Wide Field and Planetary Camera 2 (WFPC2) is the
"workhorse" camera for Hubble. It provides us
with pictures of the universe on a grander scale
than any camera to date. The camera can detect
stars over one billion times fainter than we can
see with our eyes. This picture shows astronauts
removing the Wide Field and Planetary Camera 1
(WFPC1), to be replaced by WFPC2. WFPC2 is shaped
somewhat like a grand piano and weighs 619 pounds.
The camera records two-dimensional images through
a selection of 48 color filters covering a spectral
range from far ultraviolet to red wavelengths.
WFPC2 has four charge-coupled detector cameras
arranged to record simultaneous images in four
separate fields of view at two magnifications.
The planetary camera provides a magnification
about 2.2 times larger than the wide field camera.
The planetary camera provides the best sampling
of the telescope's images at visible wavelengths
and is used whenever the finest spatial resolution
uses charge-coupled detectors (CCDs) to take data.
CCDs have finer resolution, better linearity,
and the ability to convert image data directly
into digital form. A CCD consists of an array
of light-sensitive picture elements (pixels) built
upon a thin wafer of silicon. Complex electronic
circuits also built on the wafer control the light-sensitive
elements. As light falls upon the array, photons
of light interact with the sensor material to
create small electrical charges (electrons) in
the material. The charge is very nearly proportional
to the number of photons of light absorbed. The
electronic circuits read out the array and send
signals that allow the reconstruction of the pattern
of incoming light, and hence, a picture.
CCDs in WFPC2 consist of 800 rows and 800 columns
of pixels, equaling 640,000 pixels in each array.
Each pixel can be thought of as a tiny square,
about 15 micrometers on a side.