The material comes from the set of 102 compressed CD-ROM's of
the Digitized Sky Survey produced at the Space Telescope Science Institute.
Each CD-ROM contains the digitization files of several plates. The Northern
hemisphere (
)
contains 644 plates, noted xe001 to xe643 and
one additional (xe1001).
The Southern hemisphere contains 894 plates noted s001 to s894.
Each plate is constituted of 784 (
)
individual scans of
pixels
. Each individual scan is labelled
with a 2-symbol extension (from 0 to r), e.g., s828.00 to s828.rr. The scan *.11 is the most South-Eastern
one, while the scan *.rr is the North-West one. We skipped all scans labelled with 1 or r (i.e., *.1x,
*.x1, *.rx, *.xr) in order to reject the extreme edge of each plate. Thus, we processed about one million
elementary scans. The size of a pixel is 1.70
.
Each elementary scan is about
.
The Northern part is digitized from E(red) plates, while the Southern
part comes from IIIa-J plates.
An example of a typical elementary scan is given in Fig. 1.
The source extraction is made from uncompressed scans in the same way as
in our previous papers
(Paturel et al. 1996;
Vauglin et al. 1999).
The sky background is assumed to be homogeneous over an individual scan. Its
mean intensity
is calculated from the maximum of the histogram of pixel
intensities. The standard deviation
is calculated by symmetrizing
the low intensity side of the histogram.
The threshold for source extraction is chosen as
.
Only the sources having more than 36 pixels are kept.
This means that the smallest objects have a size of
pixels, i.e.,
.
Further we impose that the number of pixels on one side of
the matrix of pixels is larger than or equal to three. If we note npx, the number
of pixels per line and nli the number of lines, this means that
,
and
npx.nli > 36.
For each object we calculate the
mean position of the matrix.
The mean is obtained by weighting each pixel with its intensity.
The J2000 equatorial coordinates are then calculated using the plate solution
calculated at the Space Telescope Science Institute. The coefficients of the 13-th order
polynomial solution are read in the header file associated which each plate.
The internal accuracy is about 3
in right ascension
and declination, near the equatorial plane.
Near the Northern pole the accuracy is about 4
.
Near the Southern pole it is about 5
.
This is in agreement with the results found by a recent study of coordinate accuracy
(Paturel et al. 1999;
Paturel & Petit 1999). Most of the uncertainty comes from the
positioning of the galaxy center.
A treatment is then applied on matrices corresponding to overlapping objects in order to separate them into their different components. The basic assumption in this treatment is that astronomical objects have a central symmetry.
First of all, we determined the number of maxima in the pixel matrix. This is done as follows: when a maximum is found, the region around its position is inhibited and the following maxima are looked for, outside this region. The inhibited region around each maximum is actually the ellipse centered on the considered maximum and tangent to the nearest edges of the matrix.
The decomposition of a matrix in several matrices is then made in the following
way: all pixels of the original matrix are considered one after the other.
For a given pixel P(i,j) we are searching for its symmetrical counterparts
with respect to the n maxima
.
If the symmetric counterpart of a given pixel P(i,j), calculated with respect to
the k-th maximum, is outside the matrix, or if it
has an intensity below the sky background, the given pixel P(i,j) is not attributed
to the object re-constructed around this k-th maximum. If the pixel P(i,j)belongs to several re-constructed objects, the pixel intensity is simply
shared with equal weight between each object. No attempt has been made to share
this intensity in a more refined way because the pixel intensities are
not additive.
The objects which result from this decomposition always have a central symmetry
in accordance with our basic assumption.
In the final catalogue a flag will remind us
a given object results from such a decomposition process.
The matrices which are truncated by the edge of the scan are also extrapolated
by symmetry if the maximum itself is not on the edge.
Copyright The European Southern Observatory (ESO)