The existence or absence of continuum flux in an IUE echellogram is
determined by the order registration module. Because interorder-overlap
flux can affect background determinations only for continuum images, in
the noncontinuum case BCKGRD does not go through a Pass 1 step.
The background estimates for these images are determined by sampling the
interorder background on the long-wavelength side (spatial) of a given
order with a one-pixel slit. A 7th degree Chebyshev solution is then
computed for this nearly continuous array of interorder pixels. Note
that tests show that some minor contamination from strongly saturated
emission lines has been detected in neighboring orders.
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.