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- 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- order is less certain than those for long-wavelength orders where uncontaminated background pixels are common.