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Hubble Space Telescope - The Design

light mirror designed primary

The Hubble Space Telescope is a large cylinder sporting long, rectangular solar panels on either side like the winding stems of a giant toy. Almost 43 ft (13 m) long and more than 14 ft (4.2 m) in diameter, this cylinder houses a large mirror to gather light and a host of instruments designed to analyze the light thus gathered.

The telescope itself is a Ritchey-Chretien Casse-grain type that consists of a concave primary mirror 8 feet (2.4 m) in diameter and a smaller, convex secondary mirror 1 foot (.3 m) in diameter that is mounted facing the primary. This pair of mirrors is mounted deep within the long tube of the Hubble's housing, which prevents unwanted light from degrading the image.

Light follows a Z-shaped path through the telescope. First, light from the target travels straight down the tube to the primary mirror. This reflects the light, focusing it on the secondary mirror. The secondary mirror reflects the light again and further focuses it, aiming it through a small hole in the center of the primary at the telescope's focal plane, which is located behind the primary. The focal plane is where the light gathered by the telescope is formed into a sharp image. Here, the focused light is directed to one of the observatory's many instruments for analysis. All data collected by the Hubble is radioed to Earth in digital form.

The Hubble's original complement of instruments, since replaced by a series of space-shuttle service missions, included the Wide Field/Planetary Camera (WF/PC1), the Faint Object Spectrograph (FOS), the High Resolution Spectrograph (HRS), and the High Speed Photometer (HSP). WF/PC1 was designed to capture spectacular photos from space. The FOS, operating from ultraviolet to near-infrared wavelengths, did not create images, but analyzed light from stars and galaxies spectroscopically, that is, by breaking it into constituent wavelengths. The FOS contained image intensifiers that amplify light, allowing it to view very faint, far away objects. The HRS also analyzed light spectroscopically, but was limited to ultraviolet wavelengths. Although it could not study very faint stars as the FOS could, the HRS operated at comparatively high precision. The HSP provided quantitative data on the amount of light emanating from different celestial objects.

Every aspect of the Hubble had to be designed for operation in space. For example, the Hubble is designed to function under radical temperature extremes. Although the vacuum of space itself has no temperature, at the Earth's distance from the Sun, an object in deep shadow cools to a temperature of —250°F ( —155°C) while an object in direct Sunlight can be heated to hundreds of Fahrenheit degrees above zero. The Hubble itself orbits the Earth every 97 minutes, spending 25 minutes of that time in Earth's shadow and the rest in direct sunlight. It thus passes, in effect, from an extreme deep freeze to an oven and back again about 15 times a day, and must be effectively insulated to keep its instruments and mirrors stable.

Another aspect of the Hubble that had to be specially designed for its orbit situation is its pointing system. Because astronomical observations often require minutes or hours of cumulative, precisely-aimed viewing of the target, the Hubble—which rotates with respect to the fixed stars an average of once every 97 minutes—must turn itself while making observations in order to keep its target in view and unblurred; ground-based telescopes must cope with a similar problem, but rotate with respect to the fixed stars at much slower keeps the Hubble aligned while it is observing a target, checking for movement 40 times per second.

Another problem for any space vehicle is the supply of electrical power. In the Hubble's case, a pair of 40 ft X 8 ft (12 m X 2.4 m) solar arrays provide power for the observatory, generating up to 2400 W of electricity. Batteries supply power while the telescope is in the earth's shadow.

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