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2.3. Cross-Correlation of Wavelength Calibration Spectra

  In this section we discuss various time-dependent characteristics of the WAVECAL echellograms used to calibrate the science data. It should be emphasized that most of the following results were initially derived for use in removing low-order time-trends in the science data within the pipeline NEWSIPS processing of science spectra. The characteristics for WAVECAL spectra discussed below have only an indirect effect on the systematics and accuracy of the zero-points of science observations.

Starting with the cross-correlation software described in $\S$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 WAVECALs 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 $\approx$ 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 $\approx$+15 km s-1 (about 2 pixels) for order m = 70 ($\lambda$1170), decreased by about -4 km s-1 during the mid-1980's, and decreased rapidly during the last 2-3 years of the mission. For m = 118 ($\lambda$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 WAVECALs 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 ($\le$ ± 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.

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Next: Systematics in IUE Parameter Up: Procedures and Calibrations Previous: The Calibration of Wavelengths