Starting with the cross-correlation software described in 2.1.
we compared wavelengths for some 50 echelle orders in all small-aperture,
optimally exposed *WAVECAL* spectra (278 for LWP, 291 for LWR, and
468 for SWP) against the reference spectra used by Garhart et al. (1997)
in the *ecreidentify* step of their calibration. The cross-correlations
showed that the zero-point shifts found for the *SWP* data have the
most complicated dependences of all three cameras and, in particular, that
they exhibit fluctuations in wavelength as well as time.
An example of the dissimilar
dependences with time at different wavelengths for this camera is
depicted in Figure 1: and requires some discussion.

**Figure 1*** Cross-correlation shifts of 415 WAVECAL
SWP echellograms during the
lifetime of the IUE for
orders at the opposite ends of the camera, centered at 1168 Å
and 1969 Å.*

In analyzing the *WAVECAL* data, we
found that it was necessary to discard the pre-1979.0 (satellite
commissioning period) *SWP* camera *WAVECAL*s
from further analysis because the zero-points otherwise showed
a very steep increase during 1978. We then fit the shifts for the
remaining sample of 415 spectra with 7th-degree polynomial both
in time and echelle order. No such complications were found for the
long-wavelength cameras. The cross-correlation results for all three
cameras are plotted with time
in Figures 2 and 3.
Figure 2 was constructed by averaging the wavelength shifts
such as those shown in Figure 1 over echelle orders.

**Figure 2 **

*Velocity shifts
of SWP camera WAVECAL echellograms during the IUE
lifetime. The solid line is a cubic polynomial fit (zero-point arbitrary)
which shows the (incomplete) extent to which time dependences were removed
from the WAVECAL wavelength zero points.
Note that at certain epochs the difference between an local mean and the
smooth curve may amount to 2 km s^{-1}. Such residuals are not
removed in the wavelength calibration of NEWSIPS.*

**Figure 3 **

*Wavelength-integrated cross-correlation shifts of WAVECAL echellograms
of the two IUE long-wavelength cameras
during IUE lifetime. Shifts represent wavelength
zero-point differences and are expressed as velocities. Solid line depicts
the fit with a linear (LWP) and cubic (LWR) function.*

The complicated dependence of the *SWP* camera wavelength zero-points
as a function of time and spatial direction (echelle order) is reminiscent
of the complicated decay of this camera's ``null flux" surface with time, as
recently documented by González-Riestra (1998) and Smith (1999).
During 1979-80 the zero-point of the *WAVECAL* spectra increased
smoothly by +15 km s^{-1} (about 2 pixels) for order *m* = 70
(1170), decreased by about -4 km s^{-1} during the mid-1980's,
and decreased rapidly during the last 2-3 years of the mission.
For *m* = 118 (1970), at the other end of the camera, the
corresponding changes in zero-point are +4.5 km s^{-1}, 0 km s^{-1},
and -2 km s^{-1}. One sees that the greatest changes occur in the
``short-wavelength" corner of the echelle surface, primarily during both
the early and late stages of the *IUE* mission. Since changes occurred
in the raw background flux at about the same epochs and in the same
region of the camera as the changes we report here,
it is possible that changes in the camera characteristics influenced the
positions of the echelle format as well as the net fluxes of these orders
as a function of time.

In contrast, the *WAVECAL* zero-points for the long-wavelength cameras
can be fit with low order polynomials, and they do not have a marked
dependence on position on the detector. The *LWP* camera
is particularly well behaved in this respect since its wavelengths
(Fig. 3a) change linearly by
+3 km s^{-1} over the interval 1980-1996.8. For the *LWR* camera
(Fig. 3b) the wavelengths change by about +4 km s^{-1}
during 1978-1980 and remain almost constant thereafter.

Low-order (cubic) fits for the zero-point dependences of the *
WAVECAL*s for the three cameras were hard-coded
into *NEWSIPS* in order to remove such trends from the science data.
For the *LWP* camera data the dependence on
time is linear, so the trends could be removed completely. For the *
LWR* camera the removal of trends is probably very good except for fitting
the rapid increase during the first 2-3 years of the mission.
A cubic polynomial cannot fully remove this early-epoch trend.
As implied above, the complex dependence for the *SWP* camera
cannot be accurately fit with a cubic polynomial function, particularly
either early or late in the mission. This can be seen by comparing
the cubic fit to the shifts in Figure 2. The differences between
this line and the zero-points of the individual *WAVECAL* spectra imply
that, independent of other error sources, *SWP* data can be expected
to have small (
± 2 km s^{-1}) epoch-dependent errors.

Aside from temporal trends in the mean shifts, one can see from
Figures 2 and 3 that the r.m.s. scatter
characteristics of zero-points among *WAVECAL* spectra also
change with time. Considering the *SWP* camera results first,
Fig. 2 shows that starting sometime in 1988-90 the scatter
decreased to only about 60% of its initial value of
± 2.5 km s^{-1}.
Also, the statistical outlying points (defined as those differing from
the epochal mean by at least 1 pixel or 8 km^{-1}) decreased in
occurrence from over 10% of all obervations to only about 4%.
It is possible that these changes are caused by the termination of
adding *TFLOOD* lamp exposures onto the *WAVECAL*
spectra. Other changes were made during this period, such as taking
these observations under more tightly defined constraints in THDA and focus,
might have contributed to the improved scatter characteristics as well.
The scatter for *LWR WAVECAL* data seems to decrease well before 1990,
so the practice of adding *TFLOOD* does not seem to be
important. Also, notice that the scatter for the
*LWP* camera actually seems to have *increased* markedly in 1990.
The suddenness of this change, particularly in the *LWP* camera,
suggests that the change in adding *TFLOOD* flux *might* have
contributed to the scatter of the *WAVECAL* zero-points for the
calibration of this camera.