Observing extended sources with the Herschel SPIRE Fourier Transform Spectrometer
Résumé
The Spectral and Photometric Imaging Receiver (SPIRE) on the European Space Agency’s
Herschel Space Observatory utilizes a pioneering design for its imaging
spectrometer in the form of a Fourier Transform Spectrometer (FTS). The standard FTS data
reduction and calibration schemes are aimed at objects with either a spatial extent that
is much larger than the beam size or a source that can be approximated as a point source
within the beam. However, when sources are of intermediate spatial extent, neither of
these calibrations schemes is appropriate and both the spatial response of the instrument
and the source’s light profile must be taken into account and the coupling between them
explicitly derived. To that end, we derive the necessary corrections using an observed
spectrum of a fully extended source with the beam profile and considering the source’s
light profile. We apply the derived correction to several observations of planets and
compare the corrected spectra with their spectral models to study the beam coupling
efficiency of the instrument in the case of partially extended sources. We find that we
can apply these correction factors for sources with angular sizes up to
θD ~ 17′′. We demonstrate how the angular size of
an extended source can be estimated using the difference between the subspectra observed
at the overlap bandwidth of the two frequency channels in the spectrometer, at
959 < ν < 989 GHz.
Using this technique on an observation of Saturn, we estimate a size of 17.2′′,
which is 3% larger than its true size on the day of observation. Finally, we show the
results of the correction applied on observations of a nearby galaxy, M82, and the compact
core of a Galactic molecular cloud, Sgr B2.
Origine : Publication financée par une institution
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