Mean shifts of wavelength zero-points as a function of wavelength extracted from a large number of IUE echellograms for 16 stars (10 Lac, Sco, Cas, Oph, UMa, Lep and the white dwarfs BD+75o325, BD+28o4211, Sirius B, WD0005+511, NGC246, REJ0623-3, EG102, Wolf 1346, GD 394, Gl191-B2B). These shifts were obtained by cross-correlating wavelengths in each high-dipersion echelle order for a number of test images of a star against a reference echellogram. Belts of ± 1 around the mean shifts are drawn from the distribution of the shifts for the individual stars.
A comparison of the velocity fluctuations resulting from the cross-correlations among different stars suggests that the precision of a zero-point determination varies as a weak power of the reciprocal of the net flux (detector counts). In addition, we find that order-to-order shifts are measurably larger for spectra of white dwarfs and rapid rotators (e.g., UMa, Cas). This fact suggests that the random cross-correlation errors increase slightly with line broadening, namely from about ± 3 km s-1 (e.g., Sco) to ± 4 km s-1 (e.g., UMa).