Flux Calibration of Broadband Far Infrared and Submillimetre Photometric Instruments: Theory and Application to Herschel-SPIRE

TL;DR

This paper develops a calibration framework for broadband far-infrared and submillimetre instruments that accounts for variations in beam profile and aperture efficiency across the passband, improving flux calibration accuracy.

Contribution

It introduces a comprehensive calibration method considering passband-induced beam variations, applicable to both point and extended sources, demonstrated on Herschel-SPIRE.

Findings

Enhanced calibration accuracy for Herschel-SPIRE.

Quantified impact of passband effects on flux measurements.

Applicable to various focal plane architectures.

Abstract

Photometric instruments operating at far infrared to millimetre wavelengths often have broad spectral passbands (central wavelength/bandwidth ~ 3 or less), especially those operating in space. A broad passband can result in significant variation of the beam profile and aperture efficiency across the passband, effects which thus far have not generally been taken into account in the flux calibration of such instruments. With absolute calibration uncertainties associated with the brightness of primary calibration standards now in the region of 5% or less, variation of the beam properties across the passband can be a significant contributor to the overall calibration accuracy for extended emission. We present a calibration framework which takes such variations into account for both antenna-coupled and absorber-coupled focal plane architectures. The scheme covers point source and extended source cases, and also the intermediate case of a semi-extended source profile. We apply the new method to the Herschel-SPIRE space-borne photometer.