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XSHOOTER Pipeline:
Recipe for Slit Nodding Observations Reduction

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For more details see the pipeline user manual at the pipelines page.

  • recipe name: xsh_scired_slit_nod
  • input raw frames:
    • SCI_SLIT_NOD_<arm> (arm = UVB/VIS/NIR)
Purpose: Processes the data up to merged 1D spectra

Description: The raw frames consist of a series of couples with object positions A and B on the slit. The recipe first sums all frames at the same nod position. Then it performs for each nodded pair a number of common data reduction steps, that are described here for the pair A, B. The frame A-B is computed to subtract the sky contribution at the first order. In order to detect both the positive cosmic ray hits (from A) and negative ones (from B) in A-B, the frames |A-B| and sign(A-B) are computed so that A-B = sign(A-B) x |A-B|. The cosmic ray hits are corrected using the Van Dokkum algorithm (van Dokkum 2001, PASP 113, 1420) on |A-B|, and the resulting frame is multiplied by the sign(A-B) frame. The frame A-B is flat-fielded and rectified, i.e. transformed from pixel-pixel space to wavelength-slit space.

Then the spectra are localized (the positive frame is used to derive the localization of A, and its opposite to derive the position of B). The rectified B-A is taken as the opposite of A-B and shifted by an integer amount of pixels along the slit. These operations are repeated for each nod pair of (co-added) frames. Next all the
rectified and shifted frames are combined with a kappa-sigma clipping into a single frame (with a larger range in slit position to encompass the "positive" and both "negative" spectra). A 1D spectrum is produced by collapsing the 2D product on a user-defined range of slit positions. The extraction slit extends over the minimal region of the sky common to all the nod postions (where one expects the positive contribution in each of the A-B frames), which should maximize the signal to noise ratio.