The characteristic stepped images of the HST's Wide Field Planetary Camera 2 (WFPC2) result from the ccd array used to record images. Four identical ccd chips record images from adjacent pieces of the sky and their pictures are combined together, like tiles, to create a mosaic.
Three of the ccds, labeled A, B, C, record images in 'wide field' mode. Each images a piece of sky one-tenth the diameter of the full moon. A fourth ccd, is used by the telescope's planetary camera (PC). It is set up to record a narrower field of view, one-fourth the area of the wide field cameras, at twice the resolution of the other chips.
When the image from the PC ccd is combined with the three images taken by the wide field ccds, the unique, stepped, chevron like appearance of full field WFPC2 images results.