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Why does my SPIRE FTS Spectrum show a discontinuity?
Posted by Mark Kidger, Last modified by Mark Kidger on 04 July 2018 02:04 PM
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Answer supplied by Ivan Valtchanov:
The SPIRE FTS is calibrated for either a perfectly centred point-like source, or for an extended source with size larger than the beam. There should not be a discontinuity in the overlap region of the two spectrometer bands SSW and SLW for these two cases. However, a discontinuity will appear for any of the following reasons: a) Point source on an extended background emission. b) The target not being a perfect point source and not a fully extended, i.e. the so called semi-extended source. c) Mis-pointing d) Very faint target Note that there could be a combination of the causes, i.e. mis-pointing and a semi-extended source. * For a), subtracting the background, as estimated from the off-axis detectors, will achieve a good match in the overlap. For many observations the background subtracted spectra for the central detectors are available as Highly Processed Data Products at the Herschel legacy area: http://archives.esac.esa.int/hsa/legacy/HPDP/SPIRE/SPIRE-S/BKGS * For b) and c) one can use the semi-extended correction tool (SECT), available in HIPE. SECT can be used to correct for the pointing, assuming a point-like source or a semi-extended source with known size, by iteratively deriving the pointing offset that gives the best match in the overlap region. You can find more details about SECT in Section 7.6 of the SPIRE Data Reduction Guide (SDRG) available here: http://herschel.esac.esa.int/hcss-doc-15.0/index.jsp#spire_drg:spire-spectroscopy * For very faint targets (case d), the calibration uncertainty on the continuum level may lead to offsets that will produce a discontinuity. | |
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