Difference: PacsCalibrationWeb (132 vs. 133)

Revision 1332016-01-08 - ElenaPuga

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PACS instrument and calibration web pages

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    • table of the point source correction factors for different wavelengths
  • 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.
  • An explanation of the data errors for any particular observation is provided in the PACS Data Reduction Guide for spectroscopy (sec. 7.6)
  • PACS spectrometer beams, version 3, can be downloaded here: PCalSpectrometer_Beam_v3.tar.gz. These beams are based on measurements of a raster with step size 2.5" around Neptune. These beam maps are useful e.g. if you want to compare to the flux distribution of your observation of a point source. Version 3 has the beam efficiencies for all IFU spaxels, and is a drastic improvement wrt version 2 as the spacecraft pointing was reconstructed more accurately. This reconstruction resulted in a non-equidistant sampling of the beam efficiency in the sky, but the beam products offered are equidistantly-sampled on a grid of 0.5 arcseconds. Each beam is normalised to the fitted peak value of the central spaxel. The WCS associated with the beam is in sky coordinates for position angle 0. The central part of the beam is the Gaussian fit to the measured beam efficiencies. This has been verified to be a very good description on the different raster observations we have of the central spaxels for wavelengths longer than 80 micron. Below 80 micron, the actual beam shows the square detector footprint, and the Gaussian approximation in the beam products v3 over-predicts the real beam efficiency by 1.5 to 2 percent. The outer part of the beams contains the interpolated values of the irregularly sampled measurements. Version 3 of the spectrometer beams are sharper than version 2, and shows the ghosts (ghosts are discussed in the Spectrometer Calibration Document ) more clearly, as well as the three-lobe structure of the Herschel telescope PSF.
  • 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. Finally, the efficiencies were calculated normalising the raster maps in the following way:
    • all the 25 beams were divided by the peak value of the central spaxel beam;
    • then, they are multiplied to match the Point Source Correction calibration file (pointSourceLoss FM_4).
  • 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.
    • SpecSpatial_BeamEfficiency_tables_v1.tar.gz: Raw measurements PACS beams - all spaxels, coarse raster measurements only: each fits files corresponds to one wavelength. Each file contains the data for all spaxels of the coarse raster measurement only. Each fits file holds an array of 3x25x25x25 where: (0,25,25,25)=y raster position, (1,25,25,25)=z raster position, (2,25,25,25)=flux normalized to the central spaxel. The second and third dimensions are the raster position indices (y and z) and the last dimension is the module number (=spaxel number).
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