observed in flight

Method provides direct in-flight emissivity measurement

Abstract

The Planck satellite in orbit mission ended in October 2013. Between the end of Low Frequency Instrument (LFI) routine mission operations and the satellite decommissioning, a dedicated test was also performed to measure the Planck telescope emissivity. The scope of the test was twofold: (i) to provide, for the first time in flight, a direct measure of the telescope emissivity; and (ii) to evaluate the possible degradation of the emissivity by comparing data taken in flight at the end of mission with those taken during the ground telescope characterization. The emissivity was determined by heating the Planck telescope and disentangling the system temperature excess measured by the LFI radiometers. Results show End of Life (EOL) performance in good agreement with the results from the ground optical tests and from in-flight indirect estimations measured during the Commissioning and Performance Verification (CPV) phase. Methods and results are presented and discussed.