This page gives a very brief description of the GALAH Survey data reduction pipeline. If you are interested in all the details of the reduction used for Data Release 4, see Kos et al. (2017). Essentially we use a bespoke iraf pipeline to collate and reduce all the HERMES spectra, and provide first estimates of the radial velocity and stellar parameters.

Reduction used in Data Release 3

This follows the usual steps for reducing multi-object spectrograph data:

  1. Raw images are corrected for bias level and flat field, and cosmic rays are removed with a modified LaCosmic algorithm (van Dokkum 2001). Scattered light and fibre-cross talk signals are removed.
  2. The wavelength solution for the extracted spectra is found via fitting of ThXe arc lamp observations.
  3. Sky spectra are modelled from the 25 sky fibres included in each field and subtracted, and synthetic telluric lines are computed using molecfit (Kausch et al. 2015; Smette et al. 2015) and removed from observed spectra.
  4. The reduction pipeline runs a cross-correlation with AMBRE spectra (De Laverny et al. 2012) to provide a first estimate of stellar parameters Teff, logg, [Fe/H], and radial velocity, (these values are labelled as x_guess in the main_spec catalogue; see the Table Schema documentation) and to normalise the spectra.

Changes in data reduction from GALAH Data Release 3

We are always working to improve our reduction. Between GALAH DR3 and GALAH DR4 there were two major improvements to the data reduction:

  1. Improved wavelength calibration for the long-wavelength end of the red camera, recovering spectra for around 15% of stars that were excluded from DR3.
  2. Better identification and exclusion of bad or mislabeled data.