1. A method for reducing sidelobe formation in an aerial image created from an attenuated phase shift mask used in photolithography, comprising the steps of: a) generating a density map for the input design having a set of nodes identified as being outside of a periphery of the input design; b) examining the aerial image using the density map to compare an image intensity of the aerial image at a plurality of locations, each location corresponding to one node of the set of nodes; c) marking a node of the density map when the image intensity at the corresponding location of the aerial image satisfies a threshold intensity criterion and a separation distance criterion to create a set of marked nodes; and d) masking each node of the set of marked nodes.