D1 stacked images and catalogues

This data has been superceded.

Go to the CADC for the most recent version.

If you use this data in a publication, please include the following acknowledgment:
"Based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institut National des Science de l'Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. This work is based in part on data products produced at the Canadian Astronomy Data Centre as part of the Canada-France-Hawaii Telescope Legacy Survey, a collaborative project of NRC and CNRS."

Summary:
Recently released Elixir-processed data for the D1 field have been stacked. ugriz images and catalogues are presented.

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Reduction procedure

Quality Control: The data were retreived from the CADC archive. The images come with a fairly accurate (0.5 arcsecond) astrometric solution and a photometric calibration. CCD01 of each exposure is inspected visually. Exposures with obviously assymetric PSF's (due to loss of tracking) or other major defects are discarded. For many exposures CCD03 was completely dead: All the images were examined quickly and the dead images discarded. No other images were examined unless a problem was found during the reduction process. A list of bad images can be found here.

The total number of usuable exposures and total exposure times is given in the following table:

BandNumber of
exposures
Total Exposure
Time (secs)
U 1610560
G 41 8280
R 6423040
I 9649920
Z 3010800

Astrometric Calibration: The AstroGwyn astrometric calibration pipeline was then run on the images. The first step is to run SExtractor on each image. The parameters are set so as to extract only the most reliable objects (5 sigma detections in at least 5 pixels).

For the first band to be reduced (I) these image catalogs were matched with the an external astrometric catalog to provide an initial astrometric solution. This external catalog was generated from the POSS II images. An image covering the D1 field was retreived from the CADC. The USNO A2 was used to calibrate this image. SExtractor was then used to extract a catalog of sources that can be used to calibrate the CFHTLS images. One advantage of this method is that it yields about 1.5 to 2 times more sources per CCD. The other is that the POSS II images were taken more recently, so that proper motion of the stars. The POSS II catalog is given here.

Elixir provides an accurate 1st order solution. AstroGwyn improves on this to provide an 2nd order solution with typical residuals of 0.45 arcseconds with respect to the POSS II. In order to register the images a internal astrometric catalog was generated by cross-referencing the catalogs for all the individual images, identifying objects common to multiple images, and averaging the positions.

For the other bands, the image catalogs are first matched to the POSS II to provide a rough WCS and then matched to a catalog generated using the first image so as to precisely register the different bands. The final astrometric calibration has an internal uncertainty of about 0.05 arcseconds and an external uncertainty of 0.3 arcseconds.

Photometric Calibration: The PhotGwyn photometric calibration system takes the object catalog from each image and cross-references them. For every pair of images, the difference in measured magnitude for objects common to both images is computed. The median difference of all these magnitude differences gives photometric offset between the two images. The Elixir headers give photometric calibrations based on standard star observations during the observing run. These calibrations are valid only under good observing conditions of course. PhotGwyn plots the difference between the Elixir zero-point and the zero-point derived from the offsets. When this difference is large, this indicates the night the data was taken was not photometric. Systematic changes over the night indicate a higher than average extinction coefficient. PhotGwyn makes it possible to identify images taken under photometric conditions and propagate the photometric zero-point to images taken under non-photometric conditions.

Coaddition: The calibrated images were coadded using the SWarp written by Emmanual Bertin of Terapix. The SWarp configuration file is given here The SWarped images are trimmed down to the original image size of 2048x4612 pixels.



Images

The images are corrected for distortion and have accurate astrometric calibrations. The WCS in the headers is good to 0.2 arcseconds RMS. The images are "flat" (no geometric distortion), and are in the usual tangential projection (TAN, not TNX) The plate scale is 0.186 arcseconds per pixel.

The images have been trimmed to the size of the original CCDs: 2048x4612. This has been centred on the area of maximum I eposure.

The photometric zero-point for each image is 30.000 in the AB system. That is to say, for each image:
  magAB=-2.5 * log10(flux in DU) + 30
No correction has been made for galactic extinction.

Although the files are all labelled with capital letters denoting the filters (UGRIZ) the passbands are the MegaCam u*g'r'i'z' system described here

The images are password protected. They are primarily for use by my collaborators. Other members of the CFHTLS community are welcome to use them as well. However, publications based on these data should include me on the author list. Also, the CADC should be acknowledged in the usual way:

Guest User, Canadian Astronomy Data Centre, which is operated by the Dominion Astrophysical Observatory for the National Research Council of Canada's Herzberg Institute of Astrophysics.

The images and the weight maps are available as multi-extension FITS files, as well as individual images. The MEF versions are given here: Each image is about 1.3 Gigabytes in size. Before you download them, condsider if downloading a single CCD's worth of data from the table a little further down thispage will fill your needs.

To examine the multi-extension FITS images, split them using the CFITSIO version of imcopy or display them with
ds9 -mosaicimage D1.I.fits
or
ds9 -mosaicimage wcs D1.I.fits
depending on your version of ds9

The data for each chip is available as a separate image from the table below; The images are sorted by CCD and filter. Click on a filter letter to retrieve the image. The image names have the following format:

D1.00.I.fits
^  ^  ^
|  |  |__ filter
|  |_____ CCD
|________ Field.

00
images
UGRIZ
weights
UGRIZ
01
images
UGRIZ
weights
UGRIZ
02
images
UGRIZ
weights
UGRIZ
03
images
UGRIZ
weights
UGRIZ
04
images
UGRIZ
weights
UGRIZ
05
images
UGRIZ
weights
UGRIZ
06
images
UGRIZ
weights
UGRIZ
07
images
UGRIZ
weights
UGRIZ
08
images
UGRIZ
weights
UGRIZ
09
images
UGRIZ
weights
UGRIZ
10
images
UGRIZ
weights
UGRIZ
11
images
UGRIZ
weights
UGRIZ
12
images
UGRIZ
weights
UGRIZ
13
images
UGRIZ
weights
UGRIZ
14
images
UGRIZ
weights
UGRIZ
15
images
UGRIZ
weights
UGRIZ
16
images
UGRIZ
weights
UGRIZ
17
images
UGRIZ
weights
UGRIZ
18
images
UGRIZ
weights
UGRIZ
19
images
UGRIZ
weights
UGRIZ
20
images
UGRIZ
weights
UGRIZ
21
images
UGRIZ
weights
UGRIZ
22
images
UGRIZ
weights
UGRIZ
23
images
UGRIZ
weights
UGRIZ
24
images
UGRIZ
weights
UGRIZ
25
images
UGRIZ
weights
UGRIZ
26
images
UGRIZ
weights
UGRIZ
27
images
UGRIZ
weights
UGRIZ
28
images
UGRIZ
weights
UGRIZ
29
images
UGRIZ
weights
UGRIZ
30
images
UGRIZ
weights
UGRIZ
31
images
UGRIZ
weights
UGRIZ
32
images
UGRIZ
weights
UGRIZ
33
images
UGRIZ
weights
UGRIZ
34
images
UGRIZ
weights
UGRIZ
35
images
UGRIZ
weights
UGRIZ


Catalogues

Photometric catalogue SExtractor's double image mode was used to get photometry in all 5 bands, using the I band as a reference. The SExtractor parameter file is give here The resulting catalogue is given below: The magnitudes are all in AB magnitudes, measured in Kron-style apertures (SExtractor's MAG_AUTO). A magnitude of 99 indicates no measurable flux. The column HLR is the half light radius (in arcseconds) of the object. A value of 99 means that the half light radius could not be measured. The column CCD is the MegaCam CCD in which the object is located. The values of the flag column are discused here.

The SExtractor segmentation images corresponding to this catalog are given here as a compressed multi-extension FITS file:

Photometric redshift catalogue
A photometric redshift catalogue has also been prepared using the gwynz photometric redshift code.

For photometric redshifts it is important to get accurate colours for the galaxy in question, rather than good total magnitudes. Therefore, magnitudes were measured in each band through matched apertures with a radius of 1 arcsecond. Pixels belonging to neighbouring galaxies are excluded from this aperture. These aperture magnitudes are given in the catalogue. The column HLR is the half light radius (in arcseconds) of the object. The column CCD is the MegaCam CCD in which the object is located. The values of the flag column are discused here. The photometric redshift is given in the column labeled z_phot. The galaxy "spectral type" is given in the final column (labeled type). This number runs from 0.00 to 1.00 with 0.00 have the typical spectral energy distribution of an E/S0 galaxy and 1.00 having the SED of an starburst galaxy. More information on the spectral type can be found at here

The quality of the photometric redshift depends on the number of photometric bands used to compute it. With all five filters, the accuaracy is pretty good, about +/- 10%. With four bands the accuaracy is less, but still acceptable. With only three bands the photometric redshifts are marginal. It is impossible to assign a phototmetric redshift with two bands or less. In this case, the photometric redshift has been set arbitrarily to -1.


Absolute magnitude catalogue
Absolute magntiudes have been calculated for every object using photometric redshifts.

The distance modulus was calculated assuming a cosmology where H0=70 km/sec/Mpc, Omega=0.3, and Lambda=0.7. The galaxy spectral type determined in the photometric redshift procedure was used to compute k-corrections. Absolute magnitudes are given in the MegaCam ugriz system and the Johnson-Cousins UBVRI system.
The HLR and CCD columns as the same as above. The type_phot column refers to the name of the best fitting spectral template. ASCII files with the spectrum of each template are given here. The a_phot column is the normalization factor required to make the spectral template match the observations. For example, if type_phot=baye34 and a_phot=2.8447E-24, multiplying the flux column of the file baye34.spec by 2.8447E-24 gives the spectrum (in W/m2/Angstrom) corresponding the object in question.
If the photometric redshift could not be properly determined, all the absolute magnitudes are set to 0.



Caveats

CCD03 was not working when some of the images were taken. The depth in CCD03 is the same for the u-band, but only about half the depth in the other 4 bands.

There was no photometric keyword information in the headers of the u-band data. The u-band data was calibrated using the average value of the zero-point and the usual extinction coefficient. Based on the behaviour of the u-band zero-points for the other Deep fields and on the stellar locus on the colour-colour diagrams, the calibration should be adequate.

Some the data was taken during the 2003B semester. However, at this point bad pixel masks are only available from CADC from 2003A. These masks were used for the 2003B data. This should not be a concern because the bad pixel locations are mostly stable. Further, since the final images are a median of a fairly large number of images, any bad values in the individual images are unlikely to have much effect on the final images.

Back to Stephen Gwyn's CFHTLS pages
Send comments/suggestions/problems to

gwyn@uvastro.phys.uvic.ca