3.1.2. Long-Wavelength Cameras:

We also compared IUESIPS-NEWSIPS wavelength zero-point differences for the two long-wavelength cameras; we believe this is the first such study of its kind. This endeavor turned out to be far more difficult because of the paucity of data over an extended time for any given star. As a result, we were unable to extract information on the time evolution of the wavelength zero-point differences for these two cameras.

To determine a IUESIPS - NEWSIPS mean difference for the two long-wavelength cameras, we considered the K2 III star $\alpha$ Boo as well as three of the four photometric calibration (``PHCAL") standards for the IUE mission, $\zeta$ Cas, $\tau$ Sco, and $\eta$  UMa. A number of echelle orders of spectra of the hot stars do not contain many lines in this spectral region, so we selected several orders interactively that gave a reasonable signal in the cross-correlation function and used them alone. The averages of the IUESIPS - NEWSIPS differences can be summarized as follows. For the LWP camera invesigation we obtained +0.5 km s^-1 for $\zeta$ Cas (126 spectra), +3.2 km s^-1 for $\tau$ Sco (91 spectra), +3.6 km s^-1 for $\eta$ UMa (165 spectra), and 4.6 km s^-1 for $\alpha$ Boo (19 spectra). For the LWR camera the corresponding differences are: +5.9 km s^-1 for $\zeta$ Cas (65 spectra), +6.9 km s^-1 for $\tau$ Sco (38 spectra), +6.5 km s^-1 for $\eta$ UMa (107 spectra), and 6.3 km s^-1 for $\alpha$ Boo (11 spectra). Taking the straight mean of these results, we have the following differences:

$\Delta$RV_LWP  =  RV_IUESIPS  -  RV_NEWSIPS  =  +3.0 ± 2 km s^-1

$\Delta$RV_LWR  =  RV_IUESIPS  -  RV_NEWSIPS  =  +6.4 ± 0.5 km s^-1

In these relations we have applied the (larger) r.m.s. values from the LWP camera to the LWR camera. The shift for the LWR camera is almost certainly significantly different from zero. As with the SWP camera, the difference(s) are likely to arise from periodic changes in the IUESIPS wavelength calibration.