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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 (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
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.