2000ApJS..126..517M

While the low-dispersion IUE New Spectroscopic Image Processing System (NEWSIPS) data products represent a significant improvement over original IUE Spectroscopic Image Processing System (SIPS) data, they still contain serious systematic effects which compromise their utility for certain applications. We show that NEWSIPS low-resolution data are internally consistent to only 10%-15% at best, with the majority of the problem due to time-dependent systematic effects. In addition, the NEWSIPS flux calibration is shown to be inconsistent by nearly 10%. We examine the origin of these problems and proceed to formulate and apply algorithms to correct them to the ∼3% level–a factor of 5 improvement in accuracy. Because of the temporal systematics, transforming the corrected data to the IUE flux calibration becomes ambiguous. Therefore, we elect to transform the corrected data onto the Hubble Space Telescope Faint Object Spectrograph system. This system is far more self-consistent, and transforming the IUE data to it places data from both telescopes on a single system. Finally, we perform a detailed error analysis of the corrected NEWSIPS data. We demonstrate that much of the remaining 3% systematic effects in the corrected data is traceable to problems with the NEWSIPS intensity transformation function (ITF). The accuracy could probably be doubled by rederiving the ITF.