The Cosmic Origins Spectrograph, or COS, doesn't produce the kind of images people associate with Hubble. Spectrographs are instruments that break light into colors and measure the intensity of each color, revealing information about the object emitting the light. The information they produce is typically plotted, creating zigzagging lines that scientists examine for clues about an object's temperature, density, velocity, chemical composition, and more.
COS focuses exclusively on ultraviolet light and is the most sensitive ultraviolet spectrograph ever produced for space. Its installation improved the telescope's sensitivity at least 10 times in ultraviolet light and up to 70 times when looking at extremely faint objects.
Getting to the point
COS is best at observing points of light, like stars and quasars, while Hubble's other spectrograph, the Space Telescope Imaging Spectrograph, is best at looking at large areas, like galaxies. Together, the two complement each other with their different areas of expertise.
COS has two channels, one to examine far-ultraviolet light, and one to examine near-ultraviolet light. Its streamlined design limits the number of times light bounces off a surface before hitting a detector. Since every time light bounces a little of it is scattered away, the design ensures that the instrument observes the greatest amount of light.
COS, installed in May 2009 during Servicing Mission 4, replaced the Corrective Optics Space Telescope Axial Replacement (COSTAR), the device that corrected Hubble's original blurred vision during the first servicing mission. Since then, all of Hubble's instruments have been designed with that correction built-in, so COSTAR was no longer needed. COS was created by the University of Colorado at Boulder, the University of California - Berkley, and Ball Aerospace & Technologies Corporation.