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Fulton T.,University of Lethbridge | Naylor D.,University of Lethbridge | Polehampton E.,University of Lethbridge | Polehampton E.,Rutherford Appleton Laboratory | And 5 more authors.
Fourier Transform Spectroscopy, FTS 2015 | Year: 2015

Asymmetries in the recorded interferograms of Fourier Transform Spectrometers (FTS) result in spectra with a non-zero phase. We describe the causes of non-zero phase particular to the Herschel/SPIRE FTS and present the phase correction processing steps employed. © OSA 2015. Source

Hopwood R.,Imperial College London | Fulton T.,Blue Sky | Fulton T.,University of Lethbridge | Polehampton E.T.,University of Lethbridge | And 12 more authors.
Experimental Astronomy | Year: 2014

Emission from the Herschel telescope is the dominant source of radiation for the majority of SPIRE Fourier transform spectrometer (FTS) observations, despite the exceptionally low emissivity of the primary and secondary mirrors. Accurate modelling and removal of the telescope contribution is, therefore, an important and challenging aspect of FTS calibration and data reduction pipeline. A dust-contaminated telescope model with time invariant mirror emissivity was adopted before the Herschel launch. However, measured FTS spectra show a clear evolution of the telescope contribution over the mission and strong need for a correction to the standard telescope model in order to reduce residual background (of up to 7 Jy) in the final data products. Systematic changes in observations of dark sky, taken over the course of the mission, provide a measure of the evolution between observed telescope emission and the telescope model. These dark sky observations have been used to derive a time dependent correction to the telescope emissivity that reduces the systematic error in the continuum of the final FTS spectra to ~0.35 Jy. © 2013 Springer Science+Business Media Dordrecht. Source

Lu N.,NHSC IPAC | Polehampton E.T.,Rutherford Appleton Laboratory | Polehampton E.T.,University of Lethbridge | Swinyard B.M.,Rutherford Appleton Laboratory | And 11 more authors.
Experimental Astronomy | Year: 2014

The Fourier Transform Spectrometer (FTS) of the Spectral and Photometric Imaging REceiver (SPIRE) on board the ESA Herschel Space Observatory has two detector setting modes: (a) a nominal mode, which is optimized for observing moderately bright to faint astronomical targets, and (b) a bright-source mode recommended for sources significantly brighter than 500 Jy, within the SPIRE FTS bandwidth of 446.7-1544 GHz (or 194-671 microns in wavelength), which employs a reduced detector responsivity and out-of-phase analog signal amplifier/demodulator. We address in detail the calibration issues unique to the bright-source mode, describe the integration of the bright-mode data processing into the existing pipeline for the nominal mode, and show that the flux calibration accuracy of the bright-source mode is generally within 2 % of that of the nominal mode, and that the bright-source mode is 3 to 4 times less sensitive than the nominal mode. © 2013 Springer Science+Business Media Dordrecht. Source

Valtchanov I.,Herschel Science Center | Hopwood R.,Imperial College London | Polehampton E.,Rutherford Appleton Laboratory | Polehampton E.,University of Lethbridge | And 13 more authors.
Experimental Astronomy | Year: 2014

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. © 2013 Springer Science+Business Media Dordrecht. Source

Benielli D.,Aix - Marseille University | Polehampton E.,Rutherford Appleton Laboratory | Polehampton E.,University of Lethbridge | Hopwood R.,Imperial College London | And 12 more authors.
Experimental Astronomy | Year: 2014

The Herschel SPIRE Fourier transform spectrometer (FTS) performs spectral imaging in the 447-1546 GHz band. It can observe in three spatial sampling modes: sparse mode, with a single pointing on sky, or intermediate or full modes with 1 and 1/2 beam spacing, respectively. In this paper, we investigate the uncertainty and repeatability for fully sampled FTS mapping observations. The repeatability is characterised using nine observations of the Orion Bar. Metrics are derived based on the ratio of the measured intensity in each observation compared to that in the combined spectral cube from all observations. The mean relative deviation is determined to be within 2 %, and the pixel-by-pixel scatter is ~ 7 %. The scatter increases towards the edges of the maps. The uncertainty in the frequency scale is also studied, and the spread in the line centre velocity across the maps is found to be ~ 15 km s - 1. Other causes of uncertainty are also discussed including the effect of pointing and the additive uncertainty in the continuum. © 2013 Springer Science+Business Media Dordrecht. Source

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