where a, b, c, d, e, f, <logr25>, <t>, <logd25> and
<de1950> are constant values given in the table here below for
each reference taken into account. The mean bt magnitude is calculated
as a weighted mean where the weight (last column) is derived from each
source of magnitude as the inverse square of the mean standard
deviation. The final actual uncertainty sbt is derived from the total
weight.
For MIGALE data extracted by Paturel et al (2000) from the DSS (a catalogue of 2,772,061 galaxy
candidates), the magnitudes are computed as follows.
Objects are extracted considering only pixel intensities above (Ibg+
3.sigmabg), where Ibg is the background mean
intensity estimated from the maximum of the histogram of pixel
intensities and the standard deviation sigmabg is calculated
by symmetrizing the low intensity side of the histogram.
An appropriate treatment was then use to clean the extracted matrixes
and separate galaxies and stars. The final apparent magnitudes are
deduced from the sum of all pixel intensities of a given source:
mDSS = -2.5.log{Sum[I(i,j)-Ibg]} +cste
The magnitudes are corrected for the mean atmospheric extinction
(assuming that the plate is taken at the meridian) and for the distance
to the center of the plate. The final bt is then given by: