The IRS extraction software replicates part of the Spitzer data reduction pipeline. A complete description of the pipeline may be found in the IRS Instrument Handbook, and only the extraction part is summarized here. Pipeline operation is controlled, in part, by configuration files referred to as “namelist” files. Calibration data are stored in standard calibration tables. The spectral extraction flow consists of five modules:
Profile
Creates a wavelength-collapsed average spatial profile of the slit used in the observation. See §2.3 for details.
Ridge:
Identifies the location for point source extraction by identifying the peak in the Profile output, or using a user-specified fractional location along the slit. See §2.4 for details.
Extract:
Extracts the one-dimensional spectrum for either point or extended sources. Flux conversion factors are based on standard width extractions. Extended source extraction is only calibrated for full slit (28 pixel width in low resolution modules). Point source extraction may also be performed using Optimal Extraction (§2.5.3). In this mode, the point source width is still employed, but extracted pixels are weighted by a standard point source profile. See §2.5 for details.
Point Source Tune:
Applies photometric “tuning” and flux conversion coefficients for point sources to the 1-D spectra from the Extract. The data are converted to Janskys, and the module corrects the slope and curvature of each order by applying the polynomial coefficients in the fluxcon.tbl file. This correction is based on an order-by-order comparison of calibration data to standard star model spectra. See §2.6 for details.
Extended Source Tune:
Applies the same steps as in Point Source Tune, but optimized for extended sources. See §2.7 for details.
The details of each module's operation are given in the corresponding module descriptions in Chapter 2.
The extraction pipeline takes as input the two-dimensional Level 1 Basic Calibrated Data (BCD) dispersed image in FITS format. Each BCD has an associated uncertainty and bitmask file which indicates individual pixel status. The output from the pipeline is a table file containing the extracted spectrum, propagated uncertainties and status flags. The naming conventions for delivered science products are discussed in the IRS Instrument Handbook.
The source spectrum incident on the array is not rectilinear in either the spectral or cross-dispersed direction. As a result, the Extract module does not extract whole pixels, but rather subdivides the array into a network of polygon-shaped sampling elements referred to as “pseudo-rectangles” that do not necessarily overlap the rectangular pixel grid. These elements allow Nyquist sampling of spectra in the dispersion direction. Extraction is performed by calculating the signal that falls within the boundary of the “pseudo-rectangles.” The extraction window can be visualized with SPICE. Light is assumed to be evenly distributed within a pixel for purposes of calculating fractional contribution.
