Difference: PacsCalibrationWeb (140 vs. 141)

Revision 1412016-07-08 - ElenaPuga

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PACS instrument and calibration web pages

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PACS spectrometer calibration

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  • The PACS Spectrometer pipeline science-ready data (Level2/Level2.5) are calibrated for extended emission.
  • The calibration of the spectrometer is based on repeated measurements of planets, asteroids, and stars. The RMS scatter of these measurements are just over 10% within any spectral band, about the same when comparing different spaxels, and similar (but higher in the red) when looking for broad-band features within any band. See the PACS Spectrometer Calibration Document for the exact numbers. These calibration certainties are independent, and should be combined when quoting calibration errors.
    • Telescope background model for ChopNod: a script to compute the telescope background model calibration tables and a technical note explaining the method used can be found here. Warning: the calibration tables computed with this script are not the same as the ones in the calibration tree. See the technical note for details.
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  • The PACS Spectrometer pipeline science-ready data (Level2/Level2.5) are calibrated for extended emission.
  • The calibration of the spectrometer is based on repeated measurements of planets, asteroids, and stars. The RMS scatter of these measurements are just over 10% within any spectral band, about the same when comparing different spaxels, and similar (but higher in the red) when looking for broad-band features within any band. See the PACS Spectrometer Calibration Document for the exact numbers. These calibration certainties are independent, and should be combined when quoting calibration errors.
    • Telescope background model for chop nod: a script to compute the telescope background model calibration tables and a technical note explaining the method used can be found here. Warning: the calibration tables computed with this script are not the same as the ones in the calibration tree. See the technical note for details.
 
  • Point source observations. To extract and calibrate the spectrum of a point source, it is necessary to use one of the tasks provided: it is not enough to add up the field-of-view of use the central spaxel only. How to do this is documented in the PACS DRG for spectroscopy.
  • PACS Spectrometer performance and calibration: The PACS Spectrometer Calibration Document v2.4 (16-June-2011) provides details on the calibration accuracy and the necessary information to optimally interpret PACS spectroscopy observations. (Please note, this document refers to the calibration status and performance of pipeline version v8.0. An update compatible with HIPE v14 release will be provided soon.) This includes:
    • flux calibration accuracies for chop nod and unchopped observations
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    • wavelength calibration, including information on a skew our native line profile develops as a point source moves off the centre of a spaxel
    • table of the point source correction factors for different wavelengths
  • An explanation of the data errors for any particular observation is provided in the PACS Data Reduction Guide for spectroscopy (sec. 7.6)
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  • PACS spectrometer beams: version 6, can be downloaded here: PCalSpectrometer_Beam_v6.tar.gz. The PACS beam efficiencies describe the relative coupling of a point source to each spaxel as a function of the source's position in the FOV. The efficiencies are the ratio of the beam profile convolved with each detectors response and with the radiation pattern of a source, and the total power received. In order to characterise the PACS beam, Neptune raster maps at certain wavelengths were observed during the mission. Coarse 25x25 raster maps with raster step size 2.5" were obtained between ODs 174 and 751 in chopped mode covering all 25 spaxels. Also, fine Neptune 5x5 raster maps with raster step size 2 were executed on ODs 1311 and 1312. The combination of four such fine rasters, offset by 1", provide very high sampling for the central spaxel beam efficiency only. All raster maps were observed with only one chop -off position (aka, asymmetric chopNod). All these measurements were registered using least squares minimization in coordinates and gain, and a synthetic beam was constructed with the coarse raster outside the area covered by fine raster and from matched fine raster inside. Finally, this synthetic beam is interpolated into a 0.5" grid. Data were processed using the telescope background normalisation scheme to obtain the telescope-normalised signal per spaxel. The WCS associated with the beam is in sky coordinates for position angle 0. The final maps are a reconstruction of what each spaxel "sees" as the planet was rastered across each detector's aperture on the sky. Note the difference to a regular source map, where we reconstruct the spatial information as a function of position in the sky, even combining information coming from different spaxels.
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  • PACS spectrometer beams: The PACS spectrometer beam efficiencies are maps of the response of each detectors on the sky. They describe the (relative) coupling of a point source to each spaxel as a function of its (the source) position in the FOV.
    • Version 6, the most up-to-date, can be directly downloaded in a tar ball PCalSpectrometer_Beam_v6.tar.gz. The corresponding calibration files are named BeamsPerSpaxelXXX, depending on the band.
    • The PACS beam efficiencies are based on Neptune raster maps at certain (14) wavelengths observed during the mission:
      • Coarse 25x25 raster maps with raster step size 2.5" were obtained between ODs 174 and 751 in chopped mode covering all 25 spaxels.
      • Fine Neptune 5x5 raster maps with raster step size 2 were executed on ODs 1311 and 1312. The combination of four such fine rasters, offset by 1", provide very high sampling for the central spaxel beam efficiency only.
      • All these measurements were registered using least squares minimization in coordinates and gain, and a synthetic beam was constructed with the coarse raster outside the area covered by fine raster and from matched fine raster inside. Finally, this synthetic beam is interpolated into a 0.5" grid.
      • All raster maps were observed with only one chop -off position (aka, asymmetric chopNod).
    • Beam efficiencies are normalised so that a point source of flux 1 at the centre of spaxel 12 has an integral of the instrument response equal to 1.
    • The WCS associated with the beam is in sky coordinates for position angle 0.
 
    • A full history of the PACS Spectrometer beam efficiencies versions can be found in PACSSBeamEfficienciesControlVersion.pdf
    • The raw data from which the PACS spectrometer beams (all versions) have been derived is also available as tables (y, z offset - signal):
      • SpecSpatial_BeamEfficiency_central_spaxel_tables_v1.tar.gz:This contains a FITS file for each wavelength measured for the central spaxel only. Raw data of the coarse and fine rasters are combined. The array dimension of the fits file is [3,npoints] where the first column gives the y raster position, the 2nd column the z raster position and the 3rd column the normalised flux measured at this raster position.
 
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