A.2.3 Pass 2: Dispersion Direction Swaths

In the second operation, Pass 2, inferred background fluxes at the order positions are sampled and assembled as arrays in the spectral direction. The fluxes from the Pass 1 solutions are used to compute a continuous Chebyshev solution for the background at each echelle order position. The generation of a fit along the positions of the echelle orders proceeds with the computation of a 7th-degree Chebyshev function interpolated for all sample (wavelength) positions. In this step it is not necessary to screen or exclude pixels or to test for data pathologies - only a single iteration is required. The Pass 2 operation tends to dilute the effects of poor solutions from a single Pass 1 swath. However, it also introduces a second smoothing into the final background surface.

Figure A.4 shows a final solution obtained for Order 95 of image SWP20931 on the B0 star HD93222. This order contains a strong Lyman-$\alpha$ feature. The spike pattern indicates samplings from the 26 swaths in Pass 1. A comparison of the residuals (comb pattern in the figure) shows that the solution for the Lyman-$\alpha$ order is less certain than those for long-wavelength orders where uncontaminated background pixels are common.