For continuum source images, a series of 25 or 26 nearly equally spaced extraction swaths (slit height of 5 pixels) are made in the spatial direction of the SIHI image, with a starting position at small spatial pixel numbers (short wavelength end). The swath sample positions were carefully selected to avoid contamination from any possible low-disperson (double) exposure. Except for the first and last few Pass 1 swaths, which form short chords along the left and right edges of the camera image, each swath samples fluxes for nearly the entire range of sample positions; that is to say that they include pixels at the spatial ends of the camera which are not affected by contaminated interorder-overlap flux. The "interorder overlap" flux is described by a PSF model described below. The accumulated effects of overlapping PSFs increase as the orders become more closely spaced. The accumulation causes the interorder overlap to become increasingly severe until the camera sensitivity falls off at short wavelengths. It is this overlap which causes local background extractions in IUESIPS to be systematically high for short-wavelength orders and which necessitated a strategy for BASISMOD to sample background fluxes in distant uncontaminated regions as well as local contaminated ones.
The fluxes sampled from interorder pixels are modified if they are affected by contamination from neighboring orders. A model PSF provides an estimate of how much the fluxes should be offset before the Chebyshev fit is made (see PSF Modeling). The PSF model itself consists of two components, first, a monotonically decreasing function out to about four pixels and, second, a "halation ramp" which extends from four to about seven pixels from the center of each order profile. Each of these components is responsible for order overlap in a particular range of echelle orders. We will refer to the image area where the monotonic portion dominates as the "Interorder-Overlap Region" (IOR). The halation component is actually an extension of the IOR. However, BASISMOD treats it separately because, unlike the IOR, its characterization is independent of the order profiles.
Figure 1 shows a mapping of the swaths extracted for a typical high dispersion IUE image. The IOR and halation-dominated portions of the Pass 1 swath are indicated in Figure 2. The initially sampled interorder fluxes in both the IOR and the halation regions are revised downward during the course of the calculations. The original and revised "working" fluxes are shown in this plot as squares and small crosses, respectively, and the flux revision for one point is shown as a downward pointing arrow. Fluxes for pixels in the halation-ramp region are affected by interorder contamination just like the triangle-inscribed IOR region, but in these cases only from the flat PSFs of the two neighboring orders. Because this overlap is constant, the algorithm may estimate it for several orders along the swath and determine a robust correction for halation. Note that the overwhelming majority of orders are subject to overlap by either the monotonic (IOR) or ramp components of the PSF.
In broad strokes, an initial estimate of the overlap for a given order is performed by computing peaks of adjacent orders above the interorder flux minimum and using the PSF to compute the fractional order flux to be subtracted from the sampled interorder fluxes. Before discussing this procedure in detail, it is necessary first to describe in more detail the different domains of a Pass 1 swath where each of the PSF components dominates.