A Study of the Wavelength Calibration of NEWSIPS High-Dispersion Spectra

In this study we cross-correlate many IUE echellograms of a variety of well-observed stars to evaluate systematic error sources in the wavelength zero points (velocities) of all three cameras. We first evaluate differences between the final archived ("NEWSIPS") and the originally processed ("IUESIPS") echellograms. These show a marked time dependence in zero point for the SWP camera resulting from several revisions of wavelength calibration coefficients used for IUESIPS. Smaller offsets are present for the LWR camera between the two processings. We also evaluated small-amplitude fluctuations in the zero points of the NEWSIPS wavelength calibration spectra themselves. In the case of the SWP camera, these variations are too complicated to have been completely removed in the NEWSIPS wavelength calibration. We also examine wavelength zero-point disparities between data obtained both through the small and large entrance apertures as well as for observations made by different target acquisitions of faint and bright stars. We also find that statistical differences between these alternative observing modes are virtually nil. For large-aperture observations, the dominant error source is the placement of the target in the aperture. These give rise to non-Gaussian, extended "tails" in apparent velocity. We also searched for spurious trends. Except for a possible trend for faint objects with SWP camera data, we cannot detect significant dependences with time. Additionally, we discovered a trend with telescope focus for data sets derived from intensive monitoring campaigns of bright stars. These exhibit a repeatable, 1 day "radial velocity variation" with a semiamplitude of nearly 3 km s–1. This pattern appears to be a by-product of fluctuations in telescope focus caused by operational procedures to maintain the ambient instrument temperature.In the second part of the paper, we measure the mean zero-point errors of NEWSIPS echellogram data with respect to laboratory results by using the Goddard High Resolution Spectrograph (GHRS) spectral atlas of the O9 V spectral standard 10 Lacertae as an intermediary reference. We find that the derived apparent velocity difference for this star is essentially zero: –1±3.5 km s–1. Several less precise comparisons lead to similar results, including cross-correlations of (1) spectra of 10 Lac and two stars with similar spectra, HD 93521 and HD 60753; (2) lines in common with the SWP camera and GHRS and STIS atlases of Arcturus and Procyon; and (3) interstellar lines in the GHRS spectrum of the white dwarf G191-B2B. The zero points of the NEWSIPS-processed long-wavelength cameras are evaluated and are also found to be nearly zero (±5 km s–1) relative to the Arcturus and Procyon atlas calibrations and relative to one another. In general, these results do not support the suggestion by Gonzalez-Riestra and coworkers that corrections should be introduced to the wavelength scales of various NEWSIPS high-dispersion data products. Despite our optimistic results, it is obvious that using small IUE data sets from large-aperture observations of arbitrarily chosen stars can contain velocity errors of at least a few km s–1.