Hubble’s Wide Field and Planetary Camera 2 (WFPC2) has a
unique feature which is responsible for the oddly-shaped images
it produces. One of its four cameras records a magnified view,
which must then be resized (scaled down) to be in proportion with
the other three images.
The magnified view falls on WFPC2's planetary camera (PC), which
is the same physical size as the three wide field (WF) cameras.
Because the image falling on the planetary camera is magnified,
that camera views a region of the sky four times smaller than
those seen by the wide field cameras, but it records four times
as much detail in that smaller region of the sky. This results
in a finer grid size in the detailed image. The added detail helps
astronomers "zoom in" on a particular section of the
object being observed.