The Herschel PACS photometer calibration - A time dependent flux calibration for the PACS chopped point-source photometry AOT mode

TL;DR

This paper develops a time-dependent flux calibration method for Herschel PACS chopped point-source photometry, improving data accuracy and consistency for scientific analysis.

Contribution

It introduces a calibration scheme based on stellar standards, addressing response drifts and systematic differences with other modes, enhancing data reliability.

Findings

Calibration accuracy reaches 1% in blue/green bands

Response drift during initial operational days is effectively corrected

Systematic calibration differences are 5-6% compared to scan map mode

Abstract

We present a flux calibration scheme for the PACS chopped point-source photometry observing mode based on the photometry of five stellar standard sources. This mode was used for science observations only early in the mission. Later, it was only used for pointing and flux calibration measurements. Its calibration turns this type of observation into fully validated data products in the Herschel Science Archive. Systematic differences in calibration with regard to the principal photometer observation mode, the scan map, are derived and amount to 5-6%. An empirical method to calibrate out an apparent response drift during the first 300 Operational Days is presented. The relative photometric calibration accuracy (repeatability) is as good as 1% in the blue and green band and up to 5% in the red band. Like for the scan map mode, inconsistencies among the stellar calibration models become visible and amount to 2% for the five standard stars used. The absolute calibration accuracy is therefore mainly limited by the model uncertainty, which is 5% for all three bands.