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ELAIS Data Release Cookbook


Extracting information from the catalogues

The main catalogues are the band merged catalogues, released under the name wfsj(field)_(no).mer. Every file containts four binary table extensions corresponding to each of the four WFC detectors. It has to noted that RA and Dec coordinates are expressed in radians.

Each merge file contains four extensions corresponding to each of the four WFC detectors.

The fluxes are aperture 'sky corrected' and are given in counts. In order to transform them to magnitudes you need to make use of the table headers. For the reference filter (r band):

EXPREF  =          5.99450E+02 / Exposure time
MAGZPTR =                24.40 / Photometric ZP (mags) for default extinction
MAGZRRR =                 0.01 / Photometric ZP error (mags)
EXTINCR =                 0.09 / Extinction for observations
AIRMASR =                 1.24 / Airmass of observation

and for each of the comparison bands:

EXPCOM1 =          5.99500E+02 / Exposure time
MAGZPTC1=                24.89 / Photometric ZP (mags) for default extinction
MAGZRRC1=                 0.02 / Photometric ZP error (mags)
EXTINCC1=                 0.19 / Extinction for observations
AIRMASC1=                 1.30 / Airmass of observation

Merge catalogue entries for the reference file, showing flux, flux error, classification, pointer and coordinates.

Merge catalogue entries for one of the comparison files, showing flux, flux error, classification, pointer and coordinates.

The magnitude is calculated as

      mag = zero - (airmass - 1.0) * extinc - 2.5 * log10 ( coreflux / exptime )

where mag is the magnitude, zero the zero point, airmass the airmass, extinc the extinction, coreflux is the core flux and exptime is the exposure time.

Likewise, the magnitude error can be calculated fom the flux error as:

      emag = 2.5 * log10(e) * coreflux_err / coreflux
      emag = sqrt ( emag * emag + zero_err * zero_err )

If more information is needed (e.g., fluxes in other apertures, isophotal fluxes, FWHM, ellipticity) then you need to read those from the single band catalogues:

REFFILE = 'r189908_cat.fits'   / Reference_file
COMFILE1= 'r189916_cat.fits'   / Comparison_file1
COMFILE2= 'r189912_cat.fits'   / Comparison_file2
COMFILE3= 'r185929_cat.fits'   / Comparison_file3
COMFILE4= 'r194670_cat.fits'   / Comparison_file4
FILTREF = 'r       '           / Camera filter
FILTCOM1= 'g       '           / Camera filter
FILTCOM2= 'i       '           / Camera filter
FILTCOM3= 'z       '           / Camera filter
FILTCOM4= 'U       '           / Camera filter

using the pointer number given in the merge catalogue. For instance the source number 2 in the merge catalogue, has a ref_pointer equal to 630, so this source is in file r189908_cat.fits (filter r) row number 630. It also as a com1_pointer equal to 523, so it is in the file r189916_cat.fits (filter g') row number 523.

The rules given above to obtain magnitudes from the fluxes in the single band catalogues apply, except that in these files the fluxes are not aperture corrected, so the corresponding corrections (apcor and precor) need to be applied (see here).


Using the catalogues and images under IRAF

World Coordinate System

The current version of IRAF (V2.12) does not recognize the ZPN projection of the header of the images. To overcome this IRAF limitation you will need to edit the header of the images as follows:

cl>hedit r99 'CTYPE1' 'RA---ZPX'
cl>hedit r99 'CTYPE2' 'DEC--ZPX'
cl>hedit r99 'WAT1_001' 'wtype=zpx axtype=ra projp1=1.0 projp3=220.0' add+
cl>hedit r99 'WAT2_001' 'wtype=zpx axtype=dec projp1=1.0 projp3=220.0' add+

i.e. change from ZPN to ZPX and add the WAT1_001 and WAT2_001 keywords. This operation can be simply reversed to get back to the ZPN projection replacing ZPX with ZPN. Scripts to perform these operations for all files *.fit in a directory can be downloaded here: and To invoke them under IRAF simply type:

cl>cl <

Image Display and Overplotting Sources

Assuming you have started up your preferred image display tool (e.g. Ximtool, SAOimage, SAOtng), you will need to make sure the display can handle the full image:

cl>set stdimage=imt4096
cl>display r999999.fits[1] 1

To overplot sources from the catalogue using the pixel coordinates, you can use the tables package to extract x, y values into an ascii file:

tt>tdump r999999_cat.fits[1]  col="X_coordinate,Y_coordinate" data=r999999_1.xy

This can then be passed into tvmark:

cl>tvmark frame=1 coords=r999999_1.xy mark=circle radii=4 color=204 intera-

will plot red circles around all sources in the catalogue, while

cl>tvmark frame=1 coords=r999999_1.xy mark=none color=205 intera- num+

will number all sources in green.

You can use tdump to create ascii versions of the catalogues, e.g.

cl> tlcol r999999_cat.fits[1] > columns
cl> emacs columns # to edit out columns you don't want
cl> tdump r999999_cat.fits[1] datafile=r999999_1.txt columns="@columns" pwidth=132

Note that RA and DEC are in radians.


Image Display and Overplotting Sources

You will need GAIA version equal or greater than 2.3-2, which must be downloaded from the Starlink web site if you do not already have it installed locally.

Startup GAIA, and select the image to open from the File menu (e.g. Select the extension you want to display.

Select your preferred colour table and scaling from the Color Map andAuto Cut menus.

Note that GAIA should have picked up the World Coordinate System (WCS) from the image headers. IF the alpha and delta boxes are empty, then you may have the older version of GAIA.

Now select Data-Servers, Local Catalogs, Load from file and enter the filename (e.g. r168802_cat.fits).

It will take a little while to plot the sources and return control to you. You will notice that only 1000 objects are plotted. You can increase this in the Max Objects box to e.g. 4000 and hit Search followed by Plot when it's found all the catalogue entries.

A nice feature of GAIA is that it plots ellipses based on the catalogue source parametrisation. The default setup plots galaxies (class=1) in cyan, cosmic rays (class=0) in green and stars (class=-1) in red.

You can also add classification information via the Options, Plot Symbols window. Select a different colour, e.g. yellow, and change the classification parameter to be $classification>1. Hit Add symbol followed by Apply and objects classified as non-stellar will be plotted in yellow. Note you may have to delete the file ~/.skycat/skycat.cfg when you wish to make changes.