Tests have shown that high-dispersion spectra from each of the three IUE cameras have characteristics that impose unique challenges for automated background extraction algorithms. The great diversity of image types in the archives prohibits implementing any strategy that makes assumptions about the behavior of source spectra in order to fix background problems, particularly in an automated processing environment.
The SWP camera has a unique confluence of conditions, a circular target ring and a blaze function shifted towards short wavelengths along the echelle-dispersion direction, which together make it impossible to sample local background at the short wavelength corner of the image. For spectra of early-type objects, these conditions almost guarantee that for the shortest-wavelength swaths in Pass 1 no pixels will be found at the short end (spatial) of the swath to use for interpolation in the IOR. In these cases the solutions will turn upwards in this region. This problem is addressed at the end of the first pass of BASISMOD by grafting a solution from a neighboring swath in place of the turned-up solution. Experience shows that this strategy is sometimes only partially successful for SWP images. As a result the final background solution in the short-wavelength corner is sometimes too high. This behavior can influence the background solutions for the blue wing of Lyman and lines of neighboring echelle orders that fall in the same blaze region to be as much as a few percent too high relative to the local continuum. This is probably the most significant systematic error produced by BASISMOD. However, we emphasize that this problem is inherent in the placement of these spectral features close to the target ring. Therefore, even customized extractions will not be able to model an accurate local background solution in this region.
Both long-wavelength cameras have the problem of a number of specific pixels being habitually converted from low DNs to highly negative FNs. In general, this is not a problem because BASISMOD clips highly aberrant fluxes before its Chebyshev fitting. However, the problem of clipping only aberrant fluxes is complicated for the LWP camera because of the turndown of the local background at the long-wavelength (spatial) end of the target. This complexity has been treated by disallowing clipping of anomalous fluxes in this region of the LWP image. Another complication is that although a 7th degree is usually sufficient to fit the flux turndown near the target ring, the solution may not adequately follow the turndown if the background fluxes are noisy or have caused the Chebyshev degree to be reduced to less than 6. Generally, there are no "notable" spectral lines in these orders, but a customized extraction can take care of this problem should the need arise.
An additional characteristic of the LWP camera fluxes is the abrupt increase in noise in the short (spatial) end of the image. It is not clear that this causes systematic background deviations, but it may cause a degraded accuracy in the background result.
The LWR camera exhibits a few patches of enhanced sensitivity that are not completely calibrated out in the PHOTOM step. The most dramatic of these is the "flare" at the long spatial and short-wavelength (dispersion) corner of late-epoch images. Inspection of many images shows an upturn in the local background in this area even during the IUE Commissioning Period. Therefore, background fluxes extracted in this area either by NEWSIPS or by customized extractions should be used with caution.