Galaxy Magnitude Offset

Even after applying the magnitude-equation correction described above, a correlation remained between the difference in the amount of correction applied to blue and visual plate pairs and the mean difference in absolute proper motions derived from the blue and visual plate pairs.  This was illustrated by Girard et al. 1998,  (A.J. 115, 855), and used as a diagnostic tool to empirically derive the value of a constant magnitude offset which compensated for the difference in magnitude equation behaviour between star and galaxy images.  The offset value used in the SPM 1.0 catalog was -0.7 mags applied uniformly to all galaxy images on both blue and visual plates.

With a larger number of fields now available, a similar (though slightly different) diagnostic analysis of the SPM 2.0 fields yielded magnitude offset values of -0.5 mags and -1.1 mags for the blue and visual plates, respectively.  The motivation for adopting these offsets is illustrated in the figures below.  The various panels show mean differences in the positions of galaxies on same-epoch blue and visual plates as a function of the difference in the amount of stellar magnitude equation correction applied to those plates.  The left panels show the x-axis (right ascension) differences and the right panels are of the y-axis (declination) differences.  The top panels are of the first-epoch plates, the lower panels are of the second-epoch plates.  Each point in the plot represents a different SPM field, and only those fields with a sufficient number of measurable galaxies are included.  If the amount of magnitude equation correction being applied to the galaxies is appropriate, the mean blue-visual differences will be zero and there will be no correlation with the amount of stellar magnitude equation correction.  This is obviously not the case in the upper figure which shows the results when no galaxy magnitude offsets are applied.

No Offsets Applied

The figure below shows the blue-visual differences which result after applying magnitude offset values of -0.5 mags and -1.1 mags to the blue and visual plates' galaxy images.  The improved agreement between blue- and visual-plate results seen in figures such as the one below, was the basis for adopting these values for the galaxy magnitude offets.

Offsets = -0.5, -1.1