During its operational lifetime (26 January 1978 to 30 September 1996) the International Ultraviolet Explorer satellite observed over 3700 objects outside the Solar System in its high dispersion mode. Because a high percentage of these objects have been regularly observed by other satellites and from the ground, a comparison of radial velocities obtained from this instrument is of considerable value to multi-wavelength and time-monitoring investigations that utilize IUE data. However stable a spectrograph might be, its calibration is susceptible to systematic wavelengths errors, usually arising from such real-world complications as the differing paths of the stellar and emission-line calibration beams through the spectrograph, dispersion nonlinearities, and in the case of the IUE, both image placement in the aperture and a wandering of its echelle spectral format on the detector surface. In this paper one of our goals will be to examine several potential sources of systematic error as a function of various instrumental parameters by means of standard cross-correlation tools. Our purpose is to give a general evaluation of the robustness of the wavelength calibration of the ``NEWSIPS" which, in contrast to its predecessor ``IUESIPS" software, is not known to produce wavelength errors as a function of time or other relevant variables.
Unlike other wavelength comparisons of the IUE calibration, our study relies on cross-correlations of like spectra, that is, of the same star. Cross-correlation studies have a high internal accuracy and offer safeguards against systematic errors in the measurement process. However, they also have the disadvantage of referring the measurements to a reference spectrum template for which the zero-point itself must be determined. Thus, a second goal will be to evaluate the ``absolute" wavelength zero-point errors of the IUE cameras in high-dispersion. This assessment has been carried out with respect to instruments on other space-borne platforms, specifically the Goddard High Resolution Spectrograph; (GHRS; operational period 1990-1997) and the Space Telescope Imaging Spectrograph (STIS; 1997-present) on board the Hubble Space Telescope. We will also compare zero-points of the IUE/SWP wavelength ``system" with ground-based measurements of reference stars and absolute (laboratory) values compiled in the literature (Brandt et al. 1998).
A list of actual camera sequence numbers for observations used in this work are available by request from the MAST. The spectra utilized were the ``absolute calibrated" flux extensions of the ``MXHI" FITS files.