Although all the WFS data products are accesible using the DQC interface we make here available pre-packaged products containing images and catalogues (single band as well as band merged catalogues) for those areas of the sky with significant coverage. We provide here the optical products for the following regions:
A single merged catalogue in both FITS and ASCII formats with double detections removed is also provided for each field.
The directory data release is HERE. The data products are described below.
This page contains the released optical images and catalogues from the Wide Field Survey observations in the ELAIS N1 and N2 regions. Each of these two regions are made up from 54 pointings covering 9 square degrees. We describe here these data and in a separate cookbook we give tips on how to obtain the information from the catalogues and images.
All the data for these regions pass our data quality criteria, i.e., they have a seeing better than 5 pixels (1.7 arcsec) and ellipticity better than 0.2. Most of the images have excellent data quality, with seeing around 1 arcsecond. The summary file (see below) lists the images and provides measurements of seeing, sky background and variance, photometric zeropoints (in the Vega system) and exposure times. This photometric calibration data is now in the image headers themselves.
Coverage of the WFS in the ELAIS N1 region. Pointings are represented as numbered brown rectangles. For simplicity only the coverage of chips #1, #2 and #4 is shown. The blue rectangles show the location of the ELAIS N1 ISO survey (small) and the SWIRE N1 survey (large).
There is a summary giving a detailed description of the images and several plots.
The catalogues are in the form of FITS binary tables with a primary HDU containing a dummy 1,1 pixel map plus a copy of the 2d image FITS header including WCS - this slight-of-hand appears to be necessary to fool IRAF into reading the tables correctly. The catalogue is in a FITS extension which contains additional header information plus a copy of the 2d image FITS header.
Note that for simplicity in writing code to access these files, all the numbers in the columns are deliberately stored as if real*4 even though some of them would more naturally be integers.
Example of extra FITS header items in the binary catalogue extension relating to catalogue production and general image properties
SKYLEVEL= ' 91.8' / Median sky brightness (counts/pixel)
An automatic 2D background-following algorithm is used to track and "remove" slowly varying background features such as image gradients etc.. The default scale size for background tracking is currently set to 128 pixels, equivalent to 43 arcsec.
SKYNOISE= ' 6.0' / Pixel noise at sky level (counts)
Robust MAD estimator for noise scaled to equivalent Gaussian rms value ie. = MAD x 1.48 after removing large scale sky background variations. MAD = Median of the Absolute Deviations about the median
THRESHOL= ' 9.0' / Isophotal analysis threshold (counts)
The default is to set this to 1.5 skynoise as a compromise between detecting close to the limit of the data and not being swamped by spurious sources. It is possible to push the data limit fainter but at the expense of a large increase in spurious sources. LSBG detection requires a more subtle add-on to the catalogue and could be added given suitable demand.
MINPIX = ' 5' / Minimum size for images (pixels)
In conjunction with the threshold above this determines how deep and how small "real" images can be. This default precludes many of the few pixel-hit cosmic rays from being considered since "real" images must have 5 contiguous simply-connected pixels in the union of the detection filter and data domains. For more details on image detection and parametersiation see http://www.ast. cam.ac.uk/~wfcsur/pubss.html - "Detectors and Data Analysis Techniques for Wide Field Optical Imaging".
CROWDED = ' 1' / Crowded field analysis flag (0 none, 1 active)
Detection algorithm tried to disentangle overlapping images or images supperposed on the "slowly" varying background of other large images (default) otherwise just straighforward isophotal detection.
RCORE = ' 3.50' / Core radius for default profile fit (pixels)
Aperture flux designed to match median seeing of survey data. It is straighforward to show that if rcore = FWHM then for typical profiles encountered the rcore flux estimate has between 80-90% of the accuracy of an idealised perfectly known PSF model method.
SEEING = ' 2.95' / Average FWHM (pixels)
An average realistic FWHM estimated directly from the stellar images on the frame. Divide by 3 to convert to arcsec.
ELLIPTIC= ' 0.04' / Average stellar ellipticity (1-b/a)
A direct estimate of the average stellar ellipticity, useful for spotting trailed frames usw.. Should not average much above 0.15 for "normal" frames, though note that images on CCD#3 always seems slightly more elongated than for the other CCDs.
CLASSIFD= T / Class flag: -1 stellar, 1 non-stellar, 0 noise
Has image morphological classifier been run ? A more detailed stellarness index is also included in the binary tables.
SATURATE= ' 41593.8' / Average saturation level in frame
An estimate directly from saturated images on the frame at what level image saturation occurs, including sky. This varies from CCD to CCD depending on the relative gains applied to bring them to a uniform flatfield reponse etc..
APCORPK = ' 2.812' / Stellar aperture correction - peak height APCOR1 = ' 0.635' / Stellar aperture correction - core1 flux APCOR = ' 0.114' / Stellar aperture correction - core flux APCOR2 = ' 0.028' / Stellar aperture correction - core2 flux APCOR3 = ' 0.000' / Stellar aperture correction - core3 flux APCOR4 = ' 0.000' / Stellar aperture correction - core4 flux
Aperture corrections in magnitudes needed to correct the assorted aperture-like measures produced in the catalogues onto the equivalent of a total flux stellar system. These constitute the components of a curve-of-growth analysis contained within the catalogues with pixel "radii": peak pixel, 1/2 x rcore, rcore, sqrt(2) x rcore, 2 x rcore, 2 sqrt(2) x rcore, to be used in the sense that corrected_photometry = 2.5*log10(flux) + apcor.
They also work well as a first order seeing correction for faint galaxies. The remaing information necessary for photometric calibration is included in the following keywords.
PERCORR = -0.006 / Sky calibration correction (mags)
this is a correction based on the median dark sky recorded in science frames compared to the median for all the CCDs and as such is an ancillary correction to the gain correction derived from the flatfield (usually twilight flats) data. This correction is to be used in the same sense as before in that
corrected_photometry = 2.5*log10(flux) + apcor + percorr
MAGZPT = 22.10 / Photometric ZP (mags) for default extinction
Nightly-derived CCD zero-point in the sense of what magnitude object gives a total (corrected) flux of 1 count/s. These ZPs are appropriate for generating magnitudes in the natural CCD+filter system based on Vega, see http://www.ast. cam.ac.uk/~wfcsur/photom.html for more details on colour equations etc.. The nightly ZPs have been derived from a robust average of all photometric standards observed on any particular night, corrected for airmass but assuming the default extinction values listed at the preceding web address. For other airmass or other values of the extinction use
ZP --> ZP - [sec(z)-1]*extinct + extinct_default-extinct
You can then make use of any of the assorted flux estimators to produce magnitudes via
Mag = ZP - 2.5*log10(flux/exptime) - apcor - percorr
Note that for the so-called total and isophotal flux options it is not possible to have a single-valued aperture correction
MAGZRR = 0.15 / Photometric ZP error (mags)
error in the zero point. If good photometric night this error will be at the level of a few percent. Values of 0.05 and above indicate correspondingly non-photometric night and worse.
To compute errors in the fluxes use the following:
error**2 = flux/gain + npixels*skynoise**2
where npixels is either the effective area ie. pi*rcore**2 for the "core" measures or the no. of pixels above the detection isophote ie. areal_profile1; gain is the final overall CCD system gain (around 3.0 but see http://www.ast.cam.ac.uk/~wfcsur/ccd.html for more accurate values); flux is whichever measure you are using but note that for "total" this formula is not accurate since "total" fluxes are derived using a rather convoluted curve-of-growth technique; and skynoise can be obtained from the catalogue fits header.
The catalogues are in the form of FITS binary tables with a standard empty primary HDU. The extension contains the merged file information.
Note that for simplicity in writing code to access these files, all the numbers in the columns are as for the catalogue files deliberately stored as real*4 even though some of them would more naturally be integers.
Each CCD has a separate merge file with sufficient header information to identify object location, which CCD, original WCS catalogue data and so on.
For each file merged, including the reference, there are 6 column entries plus two additional columns explicitly listing RA and Dec in radians. Hence the no. of matched files can be deduced from (#TFIELDS-2)/6, or by interpreting the FITS header directly.
The reference file, against which the comparison files are matched, has: standard coordinates in arcsecs with respect to TPA and TPD (see header) from which celestial coordinates can be directly computed, or you can use the RA and Dec columns at slightly reduced precision directly - the last 2 coloumns in the file; further entries are an aperture and sky-corrected 'core' flux, a flux error, the object morphological classification, and a pointer to the original catalogue row no.
Comparison files have: differential standard coordinates with respect to reference file in arcsec, an aperture and sky-corrected 'core' flux, the flux error, the object morphological classification, and a pointer to the original catalogue row number.
Finally as mentioned earlier, the last two columns of the file are RA and Dec in radians, equinox J2000, at whatever epoch is in the appropriate observation date keyword.
The aperture and sky-corrected flux values referred to are the original catalogue recommended core flux entry corrected for aperture loss for stellar images. This seems to give a sensible galaxy aperture flux too. The fluxes are also normalised by a 'dark sky' correction factor to compensate for the slight QE colour differences between the CCDs. This manifests itself as a slight imbalance (~1%) between twilight flatfield gain correction factors and the 'dark sky' derived gain factors - particularly for the i, I and z passbands. 'Dark sky' is a better match to the majority of astronomical objects.
All images, whether matching or not, are included in the merged catalogue file.
The WCS information enables pixel coordinates from the original file to be directly computed and the various flux and filter information in conjunction with the zero-points and extinction values enable magnitudes etc... to be derived.