Data Properties

Engineering Wircam data was received from CFHT as a FITS MEF file containing two extensions. This data was uncalibrated (no astrometric or photometric solutions were provided). However, the images were already preprocessed (comprising debiassing, flat-fielding, preliminar sky subtraction). Calibration images (flat-field, dark, bad-pixel mask) were also provided. Many bad pixels was present, hence masking had to be applied throughout the processing.

Here is an example of an input J-band image image.

Astrometric calibration

After adding basic astrometric keywords, astrometric calibration was performed with SCAMP using 2MASS as reference catalog, in a two-step process. This two-step process was necessary because intital astrometric solution was far from the real one. In the first step, we set SCAMP's MOSAIC_TYPE parameter to 'LOOSE' to determine the relative position of each detector in the array. This information is encoded in '.head' files which are then used in the second step, where the MOSAIC_TYPE parameter is set to 'FIX_FOCALPLANE' to determine a global astrometric solution.

Photometric calibration

Approximate photometric zero-points were added to each extension. Photometric calibration was then computed using 2MASS images which overlapped the field. As they are already calibrated, 2MASS images were labelled as photometric fields. Unfortunately, these engineering detectors has very different zero-points and for this reason the images were split and photometric solution was performed separately for each extension (the current version of SCAMP assumes that the difference in zero-points between detectors in a given focal plane is fixed).

Double pass processing

Final images were obtained using a two pass sky-subtraction process. A first stack was produced: an 'OBJECT CHECKIMAGE' is derived from it, and then projected over each input image, in order to mask bright objects in the sextractor background computation.

Sky-subtracted images are then produced and stacked.

Note that BILINEAR RESAMPLING_TYPE parameter had been used to avoid propagation of bad pixels.

Final Product

Final catalogues were produced with a modified version of qualityFITS: they were produced directly from qualityFITS together with quality assessment pages (see for example 9b9aa_J.fits, for J-band image, and 9b9aa_Ks.fits, for Ks-band image). Flags for masked zones are included in the catalogs.

The 'Release' directory contains images, catalogs, and metadata we send to the P.I., in particular

• qualityFITSin
  • band: quality assessment for input images; summaries are presented in the .html files image_name/qualityFITS/index.html
• chi2image: coadded chi2 image using all band images, its weight image
• FinalImages
  • band: median coadded image, its weight image, .ldac catalog made on chi2 image detections,
    • headers: astrometry and photometry information used for coadding,
    • image_name: quality assessment for final images and catalogs (summary in qualityFITS/index.html).
• ASCII_catalog: ASCII version of the .ldac catalogs
• plots
  • astrometry : about the astrometric solution
  • photometry : internal photometric errors
  • gal_counts : preliminar star-galaxy separation, galaxy and star counts based on that.