The WFPC2 actually has four separate cameras—three Wide Field (WF)
cameras and one Planetary Camera (PC), known as PC1. As this illustration
shows, each camera images one quarter of the scene being photographed.
All except the PC1, that is. It records a magnified view of its portion, which
allows astronomers to see finer detail in that corner of the overall image. The
final picture would look strange this way, so we need to scale the PC1 image down
in size to proportionally match the other three images. (See previous picture for
resized PC1 image.) Now you see why all pictures taken by Hubble's WFPC2 instrument
have a stair-step shape.
Let's back up and start at the beginning. A warning: the raw image beamed down
from Hubble is a far cry from its polished final version.