This preprocessing algorithm determines if large-aperture spectral data
should be extracted either as a point or an extended source, as such
information is needed in order to define certain extraction parameters.
A quick rotation of the image to align the spectrum with the horizontal
direction is performed using a nearest neighbor resampling algorithm and
a two-dimensional (2-D) portion of the spectral/background region is
sampled. Starting and ending at a point several pixels above and below
the center of the spectrum, the DN values for every pixel along each
line of the rotated image are averaged together. The resultant
one-dimensional array of numbers represents an average cross-cut of the
spectrum and by counting the number of pixels that are above the average
background, NEWSIPS determines whether the spectrum is a point or
extended source. If the number of such pixels exceeds a certain value
(Table 4.6), then the spectrum is considered to be extended;
The source-type determination algorithm only sets a flux/no-flux flag for small-aperture data, as all exposures acquired in this aperture are considered to be a point source for the purposes of flux extraction.