Derivation of sideband gain ratio for Herschel/HIFI^⋆

¹ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
e-mail: dokester@home.nl
² I. Physikalisches Institut der Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
³ European Space Astronomy Centre, Camino Bajo del Castillo s/n, Urb. Villafranca del Castillo, 28692 Villanueva de la Cañada, Madrid, Spain

Received: 19 August 2016
Accepted: 19 November 2016

Abstract

Context. Heterodyne mixers are commonly used for high-resolution spectroscopy at radio telescopes. When used as a double sideband system, the accurate flux calibration of spectral lines acquired by those detectors is highly dependent on the system gains in the respective mixer sidebands via the so-called sideband gain ratio (SBR). As such, the SBR was one of the main contributors to the calibration uncertainty budget of the Herschel/HIFI instrument.

Aims. We want to determine the HIFI instrument sideband gain ratio for all bands on a fine frequency grid and within an accuracy of a few percent.

Methods. We introduce a novel technique involving in-orbit HIFI data that is bootstrapped onto standard methods involving laboratory data measurements of the SBR. We deconvolved the astronomical data to provide a proxy of the expected signal at every frequency channel, and extracted the sideband gain ratios from the residuals of that process.

Results. We determine the HIFI sideband gain ratio to an accuracy varying between 1 and 4%, with degraded accuracy in higher frequency ranges, and at places where the reliability of the technique is lower. These figures were incorporated into the HIFI data processing pipeline and improved the overall flux uncertainty of the legacy data from this instrument.

Conclusions. We demonstrate that a modified sideband deconvolution algorithm, using astronomical data in combination with gas cell measurements, can be used to generate an accurate and fine-granularity picture of the sideband gain ratio behaviour of a heterodyne receiver.