THE SOUTHERN PROPER MOTION PROGRAM - I - MAGNITUDE EQUATION CORRECTION

As with virtually all photographic plate material, the astrograph plat es from which the Southern Proper Motion (SPM) catalog is being compil ed suffer from magnitude-dependent systematic shifts in the image posi tions, i.e., a magnitude equation. If left uncorrected, these would ad versely affect both the primary goals of the SPM catalog, the absolute proper motions, and the faint secondary reference frame. Using the di ffraction-grating images present on the plates, a procedure is adopted that allows a spatially variant magnitude equation correction to be d erived individually for each SPM place. Application of the method to s imilar to 120 SPM fields, primarily around the south Galactic pole and the -30 degrees and -35 degrees declination zones, demonstrates its e ffectiveness. Indeed, the method proves superior, in the case of the S PM, to a direct calibration of the magnitude equation using the newly available Tycho Catalogue. The result of the diffraction-grating techn ique is final SPM positions and proper motions that are largely magnit ude equation-free, as verified by numerous internal and external check s.