Correcting the extended-source calibration for the Herschel-SPIRE Fourier-transform spectrometer

Journal
Date Issued
2018
Author(s)
Valtchanov, I.
•
Hopwood, R.
•
Bendo, G.
•
Benson, C.
•
Conversi, L.
•
Fulton, T.
•
Griffin, M. J.
•
Joubaud, T.
•
Lim, T.
•
Lu, 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.
•
van der Wiel, M. H. D.
•
Wu, R.
DOI
Abstract
We describe an update to the Herschel-Spectral and Photometric Imaging Receiver (SPIRE) Fourier-transform spectrometer (FTS) calibration for extended sources, which incorporates a correction for the frequency-dependent far-field feedhorn efficiency, ηff. 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 thespectrometer long wavelength band (447-1018 GHz; 671-294 μm) and 1.4-1.5 in the spectrometer short wavelength band (944-1568 GHz; 318-191 μm). The correction was implemented in the FTS pipeline version 14.1 and has also been described in the SPIRE Handbook since 2017 February. 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 ηff, the synthetic photometry and the broad-band intensities from SPIRE photometer maps agree within 2-4 per cent - 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.
Volume
475
Issue
1
Start page
321
Uri
Url
Issn Identifier
0035-8711
Ads BibCode
2018MNRAS.475..321V
Rights
open.access
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