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FORS2 Pipeline:
Recipe for Spectro-Polarimetric Science Data

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  • recipe name: FORS_PMOS_SCIENCE
  • input frames: SCIENCE_PMOS, STANDARD_PMOS
  • associated templates: FORS2_pmos_obs_off_fast, FORS2_pmos_obs_off
purpose: processes SCIENCE data

description: This recipe is used for reducing FORS2 PMOS, scientific spectra applying the extraction mask and the normalised flat field created by the recipe fors_pmos_calib. The slit spectra are bias subtracted, flat fielded if requested, and remapped eliminating the optical distortions. The input wavelength calibration can optionally be adjusted to a number of reference sky lines. Finally, objects are searched and extracted from all the slit spectra. Then the appropriate polarimetric parameters (for linear and circular polarization) are derived by combining the spectra from the various exposures. This recipe is also used to reduced spectroscopic standard star observations. In case of standard star observations the SCI acronym in the products names should also be read STD. The pipeline reduces sets of 2 or 4 frames for circular polarimetry and sets of 4 or 8 frames for linear polarimetry for versions up to 4.7.3. Later versions have no such limit.

Pipeline versions up to 4.7.3 also had the following bug: If there was more than one active slit, and at least one slit had a number of objects different from the other slits, then the association between spectra belonging to the same object (in the two corresponding beams) was ALWAYS wrong. As long as the number of objects per slit is the same for all slits (including the case of only 1 active slit) the association was correct.

In the case of standard stars the recipe also determines for polarized standard the polarization angle and the offset in polarization. For unpolarized standard stars only the absolute polarization is determined.

products (for naming rules see here):

  • sky line offset table containing the observed sky lines offsets that were used for adjusting the input wavelength solution. This table may be very useful for judging what would be the most appropriate modeling of the observed offsets, and to what extent the input wavelength calibration really needs to be adjusted. (PSOP for STD observations; SSOP for SCIENCE observations)
  • dispersion coefficients table updated using reference sky lines (PDCP for STD observations; SDCP for SCIENCE observations)
  • wavelength map image, an upgraded version of the wavelength map; produced only in case the adjustment of the wavelength solution to the sky lines is requested (PWSP for STD observations; SWSP for SCIENCE observations)
  • mapped science data (all these products are normalized with their exposure time)
    • image with rectified and wavelength calibrated slit spectra (PMAP for STD observations; SMAP for SCIENCE observations)
    • image with rectified and wavelength calibrated slit sky spectra. This image contains the modeled sky that was subtracted from the scientific data (PMKP for STD observations; SMKPfor SCIENCE observations)
    • image with rectified, wavelength calibrated and sky subtracted slit spectra. This image matches in size the MAPPED_ALL image, and is produced only if any kind of sky subtraction (global and/or local) is requested (PMSP for STD observations; SMSP for SCIENCE observations)
  • object table which is an expansion of the input slit location table, where the positions and the extraction spatial intervals of the detected objects are also included. Here the spectra of each object in the two observed slits for each mask slit are identified and paired. This table is produced only if any kind of sky subtraction (global and/or local) is requested, otherwise no object detection or extraction is attempted (POTP for STD observations; SOTP for SCIENCE observations)
  • reduced and extracted spectra (all these products are normalized with their exposure time)
    • image with extracted objects spectra. Extracted spectra are written to the image rows listed in the object table (PRSP for STD observations; SRSP for SCIENCE observations)
    • image with errors (one sigma level) corresponding to the extracted objects spectra. This image matches the reduced SCI/STD image. (PREP for STD observations; SREP for SCIENCE observations)
    • image with sky corresponding to the extracted objects spectra. The sky is extracted in the same way as the objects, e.g., if optimal weights were applied to the object extraction, the same weights are applied to the sky extraction. This image matches the reduced SCI/STD image (PRKP for STD observations; SRKP for SCIENCE observations)
  • polarimetric parameters (angle, L, Q, U, V) depending on the type and number of observations)
  • unmapped data (not for LSS-like observations; these products are not normalized to 1 second exposure time)
    • image obtained by interpolating the sky signal trend along the spatial direction, directly on the CCD frame (SUKP)
    • image with the sky subtracted scientific spectra on the CCD frame (SUSP)

 
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