Of course if the input error frames are all zero, no correction for bias is performed and all output errors frames are set to zero. The output images can be rebinned in the two axis directions by an integer number. This permits the error per bin to be reduced or the sampling to be matched to the resolution of the polarization maps. Individual pixel values in the input images can be rejected from inclusion in binned data by setting the sigma rejection parameter to a low value. Some care should be exercised here else valid points will be deleted from inclusion in the sum of a binned region leading to a bias in Stokes parameter determination.
Largish values are useful for rejecting obvious artefacts such as hot pixels or cosmic rays.
For unbinned data this parameter is ignored. Output points with polarization error greater than this bound have pixel values set to zero in all the output images. The number of output pixels whose polarization error exceeded the maximum polarization error bound is also printed to the terminal. WFPC2 Instrument Science Report 97-11. See the ACS Instrument Handbook, Chapter 5 for details.