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This is the last of three papers describing an `absolute' calibration of theGONG magnetograph using and end-to-end simulation of its measurement process.The simulation begins with a MURaM 3D MHD datacube and ends with a `syntheticmagnetogram' of the corresponding magnetic field values as they would beobserved by GONG. We determine a calibration by comparing the syntheticmagnetic field measurements with the MURaM magnetic field values that producedthem. The previous two papers have described the GONG measurement process (bothinstrument and data processing), our simulation of it, and the theory ofmagnetogram comparison and calibration. In this paper, we address some finalpoints on calibration, combine all of this work into a set of calibrationcurves, and consider the results. We also review the results of the previoustwo papers for locality of reference. Our calibration indicates that GONGmagnetograms underestimate weak flux by a factor of over 2 near disk center,but that factor decreases to about 1 as the line-of-sight approaches the limb.A preliminary investigation of the generalizability of these results suggestsother instruments will be affected in a similar way. We also find that somedifferences in previous magnetograph comparisons are artifacts of instrumentalresolution which do not reflect an intrinsic calibration difference, and themeasurements are more similar than sometimes thought. These results aredirectly applicable to question of solar wind prediction model accuracies,particularly in the search for the cause of the common discrepancy betweenpredicted solar wind magnetic flux at 1 AU and values measured in situ bycurrent satellite missions.

Calibrating GONG Magnetograms with End-to-end Instrument Simulation III: Comparison, Calibration, and Results