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Relative pointing offset analysis of calibration targets with repeated observations with Herschel-SPIRE Fourier-transform spectrometer

Valtchanov, Ivan and Hopwood, Rosalind and Polehampton, Edward and Benielli, Dominique and Fulton, Trevor and Imhof, Peter and Konopczyński, Tomasz and Lim, Tanya and Lu, Nanyao and Marchili, Nicola and Naylor, David and Swinyard, Bruce (2014) Relative pointing offset analysis of calibration targets with repeated observations with Herschel-SPIRE Fourier-transform spectrometer. Experimental Astronomy, 37 (2). pp. 207-223. ISSN 0922-6435.

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We present a method to derive the relative pointing offsets for SPIRE Fourier-Transform Spectrometer (FTS) solar system object (SSO) calibration targets, which were observed regularly throughout the Herschel mission. We construct ratios R_(obs)(ν) of the spectra for all observations of a given source with respect to a reference. The reference observation is selected iteratively to be the one with the highest observed continuum. Assuming that any pointing offset leads to an overall shift of the continuum level, then these R_(obs)(ν) represent the relative flux loss due to mispointing. The mispointing effects are more pronounced for a smaller beam, so we consider only the FTS short wavelength array (SSW, 958–1546 GHz) to derive a pointing correction. We obtain the relative pointing offset by comparing R_(obs)(ν) to a grid of expected losses for a model source at different distances from the centre of the beam, under the assumption that the SSW FTS beam can be well approximated by a Gaussian. In order to avoid dependency on the point source flux conversion, which uses a particular observation of Uranus, we use extended source flux calibrated spectra to construct R_(obs)(ν) for the SSOs. In order to account for continuum variability, due to the changing distance from the Herschel telescope, the SSO ratios are normalised by the expected model ratios for the corresponding observing epoch. We confirm the accuracy of the derived pointing offset by comparing the results with a number of control observations, where the actual pointing of Herschel is known with good precision. Using the method we derived pointing offsets for repeated observations of Uranus (including observations centred on off-axis detectors), Neptune, Ceres and NGC 7027. The results are used to validate and improve the point-source flux calibration of the FTS.

Item Type:Article
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URLURL TypeDescription Paper ReadCube access
Valtchanov, Ivan0000-0001-9930-7886
Lu, Nanyao0000-0002-8948-1044
Additional Information:© 2013 Springer Science+Business Media Dordrecht. Received: 30 June 2013. Accepted: 26 September 2013. Published online: 19 October 2013. SPIRE has been developed by a consortium of institutes led by Cardiff University (UK) and including Univ. Lethbridge (Canada); NAOC (China); CEA, LAM (France); IFSI, Univ. Padua (Italy); IAC (Spain); Stockholm Observatory (Sweden); Imperial College London, RAL, UCL-MSSL, UKATC, Univ. Sussex (UK); and Caltech, JPL, NHSC, Univ. Colorado (USA). This development has been supported by national funding agencies: CSA (Canada); NAOC (China); CEA, CNES, CNRS (France); ASI (Italy); MCINN (Spain); SNSB (Sweden); STFC (UK); and NASA (USA).
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Canadian Space Agency (CSA)UNSPECIFIED
National Astronomical Observatories, Chinese Academy of Sciences (NAOC)UNSPECIFIED
Commissariat à l’Energie Atomique (CEA)UNSPECIFIED
Centre National d'Études Spatiales (CNES)UNSPECIFIED
Centre National de la Recherche Scientifique (CNRS)UNSPECIFIED
Agenzia Spaziale Italiana (ASI)UNSPECIFIED
Ministerio de Ciencia e Innovación (MCINN)UNSPECIFIED
Swedish National Space Board (SNSB)UNSPECIFIED
Science and Technology Facilities Council (STFC)UNSPECIFIED
Subject Keywords:SPIRE, Fourier transform spectrometer, Calibration, Spectroscopy, Pointing, Herschel observatory, Uranus, Neptune, Ceres, NGC 7027
Issue or Number:2
Record Number:CaltechAUTHORS:20140902-095620070
Persistent URL:
Official Citation:Valtchanov, I., Hopwood, R., Polehampton, E. et al. Exp Astron (2014) 37: 207. doi:10.1007/s10686-013-9356-z
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:49098
Deposited By: Ruth Sustaita
Deposited On:02 Sep 2014 19:45
Last Modified:03 Oct 2019 07:10

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