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HyperLEDA documentation: parameter: bt

Apparent total B-magnitude: bt

bt is the apparent total B magnitude reduced to the RC3 system.

The reduction of a given magnitude m to bt is given by:

bt = a.m + b + c.(logr25 - <logr25>)
+ d.(t - <t>)
+ e.(logd25 - <logd25>)
+ f.(de1950 - <de1950>)

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 (I_bg+ 3.sigma_bg), where I_bg is the background mean intensity estimated from the maximum of the histogram of pixel intensities and the standard deviation sigma_bg 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:

m_DSS = -2.5.log{Sum[I(i,j)-I_bg]} +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:

bt = m_DSS,cor +30.6

Reference table for B-magnitude homogenization

HyperLeda

Questions: prugniel@obs.univ-lyon1.fr