10.1.2 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 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-$\alpha$ 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 Ly$\alpha$ order is less certain than those for long-wavelength orders where uncontaminated background pixels are common.

Figure 10.3:   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.