Analyses of presumably featureless SWP spectral data of white dwarfs have revealed a distinctive absorption artifact at 1515Å. Although this ``feature'' is now known to be present in a variety of the SWP spectra, its presence in the featureless white dwarf spectra analyzed for flux calibration studies was the catalyst for further investigation and allowed the absolute determination of this ``feature'' as an artifact (De La Peña 1992).
Figure 6.1 is a plot of the SWP spectrum of the white dwarf, G191-B2B, a featureless continuum source. The spectra were produced with the standard extraction method of the NEWSIPS image processing system, a Signal Weighted Extraction Technique (SWET).
White Dwarf SWP spectra showing the 1515Å artifact.
Upon further investigation, the 1515Å feature was found to be present in both NEWSIPS and IUESIPS data. Recall that NEWSIPS is using a SWET extraction and the SWP ITF (Epoch 1985) was constructed in raw space. IUESIPS uses a standard boxcar extraction and the SWP ITF (Epoch 1978) was constructed in geometrically corrected space. Therefore, this artifact is independent of the extraction method, and both the ITF epoch and creation method. Despite the major differences in the image processing techniques employed by the two systems, the feature is apparent in both datasets. In addition, not only is this artifact found in these recently obtained white dwarf observations (1991), but it also can be found in extracted spectra acquired as early as 1978.
Using the data from a specific SWP image in which this absorption feature is present, the vector displacement information was used to trace the pixels in the extracted/resampled (MX/SI) images back to the linearized and raw images (LI and RI, respectively), and ultimately back to the ITF. There appears to be a group of ``lazy'' pixels in the ITF at the computed location. These pixels lack sensitivity in the lowest levels of the ITF in that they require more illumination than their neighbors before they ``react.'' Unfortunately, there are several adjacent pixels along the edge of the low-dispersion spectral swath which appear to be less sensitive than the neighboring pixels at low illumination levels. It is simply an unfortunate circumstance that these pixels reside along the edge of the spectral swath, and therefore, manifest themselves as a flux deficit in the extracted data. Because of the difficulty in accurately identifying the pixels contributing to the 1515Å artifact due to the variable placement of the spectrum on the image, this artifact is not flagged.