Electromagnetic radiation is another name for the light that all objects radiate. Photons, the basic elements of this light, contain no mass and travel at the speed of light in a vacuum. Photons also travel in waves.

The electromagnetic spectrum (EMS) is the name used to define and organize all types of electromagnetic radiation according to its wavelength, which is measured in nanometers (abbreviated "nm").

Listed below are the categories of light ranging from the shortest wavelength to the longest. Celestial objects radiating this type of light are also included.

• Gamma ray and X-ray (wavelength 0.01 – 10 nm) ; nuclear reactions on pulsars, quasars, and black holes (very hot objects) that occurred at the beginning of known time
• Ultraviolet (wavelength 10 – 380 nm); young hot stars
• Visible Light (wavelength 400 – 700 nm); the range of light waves humans see
• Infrared (wavelength 710 – 1000 nm); glowing dust in our galaxy
• Microwave and Radio (wavelength over 1000 nm) ; hydrogen clouds and cool remnants of supernova explosions

Go to the European Space Agency's site to learn more about the electromagnetic spectrum.

Since the human eye is equipped to see only those objects whose wavelengths range between 400 and 700 nanometers, we must rely on other types of technology to create visible images of objects radiating light outside our range. Digital cameras are accomplishing this task.

Light waves travel at very high velocities. In order to form a photographic image, the photons comprising the light waves need to be slowed down and redirected onto another medium that collects them. Cameras use an optical lens (usually a curved piece of glass) to slow down and refocus the photons. In a film-based camera, the photons are focused onto a sheet of film. In a digital camera, the photons are focused onto a processor called a CCD (charged-couple device). The CCD, invented in 1969, has replaced the function of film and has taken photographic imaging to much greater heights, especially in the field of astronomy.

A CCD is an image processor comprised of silicon chips that are light sensitive. They used to be sensitive to only infrared, visible and ultraviolet light. Now, they can detect x-ray light (through a cooling process) and gamma ray light (through a process known as Compton scattering).
CCDs contain thousands of pixels (photon detectors) which capture the activity of the photons as they emerge from the lens. Every time a photon hits a pixel, an electron is released. The charged particles then move to form analog currents, which an image processing program on a computer can then convert into digital images, visible to the human eye. If a color picture is desired, a beam may be used to divide the light up and push it through the various color sensors (usually light’s three primary colors of blue, red and green are used). In such a case, each pixel location will contain a record of the amount of each of the three primary colors within that pixel.
(See http://www.electronics.howstuffworks.com/digital-camera3.htm (6/07)).

CCD based cameras, QUEST (See http://www.physics.yale.edu/quest/palomar.html (6/07)) and SDSS (See http://www.sdss.org (7/07)) now work in collaboration with telescopes residing on satellites to capture images of the universe in multiple wavelengths of light. Astronomers prefer using CCDs over film for various reasons. It enables them to record very faint objects in a short amount of time, the CCD itself may be reused multiple times to collect additional images, and the CCD based images may be previewed, edited and customized with various colors that will reflect the data presentation needs of the astronomer. Film does not have these qualities. Thus, what was only visible to those looking through a filtered telescope residing on a satellite is now able to be transformed into an image that can be sent all around the world. Being able to create images of the universe in multiple wavelengths gives astronomers (and us) a more complete picture of not only what lies beyond earth, but also what occurred at the very beginning of time.

The above document was written by the webmaster on July 2, 2007.