4.6 Source-Type Determination

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;

 

 

Camera Low High

LWP 15 8

LWR 15 9

SWP 15 11

otherwise it is considered a point source. The determination of point or extended source for low-dispersion large-aperture data is used to activate the detilting algorithm (see Chapter 7.2.3) in the geometric correction step for certain images and to set the width of the spectral extraction swath. If no flux is detected in the large aperture, the source type is set by default using an object class look-up table. Object classes 1-3 and 6-8 default to extended-source extraction. All other object classes default to point source extraction, with one notable exception. If a low-dispersion large-aperture spectrum is designated a multiple or trailed exposure, it will always be extracted as extended, regardless of the results of the automated spectral width determination.

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.