Hubble Telescope Functions

Space Telescope Imaging Spectrograph (STIS)

It's one thing to look at the light from a celestial object -- but how can you tell what the object is made of? The colors, or spectrum of light, coming from a star or other celestial object is a chemical fingerprint of that object. The specific colors tell us what elements are present in the object, and the intensity of each color tells us how much of that element is present. So to identify the colors, the specific wavelengths of light, the STIS separates the incoming colors of light much like a prism makes a rainbow.

In addition to the chemical composition, the spectrum can tell us about the temperature and motion of a celestial object. If the object is moving, the chemical fingerprint can be shifted toward the blue end (moving toward us) or the red end (moving away from us) of the spectrum. For example, the STIS slit is centered over the core of galaxy M84 (the blue rectangle in the left side of the figure below). If there were no movement, then the spectrum should be the same across the entire area of the slit. However, the light in the center of the slit is blue- and red-shifted, which indicates that this particular area (within 26 light years of the core) is spinning at a speed of 800,000 mph (400 kps). Astronomers calculated that, to cause such a spin, a massive black hole (~300 million solar masses) must be present in the galaxy's core.

Photo courtesy NASA / STScI WFPC2 (left) and STIS (right) images of galaxy M84. The slit of the STIS is centered over the area shown in the blue rectangle on the left.

Advanced Camera for Surveys (ACS)
Hubble's Faint Object Camera (FOC) was replaced in March 2002 -- it now boasts the Advanced Camera for Surveys (ACS), which, according to CNN.com, offers 10 times the optical clarity of the FOC. To learn about the ACS, check out the Space Telescope Science Institute and the Ball Aerospace ACS page. For example, when aimed at Betelgeuse (a red star in the shoulder of the constellation Orion), the FOC can actually image the surface of the star. This is the first time that the surface of a star other than our sun has been imaged. From the image, scientists determined that Betelgeuse has a mysterious hot spot on its surface that is 2000 degrees Kelvin hotter than the rest of the star's surface.

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Photo courtesy NASA / STScI Credit: Andrea Dupree (Harvard-Smithsonian CfA), Ronald Gilliland (STScI), NASA and ESA FOC image of the star Betelgeuse (left) in the constellation Orion (Betelgeuse is marked by the yellow X).

Fine Guidance Sensors (FGS)
The FGS are used to point the telescope and to make detailed, precise measurements of the positions of stars, the separation of binary stars and the diameter of stars. There are three FGS in the Hubble; two are used to point the telescope and keep it fixed on its target, looking for "guide" stars in the HST field near the target. When each FGS finds a guide star, it locks on to it and feeds information back to the HST steering system to keep that guide star in its field. While two FGS are steering the telescope, one is free to make astrometric measurements (star positions). Astrometric measurements are important for detecting planets, because orbiting planets cause the parent stars to wobble in their motion across the sky.