2.1 Outline of Process

  In NEWSIPS the background-sampling is done from the geometrically-corrected, resampled image. For high dispersion this is the ``SIHI" image in which echelle orders run strictly parallel along the spectral (``sample" ) direction and are separated from one another in the spatial (``line") direction by a cross-dispersing grating. In the image processing the resampling step includes the operations of image derotation, order ``desplaying" (rectilinearizing), global translation in the spatial direction, and correction for small-scale systematic ``wiggles" in the individual orders. Figure 2 depicts the resulting rectilinear format of the echelle orders and the paths of the extraction slits discussed below.

Figure 2: Road map of the background extraction swaths on the high-dispersion echellogram SWP08683 taken on $\tau$ Sco in 1979. Dotted lines running in the vertical direction indicate positions of extraction ``Pass 1" swaths in the BCKGRD module of NEWSIPS. Raw fluxes sampled are taken along these lines within the target rings and between echelle-order fluxes. The reconstructed background solutions based upon the Pass 1 extractions are 7th degree Chebyshev polynomial functions. These are placed in the positions of these orders (solid lines) in ``Pass 2" of the algorithm. The Lyman $\alpha$ absorption line is located at coordinates (144, 195).

For non-continuum high-dispersion images, BCKGRD extracts fluxes from pixels parallel to the illuminated order. For high-dispersion images with significant continuum flux the algorithm extracts raw background fluxes from pixels along a spatial direction, as indicated by the dotted lines in Fig. 2. For SWP images, 26 extraction paths, or ``swaths," are taken at roughly 27-pixel intervals (25 swaths are taken for LWP/LWR images). In this initial set of extractions a slit five-pixels tall is used. Fluxes from two types of pixels are excluded in this initial extraction. The first group are those pixels which are likely to lie within or at the edges of an illuminated order. These pixels are predetermined from a model of the line positions for each echelle order (this correction is facilitated by the spatial shifting of the image as a part of the geometrical correction process). The second group are pixels which the ``RAW_SCREEN" module has previously identified as having inferior data quality at an earlier stage of image processing. After correction for interorder overlap by a model point spread function (``PSF") in the spatial direction, a Chebyshev solution is fit through the available data and evaluated at all possible line positions. After completing the calculation of these solutions for all 26 swaths (25 for LWP and LWR cameras), a set of Chebyshev solutions is determined by interpolation from the Pass 1 results along the spectral direction and at each of 60 line positions (for SWP) corresponding to an echelle order. This set of ``Pass 2" results constitutes the evaluation of a 60 × 768 pixel background ``surface." Note that the procedure consists of two sets of calculations of one-dimensional flux arrays and not a simultaneous fit to a two-dimensional surface. Each of these steps will be described next in greater detail.