| Stacked Images | Median (or other SWarp COMBINE_TYPE if requested) of all available images, weighted accordingly. Projection TAN (other projections on request). Astrometric reference catalogue used 2MASS (or others if requested). Equatorial Coordinate system, J2000 (ICRS, tight to the 2MASS astrometric system). Pixel-scale 0.3''/pxls (different pixel-scale on request). | ||||||
| Mag. ZP | CFHT provides for each image and extension the photometric zero-point (in ADU per second at airmass 1) with respect to 2MASS Vega system. Overlapping sources on all exposures in a given field for a given filter are compared, and the images are then scaled to the defined zero-point of 30. This step is done by SCAMP.
The magnitudes of objects in the final stacks is computed as follows: m=30-2.5log(counts). The magnitude system is instrumental Vega because of the calibration on 2MASS. | ||||||
| Effective FOV | Effective area is derived by subtracting from the field coverage all the flagged areas defined using the flag image (see below). | ||||||
| Weight map images | FITS weight map image. There is one weight map image for each FITS image. Input weight maps are generated during the QualityFITS-in process (by WeightWatcher) and output weight maps are produced during the image stacking using SWarp. Weights are proportional to the inverse of the local sky background flux variance. The input weight-maps are generated using flat field and bad pixel mask FITS images delivered by CFHT. When used during co-addition, the weighting scheme takes into account the image-to-image flux rescaling, as described in the SWarp User's manual. | ||||||
| Flag map images | FITS flag map image. Produced during the QualityFITS-out process (by WeightWatcher), they are taken into account in the production of catalogs. Objects detected by SExtractor are flagged (if IMAFLAG_ISO is selected in the .param file) if at least one pixel in the object is flagged in the flag map.
The bits used are the following :
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| Galaxy counts | Galaxy number counts per 0.5 mag, per deg2 (in logarithmic scale), computed for the effective area of each field, i.e., after the selection of not flagged objects, and derived from the catalogue produced for quality assessment. Magnitudes are SExtractor MAG_AUTO. No extinction correction has been applied. The magnitude system is instrumental Vega ; there is one plot per filter. | ||||||
| Astrometric quality assessment plots | SCAMP takes as input SExtractor catalogues and produces as output WCS images headers (.head files) that can be read by SWarp to re-scale, re-sample and co-add images. The first task for SCAMP is to examine the image headers and group exposures by sky area, "astrometric context" and "photometric context". An "astrometric context" defines a set of exposures that have been observed with a common, mechanically stable configuration. For the WIRCam CFHT data, these are exposures which share the same filter and QRunID. There is one astrometric solution per astrometric context. Likewise, a "photometric context" characterises a set of exposures with the same spectral response, that is, with the same filter. A subset of the reference astrometric catalogue (2MASS) surrounding each identified sky area is automatically downloaded from CDS. Cross-correlation is used to match each exposure with respect to the reference catalogue: pixel scales, frame position angles, and shifts in RA-DEC are determined within around 1 arc-second. Pattern matching is followed by a cross-identification with a search radius of 2" between all overlapping detections. SCAMP computes a polynomial model of WIRCam image distortions (a third or fourth degree is used) by minimising a weighted, quadratic sum of differences in position between both overlapping detections and matched sources from the 2MASS astrometric reference catalogue. Since the catalogue has a typical accuracy of 0.2 arc-second, which is insufficient, combining the constraints given by the relative positions of overlapping detections and those from the reference catalogue provides both a very accurate adjustment of exposures on small scales, and a correct positionning on the sky on larger scales. Residual position errors provide an approximate quality of the final astrometric solution. | ||||||
| Mask files | Built using the "polygon" option of the DS9 visualisation tool to create a '.reg' file which describes, in image co-ordinates, the regions of the images which should not be used for scientific analysis. They mask the field boundaries, saturated stars, satellite tracks and image defects, if any. It is an ASCII file: polygons may be added or removed according to user wishes. It is important that users check the masks and modify them according to their scientific needs. | ||||||
| Catalogues | These catalogues ('.cat' files for ASCII format, '.ldac' files for FITS format) are extracted on the stacked image using weight and flag map for each filter. Note that these catalogues have not been corrected for galactic extinction. Columns of catalogues are defined in the '.param' file. | ||||||
| Qfits-out | QualityFITS quality assessments information on final stacked image. An html summary contains PNG images of the stack and their attached weight and background maps, a PSF map image of the field (10' resolution), a preliminary estimate of galaxy and stellar number counts (rough galaxy-star separation, no mask used to avoid defects and very noisy parts of the detectors in order to use the effective field area), with the selected objects shown on rh-mag diagrams. A summary FITS table ('_diag.fits') provides information on stacked images. | ||||||
| Configuration files | The configuration files used for SCAMP, SWarp and SExtractor. |
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