VMC Data

Raw data are public worldwide immediately after passing quality control tests at ESO via the ESO archive .
The VMC-ESO data release is available at: VMC@ESO. The VMC data are also released at the VISTA Science Archive in Ediburgh at: VMC@VSA .
Deep VMC images for each tile are viewable at: VMC-Images.
A final release with global photometric and astrometric calibration will be made upon completion of the survey.

The main VMC data products refer to pawprints and tiles and include:
  • instrumentally corrected, stacked frames in all the relevant wave bands, with photometric and astrometric calibrations in the headers
  • confidence maps
  • aperture-matched object catalogues with multi-band photometric, astrometric and morphological measures
  • These data are calibrated using standard night-by-night calibration procedures.

    Other high-level science products that may be available are:
  • PSF photometry catalogues, results of artificial star test experiments, images of the completeness and photometric errors as a function of position in colour-magnitude diagrams
  • catalogues of known variables (i.e. RR Lyrae stars, Cepheids, late-type giants and eclipsing binaries)

  • Reduction
    The VMC data are reduced using the VISTA Data Flow System (VDFS), operated by the VDFS team, and augmented by individuals from the VMC team, especially for Product Definition and Quality Control. The VDFS is a collaboration between the UK Wide Field Astronomy Units at Edinburgh (WFAU) and Cambridge (CASU) coordinated by the (VISTA-PI) and funded by STFC.

    CASU: pipeline processing
    CASU is responsible for the VDFS pipeline processing component which has been designed for VISTA. It has been scientifically verified by processing wide-field mosaic imaging data from WFCAM@UKIRT and is now routinely used to process up to 250 GB/night of data. The pipeline is a modular design allowing straightforward addition or removal of processing stages and will have been tested on a range of input VISTA datasets. The standard processing is on a night-by-night basis with data products defined by the overall OB structure. Those important for the VMC survey are: - non-linearity, dark, flat, fringe, cross-talk and systemic noise correction; - sky subtraction (tracking and homogenisation during image stacking and mosaicing, the latter to remove unexpected 2D systematic effects from imperfect multi-sector operation of detectors; - assess and delay with image persistence from preceding exposures if necessary; - combination of dithered images and of tile pattern; - point source extraction; - astrometric and photometric calibration (the latter put in an internally uniform system as well as in an optimised system obtained by monitoring suitable pre-selected standard areas covering the specific VMC survey area); - shape and data quality information; - bad pixel handling, propagation of error arrays and effective exposure times by use of confidence maps; - realistic errors on selected derived parameters for images and catalogues; - nightly extinction measurements in relevant pass bands; - pipeline software version control. The processing history just described is recorded directly in FITS headers.

    WFAU: science archiving
    The Science Archive (SA) ingests the products of the pipeline processing into a database and then curates them to produce standardised data products. The most important processes for the VMC survey are: - individual passband frame association and source association to provide multi-colour, multi-epoch source lists; - global photometric calibration (using 2MASS); - cross-association with external catalogues (list driven matched photometry); - automatic stacking and source extracting for overlapping tiles in areas of reduced exposure (to be implemented); - deeper stacking in specified fields; - quality control procedures, as required by the public survey consortium, and supported by the archive team members. These features are available in the context of a continually updating survey dataset from which periodic releases are made. A point-and-click web form as well as full access to Structure Query Language constitute the dual (simple and sophisticated) end-user interfaces for the data. A generalised relational model for survey catalogue data has been developed in the VDFS. The key features to note are the normalised design with multi-wave band catalogue data that allow the user to track right back to the individual source images and merged-source tables that present the user with a generally applicable science-ready dataset. The SA has a high-speed query interface, links to analysis tools such as TopCat, and advanced new VO services such as MySpace. Data products are being successfully ingested into the WFCAM-SA in Edinburgh, with the first data release in July 2006. VMC is intrinsically a multi-wavelength project and most science will come from the linking of VISTA data with other survey data; the SA is designed to enable such links.

    The calibration, including sky subtraction, of VMC data is performed in a similar way as it is currently done for the UKIDSS survey. Both the photometric and astrometric calibration are linked to the homogeneous 2MASS PSC which has an astrometry as accurate as 0.1 arcsec and is consistent to a global photometry of 1% in any wave band.

    Photometry & Astrometry
    The VMC data are calibrated to magnitudes in the Vega system. Calibration on the sky is achieved using observations of 2MASS stars within each VMC field (there are plenty of unsaturated ones in every exposure of the Magellanic System), which does allow to deriving photometric calibration even during non-photometric conditions, including colour equations for transformation from the 2MASS system to the VISTA system. The latter also corrects for a small dependence on Galactic extinction (Warren et al. 2007). The procedure (Lawrence et al. 2006) is to cross-match objects detected by the pipeline with 2MASS unsaturated sources as accurate as ~0.1 mag in J and Ks, and to transform the photometry of these stars into the VISTA YJKs system using empirically derived colour terms. After correcting counts for the known radial variation in pixel scale, the average of these stars gives a global per-frame zero-point. Tests against observations of UKIRT faint standards indicate that this procedure gives a JKs photometric system accurate to 2%, this accuracy becomes 2-4% in Y. However, VDFS is developing a way to reduce the latter to also 2%; it also aims to achieve a goal of 1% in any wave band. These numbers refer to data obtained in one night and in one pawprint.
    In particular, the calibration of the Y filter obtained using 2MASS will be complemented with the observations performed regularly by ESO. The calibration plan lists 1 hour per night to be spent on the observation of standard star fields in any filter as well as a quarterly monitoring (over the first 2 years) around the primary standards to establish the photometric pedigree and accuracy of secondary standards with a goal of 0.005 mag rms.
    The use of 2MASS for the many non-variable objects detected by VMC assures a homogeneous calibration at each data release, which will include stack of observations at a given filter for a given tile as well as individual catalogues for each epoch. It is also envisaged to combine adjacent tiles at the moment of a given data release, the catalogue extracted from the resulting image will include the Y overlap of tiles down to the same depth as the centre and will be homogeneously calibrated using 2MASS as above. This task is being implemented by the VDFS team. Afterwards the VMC team will check and refine the calibration among tiles using tile overlaps as defined in the observing strategy.
    Similarly, the astrometry is as good as 0.1 arcsec.

    Sky subtraction
    Sky subtraction is performed using all observations obtained during the same night, in the same filter, building a series of sky subtraction frames as is done for UKIDSS. This procedure works well also for crowded fields provided that some observations of un-crowded fields are obtained during the same night.