7.3. PACS spectroscopy standard data processing
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7.3. PACS spectroscopy standard data processing

7.3.1. PACS spectroscopy processing steps

  1. Raw telemetry data is decompressed and stored as science (rotating raw ramps of 3 pixels and slope fitted ramps stored in Frames objects) and housekeeping data
  2. Bad/saturated/low quality pixels and readouts are masked
  3. The masked science data are reduced to [Volt/sec] from [ADU/reset interval]
  4. Major observation blocks (like e.g. nod positions, grating scan directions) are summarized
  5. The chopper plateaux are cleaned from transition values and the chopper angle is converted to an the angle on the sky
  6. Grating movements are cleaned from transition values
  7. Spacecraft pointing is associated to each science frame
  8. Wavelength is associated to each pixel in each data frame
  9. The sky coordinates are calculated for each pixel
  10. The S/C on-board time is converted to UTC
  11. Signal glitches are searched and masked
  12. Scale signal to standard capacitance
  13. Determine response and dark from calibration blocks
  14. Correct signal non-linearities
  15. Divide signal by the relative spectral response function
  16. Divide signal by the absolute response, get units in [Jy]
  17. Subtract off-source signal from on-source signal by pairwise differencing
  18. Create a PacsCube from PACS Spectrometer Frames
  19. Compute wavelength grid for rebinning signal in PacsCube
  20. Rebin data on the wavelength grid, get 5x5xlambda PacsRebinnedCubes for nod A and B positions
  21. Project rebinned cubes onto an mxn spatial grid and combine nod positions, the result is the final MxNxlambda PacsProjectedCube

7.3.2. Spectroscopy processing levels and data products

There is a Herschel-wide convention on processing levels of the different instruments.

  • Raw Telemetry : All telemetry packets produced by the instrument in the course of the observation. In PACS IA, we store/manipulate this level as a PacketSequence .

  • Level 0 data: Telemetry data as measured by the instrument, minimally manipulated and stored as Data Frames. For PACS spectroscopy, this level is stored/manipulated in a DataFrameSequence : a sequence of PACS dataframes, which are decompressed SPU buffers. What is contained in every decompressed SPU buffer depends on the SPU reduction mode. Typically there are several reduced readouts for every active detector (averaged ramp readouts or fitted slopes), 256Hz readouts for a few selected pixels and mechanism/status information sampled at 256Hz by the DecMec, the so-called DMC Header.

  • Level 0.5 data : The information contained in the Level 0 data is not sufficient to be able to process those data. The level 0.5 data is a bundle of Level 0 data and the data needed to fully process those data: auxiliary data for the timespan covered by the Level 0 data, such as the spacecraft pointing (attitude history), the time correlation, selected spacecraft housekeeping, etc... It is also possible to include in this bundle the level 0 data of 'associated' observations - e.g. flatfields or photometric checks taken throughput the operational day.

  • Level 1 data: Detector readouts calibrated and converted to physical units, in principle instrument and observatory independent. For PACS spectroscopy this is an oversampled 5x5xn cube with flux densities, associated wavelengths and sky coordinates for every flux density.

  • Level 2 data: Further processed level-1 data to such a level that scientific analysis can be performed. For optimal results many of the processing steps involved to generate level-2 data may require human interaction, based both on instrument understanding as well as understanding of the scientific aims of the observation. These data products are at a publishable quality level and should be suitable for VO access.

    This level of PACS spectroscopy data consists of an image cube, the depth of the cube being the wavelength/frequency. Each layer in the image cube has the same sky projection. This cube is constructed by re-sampling the integral field cube (oversampled in wavelength, different projection per layer due to distortions) onto the same sky / wavelength grid of the instrumental resolution.

    Level 2 data will can also contain a set of 1-dimensional spectra, Nyquist sampled to the instrument resolution, combining the data for one spatial pixel over different scans, applying noise filters, etc...

  • Level 3 data: These are the publishable science products where level-2 data products are used as input. These products are not only from the specific instrument, but are usually combined with theoretical models, other observations, laboratory data, catalogues, etc. Their formats should be VO compatible and these data products should be suitable for VO access.

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