1. A passive optical system for background suppression in staring array imagers, said system comprising: a dual focus objective lens and polarizing means for providing a linearly polarized image in a first focal image plane and an opposite linearly polarized image in a second focal image plane; a detector array positioned in said first focal image plane closest to said objective lens for receiving a sharp focused linearly polarized image of a high spatial frequency feature target and low spatial frequency background therein and an unsharp defocused opposite linearly polarized image in said second focal image plane which contains a blurred image of said high spatial frequency feature target and an image that contains a faithful reproduction of said low spatial frequency background as in said sharp focused linearly polarized image in said first focal image plane, said focused and defocused images received in each pixel site of said detector array; difference sensing means for sensing said focused and defocused images at each pixel site to provide output signals proportional to the difference between said focused and defocused opposite linearly polarized images; and means for electronically processing said output signals from each of said pixel sites to algebraically subtract said backgrounds and produce an enhanced contrast target signal for said staring array imager.