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.
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
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: zpn_to_zpx.cl and zpx_to_zpn.cl. To invoke them under IRAF
simply type:
cl>cl < zpn_to_zpx.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:
cl>tables
ta>ttools
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.
GAIA
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. r168802.fit). 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.
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.
