2.3. Final Background Solutions (Pass 2)

  As a second operation, called Pass 2, background fluxes are taken at line positions of the echelle orders from Pass 1 solutions along the spatial axis. The generation of a fit at the positions of the orders proceeds with the computation of a 7th-degree Chebyshev polynomial interpolated for all sample (wavelength) positions. In this step it is not necessary to screen pixels or to test for pathologies because the solutions for positions to be sampled are guaranteed from the output of the first step. Note also that the result of this pass is always a polynomial of degree 7. Therefore, this pass operates without problems in an automated environment, but of course at a small price of introducing a secondary smoothing in the background surface. Figure 4 shows the final background solution for order m = 95 ($\approx\lambda$1450) taken from an image of an early-type star.

Figure 4: A sample background solution in an SWP high-dispersion image along an echelle order, m = 95. The short vertical lines (``combs") connected to the final solution (smoothed line) are the intersections of Pass 1 solutions at the line values for the particular order shown. The central line is the 7th degree Chebyshev fit to these points. The long vertical lines connecting the zero flux level designate the positions of the first and last extraction swaths of Pass 1 in BCKGRD processing. The flux scale is again in units of IUE Flux Numbers per pixel.