Spitzer Documentation & Tools
IRS Instrument Handbook

5.6  Description of Modules in the Background Subtraction Pipeline

This pipeline consists of only one module, IMSUBTRACT. It operates on only SL and LL, and only on IRS Staring Mode observations. Its inputs are co-added files coa2d.fits. It mutually subtracts the observations in a pair of nod positions, in order to suppress the sky background signal. Sky-subtracted images produced in this way should only be used for point source observations.

5.6.1             IMSUBTRACT

This module subtracts two FITS files, and also operates on the corresponding uncertainty and mask files. All FITS files must have identical dimensions. In this case, all FITS files are 128128 2-dimensional images. The header from the first input image is copied to the output image.

The output primary image bksub.fits is computed by subtracting the second image from the first. An output pixel will be NaN if either of the corresponding input pixels is NaN.

The output uncertainty image bkunc.fits is computed by combining the two input uncertainty images in quadrature. An output pixel will be NaN if either of the corresponding input pixels is NaN.

The output mask file bkmsk.fits will have a bit set if either or both of the input files bmask.fits has it set at the corresponding pixel.