An alternative method of evaluating mean radial velocity errors for the long-wavelength errors is to cross-correlate the fluxes of IUE spectra with those of HST atlases of Procyon and Arcturus. Note that the suitability of this technique relies upon our previously tying the zero-points of these atlases to the 10 Lac atlas via IUE data. This method is reliable in principle, but in practice it has the drawback that rather few IUE observations are available for these particular stars. In the case of LWR camera for Arcturus, we were able to cross-correlate 9 orders in just three spectra and found a difference, RVLWR - RVSTIS, of +2.7 ± 4 km s-1. These quoted errors and others quoted below include both the r.m.s. velocities for individual orders and the r.m.s. arising from the small numbers of observations. We cross-correlated seven available spectra with the Procyon atlas and found a difference of -3.5 ± 6 km s-1. A comparison between the Arcturus atlas and the LWP datasets is not possible because the IUE observations were made with the star image placed in various ``nonstandard" positions in the large aperture. For the Procyon atlas (3 orders; 4 LWP spectra) the difference, RVLWP - RVGHRS, is +7.9 ± 6 km s-1. However, this difference is noticeably affected by a wavelength shift of one spectrum (LWP 13112) with respect to the three. Although we could find no a priori reason to exclude the LWP 13112 spectrum from our analysis, its omission would result in an RVLWP - RVGHRS difference of +2.4 km s-1. This would bring the LWP scale fully into agreement with the essentially null difference found in the preceding paragraph. Additionally, we compared the mean zero-point difference between the LWP and LWR cameras by cross-correlating many spectra in 59 orders of six stars. These results are summarized in Table 4. The entries in this table give a mean difference of only +0.4 ± 3 km s-1, which is shown as the last entry in Table 3. T his agreement suggests again that the zero-points of these two cameras agree to within errors of about ± 5--6 km s-1.
To summarize all these comparisons, the radial velocity differences of the three IUE cameras, inter alia, are zero to within the measurement errors.
|Star||Mean RV||Number||Star||Mean RV||Number|
|(km s-1)||(km s-1)|
|10 Lac||1.0||3, 13||RR Tel||-3.5||21, 14|
|BD +75o325||3.4||6, 67||Sco||3.3||24, 75|
|BD +28o4211||-3.1||13, 45||HD 11636||1.3||12, 2|