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 wavelength positions. 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 10.3 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 Ly order is less certain
than those for long-wavelength orders where uncontaminated background
pixels are common.
Final background solution for SWP20931, Order 95
(smooth solid line). The comb structure connected to the solution
reflects the solutions for the various Pass 1 swaths sampled at the line
position of this order.