Correcting the extended-source calibration for the Herschel-SPIRE Fourier-Transform Spectrometer

• Published in: arXiv.org 2018-01
• Main Authors: Valtchanov, Ivan, Hopwood, R, Bendo, G, Benson, C, Conversi, L, Fulton, T, Griffin, M J, Joubaud, T, Lim, T, N Lu, Marchili, N, Makiwa, G, Meyer, R A, Naylor, D A, North, C, Papageorgiou, A, Pearson, C, Polehampton, E T, Scott, J, Schulz, B, Spencer, L D, M H D van der Wiel, R Wu
• Format: Article
• Language: English
• Subjects: Broadband; Calibration; Fluxes; Fourier transform spectrometers; Line spectra; Mapping; Photometry; Spectral resolution
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1708.09203

We describe an update to the Herschel-SPIRE Fourier-Transform Spectrometer (FTS) calibration for extended sources, which incorporates a correction for the frequency-dependent far-field feedhorn efficiency, \(\eta_\mathrm{FF}\). This significant correction affects all FTS extended-source calibrated spectra in sparse or mapping mode, regardless of the spectral resolution. Line fluxes and continuum levels are underestimated by factors of 1.3-2 in the Spectrometer Long-Wavelength band (SLW, 447-1018 GHz; 671-294 \(\mu\)m) and 1.4-1.5 in the Spectrometer Short-Wavelength band (SSW, 944-1568 GHz; 318-191 \(\mu\)m). The correction was implemented in the FTS pipeline version 14.1 and has also been described in the SPIRE Handbook since Feb 2017. Studies based on extended-source calibrated spectra produced prior to this pipeline version should be critically reconsidered using the current products available in the Herschel Science Archive. Once the extended-source calibrated spectra are corrected for \(\eta_\mathrm{FF}\), the synthetic photometry and the broadband intensities from SPIRE photometer maps agree within 2-4% -- similar levels to the comparison of point-source calibrated spectra and photometry from point-source calibrated maps. The two calibration schemes for the FTS are now self-consistent: the conversion between the corrected extended-source and point-source calibrated spectra can be achieved with the beam solid angle and a gain correction that accounts for the diffraction loss.