HARVESTER
Contents
Data products
Data flow
Sky subtraction
The ARRAKIHS Harvester is the dedicated data reduction pipeline of the ARRAKIHS mission, designed specifically to preserve and accurately recover extremely low surface brightness (LSB) signals that are typically lost in standard astronomical processing. LSB science requires exceptional control of sky modelling, background subtraction, and instrumental systematics. Even small residual errors can remove or distort the faint structures that trace galaxy formation, accretion processes, and diffuse stellar halos. The Harvester is therefore optimised to maintain these faint signals throughout the full processing chain.
Data Products
The Harvester processes the following steps of the ARRAKIHS data flow, from single exposures to final science products:
- Level 1 (L1): FITS formatted single exposures of science and calibration targets (input, similar format to ATREIDS outputs).
- Level 2 (L2): Cleaned and calibrated science exposures.
- Level 3 (L3): Deep co-added images per target.
- Level 4 (L4): Scientific catalogs and derived products.
- In-flight reference files: Processed calibration exposures used in the creation of L2.
Starting from L1, the pipeline generates in-flight reference files and calibrated science exposures (L2) products through a full calibration sequence including bad pixel masking, bias and dark subtraction, flat-fielding, non-linearity corrections, cosmic ray and satellite trail removal, astrometric refinement, and photometric calibration. A key aspect of the processing is the treatment of extended point spread functions (PSF) and optical artefacts such as internal reflections (ghosts), which are modelled and corrected to avoid contamination of faint diffuse emission. Deep coadds (L3) are built for every filter and target by co-adding all
L2 exposures of the same filter and target, with different warping and coadding strategies available (e.g. PSF-matched or optimised for low surface brightness recovery). Finally, L4 products consist of scientific catalogues and derived measurements.
Besides the data products derived by Harvester,the AMC science team will produce the high level data product known as (Level 5), which will remove unresolved background sources from the coadds of Level 3.
Sky subtraction for LSB science.
A central goal of the Harvester is the robust recovery of extremely faint, extended emission. This requires a careful separation between instrumental background and true astrophysical signal.
Sky modelling and subtraction are therefore implemented with particular care to avoid over-subtraction, which can artificially remove diffuse structures such as galaxy halos or tidal streams. The pipeline is designed to minimise these effects by combining instrument modelling, masking strategies, and large-scale background estimation techniques.
The resulting processing strategy is continuously validated using simulations and representative observational datasets, ensuring consistency and stability across different observing conditions.
Software framework
The Harvester is developed by a team at CEFCA, under the supervision of M.Akhlaghi. It is open-source software under the GNU GPL v3 license and will be released alongside ARRAKIHS data products in every data release. It follows a modular and reproducible workflow design, allowing individual stages to be executed independently or as part of full end-to-end processing. The pipeline is built on established tools such as Gnuastro and the Maneage environment system, ensuring long-term reproducibility and portability.