bias If computing polarized intensity, do NOT remove the Ricean bias in the image. By default, the bias is removed to first order with P = sqrt(P_obs**2 - sigma**2) You should have a very good reason for using this option. See VLA memo no. 161 by Patrick Leahy for more details of bias removal. zero When the output pixel is clipped, by setting CLIP(1), setting OPTIONS=ZERO will cause the output polarized intensity image (not the position angle image) pixel to be set to 0.0 rather than being masked out. This is very important if you are interested in doing statistics on a region of low polarized intensity S/N ratio. If you leave the region masked rather than zeroed, you will bias the statistics in that region -- zero is a better estimate of the pixel than just excluding it from the statistics (provided the clip level is sufficiently small). Residual bias in the statistical results from the area then depend upon how well the bias remover works and at what level clipping was performed. See VLA memo no. 161 by Patrick Leahy. radians Output the phase image in radians instead of degrees. microns Output the phase image as equivalent surface error in microns. relax Only warn about image axis descriptor mismatches instead of giving a fatal error bmfit Fit focus and pointing offsets to aperture E-field maps.