Source-linked AI summary
The Kilo-Degree Survey
Jelte T. A. de Jong, Gijs A. Verdoes Kleijn, Konrad H. Kuijken, Edwin A. Valentijn, KiDS, Astro-WISE consortiums
TL;DR
KiDS addresses the need for a large Southern optical survey and for coordinated handling of its substantial data products. It combines wide-field imaging and multi-band science planning with Astro-WISE’s distributed, data-centric processing and archive, producing shareable survey products that can be improved and reprocessed as calibrations evolve.
Problem
KiDS addresses the limited availability of large-scale Southern optical surveys and the challenge of coordinating processing, calibration, and archiving for a distributed survey team.
Method
The survey combines VST optical imaging with VIKING near-infrared coverage and uses Astro-WISE to share, calibrate, quality-control, process, and archive data products through data lineage and access management.
Results
Astro-WISE supports improved survey products by pooling calibration data, enabling quality control, and reprocessing only data objects affected by changed calibrations or methods.
Takeaways & Limitations
KiDS data can be shared with the team and wider community through a flexible, controllable live archive, while supporting products such as photometric redshifts and galaxy morphometry.
Takeaways & Limitations
The survey’s object-selection rule treats newer calibration data as better when choosing applicable calibration items.
Abstract
from arXiv · showhide
The Kilo Degree Survey (KiDS) is a 1500 square degree optical imaging survey with the recently commissioned OmegaCAM wide-field imager on the VLT Survey Telescope (VST). A suite of data products will be delivered to the European Southern Observatory (ESO) and the community by the KiDS survey team. Spread over Europe, the KiDS team uses Astro-WISE to collaborate efficiently and pool hardware resources. In Astro-WISE the team shares, calibrates and archives all survey data. The data-centric architectural design realizes a dynamic 'live archive' in which new KiDS survey products of improved quality can be shared with the team and eventually the full astronomical community in a flexible and controllable manner.
1 Introduction
KiDS addresses the need for a large Southern optical survey by imaging 1500 square degrees in four filters, complemented by VIKING near-infrared observations. Astro-WISE supports distributed processing, collaboration, calibration, and public archiving for the survey.
- 1500 square degrees in u, g, r and i will be imaged by KiDS over 3–4 years.Together with VIKING observations in ZYJHK, the surveys will provide a sensitive 9-band multi-colour survey.
- KiDS helps fill the Southern-sky gap left by predominantly Northern surveys such as SDSS and PanSTARRS.The survey uses ESO’s VST, a dedicated Southern optical survey telescope.
- Astro-WISE provides distributed processing, quality control, and public archiving for VST surveys.The KiDS team uses web-based collaborative interfaces as a single virtual team distributed across countries.
- The paper discusses KiDS’s observational set-up and primary science goals alongside Astro-WISE’s role in achieving them.
2.1 Instrumental set-up
KiDS uses the VST’s OmegaCAM, a large-format optical camera, to image two survey patches whose layout is shown for the Northern and Southern fields.
- OmegaCAM is a 268 Megapixel camera with a 1°×1° field of view on the 2.6-m VST.Its focal plane contains 32 2048×4096 pixel CCDs, producing a 16k×16k-pixel array at 0.214 arcseconds per pixel.
- KiDS comprises Northern and Southern patches, with their detailed layout presented in Fig. 1 and Table 1.
2.2 Observational survey set-up
The survey layout and observing strategy are designed to provide broad, uniform, multi-band coverage while supporting redshift, lensing, and proper-motion science.
- 1500 square degrees are covered by two patches spanning galactic latitudes from 40° to 90°.The 10° strip width is intended to sample the three-dimensional structure of the universe well.
- A median galaxy redshift of 0.8 targets studies of galaxy-population and matter-distribution evolution over roughly half the universe’s age.
- Queue scheduling matches bands to seeing conditions, reserving the best seeing for deep r-band exposures and poorer seeing for u.Exposure times and observational constraints are listed in Table 2.
- Five dithered observations in g, r and i, and four in u, bridge OmegaCAM’s CCD gaps.The dither pattern uses 25″ steps in X and 85″ steps in Y.
- Restricting r-band observations to a maximum airmass of 1.3 limits spectral dispersion to <0.2″ for weak-lensing analyses.
- A repeat g-band pass with at least a 2-year baseline enables proper-motion measurements at 40 mas yr−1 and better for sources with signal-to-noise 10.
2.3 Scientific goals
KiDS and VIKING target weak-lensing, photometric-redshift, and galaxy-evolution science across a large, deep survey area. Their data support studies of dark energy, BAO, galaxy–mass relations, galaxy and cluster evolution, environments, and the Milky Way halo.
- Dark energy: Weak gravitational lensing and photometric redshifts are KiDS and VIKING’s central probes of matter distribution and cosmic expansion.Weak lensing requires systematics and photometric-redshift biases controlled near the 1% level.
- Dark energy: KiDS photometric redshifts can reach the 0.03 × (1 + z) rms accuracy needed for BAO detection, with w measured to approximately 5% in simulations.The simulations use the full nine-band ugrizYJHK coverage planned for KiDS and VIKING.
- Structure of galaxy halos: KiDS enables galaxy–galaxy lensing analyses across galaxy types and redshift bins, using many pairs, accurate shapes, and known foreground large-scale structure.These capabilities support measurements from inner halo scales through galaxy–mass correlations and larger-scale structure.
- Evolution of galaxies and clusters: An estimated 10^8 galaxies with median redshift z = 0.8, including approximately 20% at 1 < z < 1.5, support studies of galaxy luminosity, stellar mass, and early-type systems.The sample is intended to trace galaxy evolution to unprecedented look-back times.
- Evolution of galaxies and clusters: KiDS is expected to provide 1–2 × 10^4 clusters, with approximately 5% beyond z = 1, while weak lensing can calibrate cluster richness against mass.The red sequence is expected to remain detectable to z approximately 1.4.
- Evolution of galaxies and clusters: KiDS-S overlaps the Pisces-Cetus and Fornax-Eridanus superclusters, enabling galaxy-property and environment studies from cluster cores through filaments.The survey also offers a deeper view of more distant Milky Way halo regions than SDSS, despite covering a smaller area.
2.4 Survey calibration and data products
KiDS combines calibrated survey observations with public basic and advanced data products distributed through ESO and Astro-WISE. Its calibration and processing plan supports uniform measurements, lensing analyses, and combined optical–near-infrared products.
- Data products: KiDS will make approximately 100,000 sources per square degree public, producing 150 million sources and 15 TB of final image data over the survey.The catalogue and image products are intended for community access.
- Calibration: Astrometric calibration uses per-tile gnomonic solutions tied to the 2MASS PSC, followed by global focal-plane modeling from dither overlaps and external residuals.Initial relative astrometry is 0.1–0.15 arcsec rms, with approximately 0.05 arcsec rms expected globally.
- Calibration: Photometric calibration is performed survey-wide in the Sloan AB system using nightly standard-field zeropoints and OmegaCAM secondary standards.The calibration program covers the OmegaCAM field of view in the u, g, r, and i bands.
- Data products: Basic products will be released through ESO and Astro-WISE within a year after each area is observed in all filters, including calibrated images, calibration frames, and single- and multi-colour catalogues.The release plan covers coadded, regridded images with weight maps and associated calibration data.
- Advanced products: Advanced products include Gaussianized-PSF images, aperture-matched colour catalogues, unsharp-masked images, and fitted morphological parameters.GALFIT and GALPHOT are implemented in Astro-WISE for source processing and database publication.
- Combined products: Selected VIKING and combined KiDS–VIKING products will be made available through Astro-WISE, although the KiDS team will not perform VIKING’s large-scale processing.This extends the archive beyond KiDS-only optical products.
3 Internet-based survey collaboration for a geographically distributed team
Astro-WISE enables geographically distributed KiDS collaborators to process, control, manage, share, and archive survey data through a shared, data-centric system. Data attributes govern access, calibration, quality status, lineage, and reproducible processing.
- Collaboration: Astro-WISE lets the distributed KiDS team process, quality-control, manage, archive, and publish survey data through shared European storage, compute, and database resources.Webservices allow team members to work through a browser, regardless of location.
- Data-centric architecture: Data objects retain calibration validity, quality assessments, provenance, and instructions for creating improved products from lower-level survey objects.Processing dependencies are recursively linked back to raw data.
- Data-centric architecture: Requested targets trigger backward database queries that identify suitable dependencies, after which the resulting forward workflow is executed.If a dependency is unavailable, the query continues toward objects closer to the raw data.
- Data management: Project and Privileges context parameters filter accessible data, while MYDB and PROJECT levels separate private experiments from team-shared baseline products.Users can publish promising MYDB products to PROJECT, while project managers can promote baseline data further.
- Quality control: Quality assessment combines automatic verification, comparison with earlier results, and manual inspection, storing outcomes as data attributes or comments.Quality information can be reviewed through webservices and customized through command-line scripts.
- Calibration control: Continuous calibration coverage allows improved calibrations to eclipse older versions and be pooled with Astro-WISE users through publication at Privileges level 3 or higher.This supports calibration of the instrument and atmosphere rather than only individual datasets.
4 From first reductions towards final data delivery
KiDS operations began with initial calibration data and early survey products. Quality control, lineage-aware reprocessing, and accumulated calibration knowledge support progression toward a complete, high-quality public data release.
- Initial reductions: KiDS survey operations officially began on 15 October 2011 with fields being calibrated using initial calibration data.Quality control was expected to improve calibration objects, pipeline configuration, and processing methods.
- Iterative improvement: Astro-WISE lineage makes it straightforward to reprocess only data objects affected by improved calibrations or pipeline changes.The resulting calibrated data support advanced products including photometric redshifts, galaxy morphometry, and source variability analysis.
- Final delivery: The KiDS team plans to progress from quick-look products to a complete, high-quality, value-added public survey dataset.The live archive captures accumulated knowledge about OmegaCAM, the VST, and KiDS data over time.